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Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology


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Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence.
Models of subduction initiation that explain similar ages between the formation of metamorphic soles and associated ophiolites (in cross-section and map view). The high temperature metamorphic sole (shown as a thick, black line) is generated from the crust of the subducting plate during subduction initiation. It may then be preserved at the base of the upper plate (future ophiolite, shown in cross-hatched pattern). Each model predicts a different age relation between the initially subducted crust, the overlying ophiolite, and the time of subduction initiation. Plate ages are schematically shown with darker shades representing older lithosphere. White arrows on subducting plate indicate relative plate motion. (A) Sinking of the subducting plate along a transform fault (TF) or fracture zone (FZ) drives extension in the upper plate, generating the future ophiolite (10). (B) Subduction initiation of distinctly older lithosphere near an active spreading center (27, 28). (C) Subduction initiation along an oceanic detachment fault near a spreading center (12). (D) Subduction initiation very close to or at a spreading center axis (13). Hacker et al. (25) proposed a variant of this in which subduction initiates across a transform or fracture zone with underthrusting directed parallel to an active spreading center axis. Both options are shown in map view. (E) Schematic of the Palawan ophiolite, its metamorphic sole, and the dated lensoid pods preserved within the sole. U-Pb zircon ages of the pods and ophiolite obtained from this study are displayed along with the metamorphic cooling age of the sole (18).
Content may be subject to copyright.
Rapid conversion of an oceanic spreading center
to a subduction zone inferred from
high-precision geochronology
Timothy E. Keenan
, John Encarnación
, Robert Buchwaldt
, Dan Fernandez
, James Mattinson
Christine Rasoazanamparany
, and P. Benjamin Luetkemeyer
Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO 63108;
Department of Earth and Environment, Boston University,
Boston, MA 02215;
Schlumberger-WesternGeco, Geosolutions-Interpretation, Houston, TX 77042;
Department of Earth Science, University of California,
Santa Barbara, CA 93106; and
Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056
Edited by W. G. Ernst, Stanford University, Stanford, CA, and approved October 11, 2016 (received for review June 20, 2016)
Where and how subduction zones initiate is a fundamental tectonic
problem, yet there are few well-constrained geologic tests that
address the tectonic settings and dynamics of the process. Numerical
weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L
(2004) Geochem Geophys Geosys 5:Q07001], but previous studies
have not favored them for subduction initiation because of the pos-
itive buoyancy of young lithosphere. Instead, other weak zones,
such as fracture zones, have been invoked. Because these models
differ in terms of the ages of crust that are juxtaposed at the site of
subduction initiation, they can be tested by dating the protoliths of
metamorphosed oceanic crust that is formed by underthrusting at
the beginning of subduction and comparing that age with the age
of the overlying lithosphere and the timing of subduction initiation
itself. In the western Philippines, we find that oceanic crust was less
than 1 My old when it was underthrust and metamorphosed at the
onset of subduction in Palawan, Philippines, implying forced sub-
duction initiation at a spreading center. This result shows that young
and positively buoyant, but weak, lithosphere was the preferred site
for subduction nucleation despite the proximity of other potential
weak zones with older, denser lithosphere and that plate motion
rapidly changed from divergence to convergence.
subduction initiation
Subduction is the major driver of plate motion (1), and pro-
cesses at subduction zones are largely responsible for the
growth and evolution of continents through accretion, collision,
and magmatism. Despite its importance, the process of subduction
initiation is still debated partly because evidence from the early
stages of subduction is often obscured by later deformation and
magmatism. The lack of geologic constraints on how subduction
zones initiate remains a significant void in our understanding of
Subduction initiation has been addressed primarily through nu-
merical modeling (29). These studies have demonstrated the need
for a weak zone in the lithosphere to facilitate subduction. Based on
this, subduction has been proposed to initiate in a variety of settings
such as transform faults or fracture zones (10), passive continental
margins (6, 11), oceanic detachment faults (12), and oceanic
spreading centers (13). Spreading centers have been the least fa-
vored, however, because the lithosphere there is positively buoyant.
Two contrasting ideas regarding the dynamics (i.e., the forces) of
subduction initiation have been explored. In spontaneoussub-
duction initiation, a plates increasing density with age may even-
tually cause it to sink into the underlying asthenosphere (10, 14),
whereas in forced subduction initiation, external plate forces are
required to initiate subduction (3, 5, 15). Oceanic plates that are at
least 10 My old are negatively buoyant (16) and may undergo
either forced or spontaneous subduction initiation. Subduction
initiation within very young lithosphere near a spreading center,
however, can only be forced because the plate is still positively
buoyant. Interestingly, some numerical models predict that despite
the buoyancy of the plate and the ridge push force, the forces re-
quired to initiate underthrusting within the young, thin lithosphere
of a spreading center are lower than within older, thicker litho-
sphere, which requires increasingly larger forces to cause down-
bending of the stronger plate (1). Self-sustained subduction, driven
by a plates negative buoyancy, might eventually be achieved after
initiation at a spreading center, if forced convergence is sustained
until older, denser lithosphere finally enters the trench.
Well-constrained geologic tests of the aforementioned models
are necessary to carry the debate forward. Because transform faults,
fracture zones, and continental margins juxtapose lithosphere of
different ages, whereas plates of equal and approximately zero age
are adjacent at spreading centers, determining where subduction
has initiated may be possible by comparing the ages of the un-
derthrust and overriding lithosphere at the time of subduction
initiation, in relation to the time of subduction initiation itself.
The timing of subduction initiation in some paleo-subduction
zones may be determined by constraining the timing of high tem-
perature metamorphismassociated with the initiation of sub-
ductionof the uppermost portions (i.e., the crust) of the initial
subducted plate. This metamorphic material may be transferred to
(or welded) and preserved underneath the mantle peridotite
hanging wall of the nascent subduction zone forearc of the upper
plate as heat from the overlying mantle, and the resulting ductile
shearing, progressively propagates down into the cold underthrust
Subduction, the process by which tectonic plates sink into the
mantle, is a fundamental tectonic process on Earth, yet the
question of where and how new subduction zones form remains
a matter of debate. In this study, we find that a divergent plate
boundary, where two plates move apart, was forcefully and
rapidly turned into a convergent boundary where one plate
eventually began subducting. This finding is surprising because,
although the plate material at a divergent boundary is weak, it is
also buoyant and resists subduction. This study suggests that
buoyant, but weak, plate material at a divergent boundary can
be forced to converge until eventually older and denser plate
material enters the nascent subduction zone, which then
becomes self-sustaining.
Author contributions: T.E.K. and J.E. designed research; T.E.K., J.E., R.B., D.F., J.M., C.R.,
and P.B.L. performed re search; R.B. contributed n ew reagents/analytic tool s; T.E.K., J.E.,
R.B., D.F., J.M., and C.R . analyzed data; J.E., D.F., C. R., and P.B.L. performed fieldwo rk;
and T.E.K., J.E., and R. B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
To whom correspondence should be addressed. Email:
This article contains supporting information online at
1073/pnas.1609999113/-/DCSupplemental. PNAS Early Edition
crust (17). These high-temperature metamorphic solesthat form
during the initiation of subduction (18, 19) are found at the base of
ophiolitessections of oceanic lithosphere now on landthat are
often thought to represent the trapped forearc (i.e., the upper plate)
of subduction zones (20, 21). The high-temperature (>700 °C)
amphibolite-granulite facies rocks preserved in metamorphic soles
(18, 22, 23) indicate that they formed during the very early stages
of subduction, before the development of depressed isotherms
that produce typical blueschist facies rocks during more mature
subduction (24). Also important, the high pressures (10 kbar)
(18, 22, 23) associated with the formation of some metamorphic
soles indicate that they formed at oceanic mantle depths during
subduction initiation rather than by some other process like oce-
anic core complex formation or intraoceanic thrusting unrelated
to subduction initiation.
Metamorphic soles commonly have metamorphic cooling ages
that are similar to the igneous crystallization ages of their overlying
ophiolites (19, 25, 26), implying that the overlying ophiolite was still
very young or formed during or shortly after subduction initiated.
However, because these cooling ages reflect the time of meta-
morphic cooling and not necessarily the time when the original ig-
neous crust formed, the similarity in ages between the metamorphic
sole and overlying ophiolite may be interpreted in several ways:
(i) subduction initiation between older lithospheric plates followed
by rapid slab rollback and seafloor spreading that generates the
ophiolite eventually preserved with the sole (10); (ii) subduction
initiation of variably older lithosphere beneath an already active
spreading center (27, 28); (iii) subduction initiation along weak
detachment faults at some distance from a spreading center (12), or
(iv) underthrusting of young lithosphere at a spreading center (13)
(Fig. 1). Without the age of the initially subducted plate, it may not
be possible to differentiate between these models.
The geochemical affinities of the ophiolite-sole pairs could vary
depending on where, and how, the subducting and overriding
plates were generated. Thus, the geochemistry of the sole and
overlying ophiolite could be similar or different and may provide
some constraint on the different models outlined in Fig. 1. Al-
though geochemical investigations of sole-ophiolite pairs may
prove useful, the age relationships discussed above can provide
further tests on the subduction initiation scenarios. Although all
four scenarios listed above predict similar ages for the initiation of
subduction (i.e., the metamorphic age of the sole) and the over-
lying ophiolite, each case predicts a different age relationship be-
tween the initially subducted crust (i.e., the metamorphic soles
protolith age) and subduction initiation. A determination of the
igneous crystallization age of the protolith of the high temperature
metamorphic sole has been the missing piece of information in all
previous studies of subophiolitic metamorphic soles and is key to
testing the various models of subduction initiation.
Cenozoic Subduction and Collision in Palawan
We applied the foregoing test to the metamorphic sole of an
ophiolite associated with a young, short-lived subduction zone in
Palawan, western Philippines (Fig. 2). The central Palawan
ophiolite was trapped in the forearc of the subduction zone that
generated the Cagayan arc (18). Subduction began at 34 Ma and
lasted for 20 My before it was terminated by microcontinent-arc
collision (18, 29, 33). The subduction zones relatively young age
Subducting Plate
Mantle upwelling
Metamorphic sole
protolith significantly
older than both the
sole and the
Metamorphic sole
protolith older than
the sole and slightly
older than the
Metamorphic sole
protolith age similar to
(or indistinguishable)
from the sole and the
ophiolite ages Ages are compatible
with Model (D)
Metamorphic sole
protolith significantly
older than both the
sole and the
detachment fault
Igneous crystallization
of ophiolite
34.1 ± 0.1 Ma
Cooling of the
metamorphic sole
through 550-400°C
34.2 ± 0.6 Ma
Igneous crystallization
of sole protoliths
35.25 ± 0.15 Ma
35.242 ± 0.062 Ma
35.862 ± 0.048 Ma
Fig. 1. Models of subduction initiation that explain similar ages between the formation of metamorphic soles and associated ophiolites (in cross-section and
map view). The high temperature metamorphic sole (shown as a thick, black line) is generated from the crust of the subducting plate during subduction
initiation. It may then be preserved at the base of the upper plate (future ophiolite, shown in cross-hatched pattern). Each model predicts a differentage
relation between the initially subducted crust, the overlying ophiolite, and the time of subduction initiation. Plate ages are schematically shown with darker
shades representing older lithosphere. White arrows on subducting plate indicate relative plate motion. (A) Sinking of the subducting plate along a transform
fault (TF) or fracture zone (FZ) drives extension in the upper plate, generating the future ophiolite (10). (B) Subduction initiation of distinctly older lithosphere
near an active spreading center (27, 28). (C) Subduction initiation along an oceanic detachment fault near a spreading center (12). (D) Subduction initiation
very close to or at a spreading center axis (13). Hacker et al. (25) proposed a variant of this in which subduction initiates across a transform or fracture zone
with underthrusting directed parallel to an active spreading center axis. Both options are shown in map view. (E) Schematic of the Palawan ophiolite, its
metamorphic sole, and the dated lensoid pods preserved within the sole. U-Pb zircon ages of the pods and ophiolite obtained from this study are displayed
along with the metamorphic cooling age of the sole (18).
| Keenan et al.
and short duration make it an ideal site to study subduction initi-
ation because it was terminated before a potentially complicated
history might have ensued.
The island of Palawan flanks the southern margin of the South
China Sea (Fig. 2). Its geology consists of ophiolite-related rocks
(31, 32) and continental crust [the North Palawan continental
terrane (NPCT)] that rifted from southeast China during the
opening of the South China Sea Basin (SCSB) (29, 34). The
Cagayan Ridge is located to the southeast of, and trends parallel
to, Palawan. It is composed of calc-alkaline volcanic rocks (33) and
is the volcanic arc associated with the subduction zone whose in-
ception is preserved on Palawan (18, 29, 33, 34). To the northwest
of central and southwest Palawan is the Palawan trough (Fig. 2), a
linear depression that, based on seismic reflection observations, is
interpreted as a downwarped segment of the southern edge of the
NPCT and contiguous proto-SCSB that is underthrust beneath the
Palawan ophiolite (35, 36). The proto-SCSB is Cretaceous oceanic
lithosphere that existed south of the NPCTsoutheast China pas-
sive margin before opening of the SCSB (29). Allochthonous
remnants of this older Cretaceous ophiolite are found in tectonic
windows beneath the 34 My old (see below) Palawan ophiolite
(18, 31, 36). Gravity data are consistent with the younger Central
Palawan ophiolite being rooted in the south (37) and thrust
northward on to the continental crust of the NPCT (Fig. 2D)(32)
in a manner similar to Tethyan ophiolites (20).
In summary, all of the available evidence (onshore geologic and
offshore drill hole, seismic, and gravity data) from Palawan and
the surrounding areas is consistent with a south-southeast dipping
Basalt / Diabase
Reverse Fault
Foliated flyschoid
metasediments; some
pillow basalts (L. Cretaceous
to E. Eocene)
Clastics and Carbonates
(Upper Eocene? - Miocene)
Alluvium/ River Deposits
(Pliocene - Recent)
15 km
Inferred Fault
Palawan NPCT
Palawan Trough
Cagayan Ridge
Borneo Celebes
Sea Basin
Palawan Ophiolite
submerged Eurasian
continental crust
oceanic crust of
marginal basins
spreading axis
NW limit of Palawan
thrust wedge
margin of
NPCT Ulugan Bay
Red River-
shear zone
Normal fault
Sagasa Point
metamorphic sole
(panel C)
118 °0 0’ 11 45
110° 120°
Dalrymple Point
100 m
Epidote amphibolite
1 -
2 -
3 -
4 -
5 -
Inferred contact
Inferred fault
Amphibolite gneiss
with hornblendite lenses
Garnet amphibolite
Hornblendite, quartzite,
kyanite schist
Mantle peridotite
Location of photos
in Figure 3
clastic rocks
10 km
Schematic NW-SE section across Ulugan Bay area (see panel “b”)
Fig. 2. (A) Present tectonic setting of Palawan island, Philippines (18, 29, 30). Rectangle outlines the area shown in B.(B) General geology of central Palawan
showing locations of sample sites. The general structure consists of an 34-Ma ophiolite (the Central Palawan ophiolite) thrust over deformed Cretaceous-
Eocene turbiditic sedimentary rocks of the NPCT. Remnants of the older Early Cretaceous proto-SCSB ophiolite are found as occasional pillow lavas in tectonic
windows in the younger Palawan ophiolite (geology from our field observations and refs. 18 and 31). (C) Geologic map of the metamorphic sole at Dalrymple
Point. See Bfor location. Background image from Google Earth (Digital Globe, CNES/Astrium). Apparent metamorphic grade decreases away from the mantle
peridotite. (D) Schematic NW-SE cross section of Palawan in the Ulugan Bay area after ref. 32.
Keenan et al. PNAS Early Edition
subduction zone that formed the Cagayan arc, which places the
Palawan ophiolite in the forearc. This subduction zone was ter-
minated when the southern edge of the rifted margin of China
(the NPCT) jammed the trench in the Middle Miocene (33),
causing obduction of the Palawan ophiolite. The question of how
subduction initiated in Palawan is unresolved. Mitchell et al. (32)
speculated that subduction may have initiated at a spreading
center, whereas the model of Encarnación et al. (18) shows initi-
ation at a transform fault or fracture zone. Both of these proposals
lack the necessary age data to properly test these models.
Age and Geochemistry of the Palawan Ophiolite and Its Sole
The central Palawan ophiolite is a relatively coherent section of
oceanic lithosphere consisting of pillowed and massive lavas, diabase
dikes, plagiogranite, gabbro, troctolite, and mantle harzburgite (31,
32) (Fig. 2). Samples of pillow lavas, dikes, and gabbroic intrusions,
as well as felsic intrusions (plagiogranite) (Fig. 3) collected from
the ophiolite, show slight light rare earth element (LREE) de-
pletions and relatively flat middle REE and heavy REE patterns,
about 10 times chondritic values, consistent with midocean ridge
(MORB)-like magma (Fig. 4A). A similar MORB to transitional
MORB-island arc basalt (IAB) or suprasubduction zone signature
is evident in other multielement plots. Tectonic discrimination
diagrams that use robust statistical tests (38) assign the data within
the MORB field (Fig. 4B) or to a transitional MORB-IAB field
(Fig. 4C), consistent with a backarc basin basalt (BABB)-type
geochemistry (39) and consistent with many other ophiolites (40)
formed at oceanic divergent-type boundaries.
Based on zircon U-Pb geochronology from a plagiogranite
sampled near Penacosa Point (Fig. 2), the best estimate for the
age of the ophiolite is 34.1 ±0.1 Ma (Fig. 1 and Table S1). The
regional distribution of rock types (Fig. 2) shows that the outcrops
of gabbroic and tonalitic intrusives in the Penacosa Point area are
located in the upper levels of the main gabbroic crustal section
transitional to the extrusive section of the ophiolite. The Penacosa
Point plagiogranite exhibits field relations that are consistent with
the felsic magma being comagmatic with the dominant mafic
magmas comprising the bulk of the oceanic crustal section here
(Fig. 3). In addition, the geochemistry of the plagiogranite is
consistent with simple fractional crystallization from the dominant
basaltic magma (Fig. 4A).
The metamorphic sole of the ophiolite is exposed in the Dal-
rymple Point area and is composed of ductiley strained garnet
amphibolites, hornblendites, amphibolites, epidote amphibolites,
quartzites, and kyanite schists with isoclinal folds and a distinct,
penetrative mineral lineation in most rocks (Fig. 2). These rocks
represent metamorphosed igneous, basaltic oceanic crust and as-
sociated sediments (cherts and mudstone). As in other meta-
morphic soles, the higher temperature garnet amphibolites and
hornblendites are located closer to the basal mantle harzburgites,
whereas lower temperature epidote amphibolites tend to be
structurally lower. At several locations, more highly strained sole
rocks enclose less deformed (or isotropic) and more competent,
irregularly shaped lensoid pods of epidote amphibolite (decimeter-
meter scale) and smaller lensoid pods of amphibolite (a few mil-
limeters to centimeters in thickness and several centimeters to
decimeters in diameter) (Fig. 3). Thermobarometric determina-
tions show that the garnet amphibolites of the sole reached peak
metamorphic temperatures of 700760 °C and pressures exceeding
9kbar(27 km depth in the mantle) (18), conditions consistent
with those that the crust of a subducting plate would be subject to
during the earliest stages of subduction (23). Critical to this study,
previous work (18) has constrained the minimum age for sub-
duction initiation by determining
Ar cooling ages on two
hornblende samples and one white mica sample (from garnet
amphibolite, amphibolite, and kyanite schist, respectively) in the
sole. These ages are indistinguishable at 34.2 ±0.5, 34.2 ±0.6, and
34.25 ±0.3 Ma, respectively (Fig. 1) (corrected for new revised
ages of the neutron flux monitors) (41), and indicate rapid cooling
of the sole to 550400 °C after reaching peak metamorphic tem-
peratures (18). Lower grade, altered greenstones, and pillow lavas
structurally beneath the metamorphic sole are exposed further
south in the Sagasa Point area (Figs. 2 and 3). These rocks are less
metamorphosed oceanic components underthrust beneath the
Palawan ophiolite and its sole sometime after the initial un-
derthrusting associated with subduction initiation.
Overall, metabasite samples from the metamorphic sole are
geochemically similar to the ophiolite in that they plot in the
MORB-like to transitional MORB-IAB fields (Fig. 4). Several
samples of the epidote amphibolite pods are depleted (23 times
chondritic values) relative to the MORB-like samples and have
positive Eu anomalies, indicating they were probably cumulate
50 um 100 um 100 um 100 um 100 um
100 um
Fig. 3. Photographs of outcrops from the central Palawan ophiolite (A) and its metamorphic sole (BF) and cathodoluminescence images of extracted zircons
from selected samples (GL). (A) Magma-mingling structures exhibited by light-colored tonalite (plagiogranite) and diorite-gabbro (darker) at Penacosa Point.
The tonalite yielded zircons with a crystallization age of 34 Ma. Pencil for scale. (B) Layered chert/quartzite and amphibolite showing sheath folds.
(C) Amphibolite gneiss with hornblendite domains exhibiting isoclinal folding. (D) Foliated and lineated epidote amphibolite, looking west; mountains
across Ulugan Bay are mantle harzburgite of the Palawan ophiolite structurally overlying metamorphic sole rocks; strike and dip symbol indicates foliation.
(E) Smaller, foliation-parallel, light-colored lensoid pods of amphibolite (sample PL-14-07). (F) Competent, light-colored pod of epidote amphibolite (with
cumulate gabbro-like REE signatures; sample PL-14-05) enclosed within the strongly foliated amphibolite. These pods yielded zircons with crystallization ages
of 35.242 Ma. Hammer for scale. (Gand H,Iand J, and Kand L) Cathodoluminescence images of zircons, showing magmatic oscillatory zoning (samples PL-14-
05, PL-14-06, and PL-14-07, respectively).
| Keenan et al.
rocks (gabbros, based on major element chemistry) of the mid-
lower crust. One of the smaller pods of amphibolite displays a
negative Eu anomaly and is slightly more enriched than the larger
epidote amphibolite pods. This sample may have crystallized from
a magma following the extraction of plagioclase. The rocks of the
sole are therefore not geochemically unlike the overlying ophiolite
and probably formed in a similar petro-tectonic setting.
Based on their geochemistry and geologic context within the
high temperature sole, we interpret the more competent pods to
be middle- to lower-level crustal rocks of the leading edge of the
subducted plate. During the period of underthrusting to sub-
duction, the underthrust crust was sheared, thinned, and ductiley
folded, resulting in the transposition of middle to lower crustal
gabbroic rocks with upper crustal basaltic rocks, both of which
were subject to the high temperature and high pressure meta-
morphism that formed the metamorphic sole (Fig. 1).
Zircons from two of the larger pods (PL-14-05 and PL-14-06)
and one smaller pod (PL-14-07) from the high temperature
chronology. The four analyzed zircons from sample PL-14-05
yielded internally and externally concordant ages with an error-
weighted mean age of 35.242 ±0.062. Three of five analyzed
zircons from sample PL-14-06 yielded internally and externally
concordant ages with a mean of 35.862 ±0.048 Ma (Fig. 1, Table
S2,andFig. S1). Three externally discordant but internally con-
cordantagesof37.00±0.16, 35.97 ±0.11, and 35.25 ±0.15 Ma
(Fig. 1, Table S2,andFig. S1) were obtained from the smaller pod.
The age of the youngest zircon, 35.25 ±0.15 Ma, is taken as the
best estimate of the final crystallization age of the protolith of this
sample. The small spread in ages seen in this sample is similar to
those revealed by high precision U-Pb geochronology in the
Samail ophiolite (44) and may be due to prolonged zircon crys-
tallization in a replenished magma chamber or assimilation of
slightly older wall rock.
Establishing that these zircons are igneous, and not meta-
morphic, is critical because the age of metamorphic zircons would
merely represent the age of metamorphic sole formation (sub-
duction initiation) instead of the crystallization age of the meta-
morphic sole protoliths. Although metamorphic zircon growth has
been shown to occur under amphibolite facies conditions (45),
cathodoluminescence imaging shows no evidence of metamorphic
overgrowths in these zircons. Instead, they are euhedral, prismatic,
and have distinct, fine, oscillatory zoning, a feature that is char-
acteristic and unique to magmatic zircons (46) (Fig. 3). Even
though Th/U ratios are not completely reliable indicators of
magmatic vs. metamorphic zircon, we note that the Th/U ratios in
these zircons (>0.1) are consistent with many magmatic zircons
(47). We are therefore confident that these zircons are igneous and
that their ages reflect the original igneous crystallization age of the
oceanic crust that was underthrust, and then metamorphosed, at
the onset of subduction.
Forced Subduction of Young, Buoyant Lithosphere
As discussed earlier, a critical test to constrain the tectonic setting
of subduction initiation is a comparison of the igneous ages of the
underthrust and overriding lithosphere in relation to the time of
subduction initiation. A positive test for subduction initiation at an
active spreading center is to find all three events very close in age.
We find that the age differences between the upper plate (the
Palawan ophiolite), the subducting plate (protoliths of the sole),
and metamorphism of the sole are less than 1My(Fig.1).The
very small age difference between formation of the sole protolith
and its metamorphism during subduction initiation leads us to re-
ject outright the models shown in Fig. 1 Aand B.Furthermore,the
model shown in Fig. 1Cis rejected for subduction initiation at
detachment faults that are far from the spreading center. We
therefore conclude that subduction must have initiated in very
close proximity to, or at, a spreading center (Fig. 1D). Our age data
do not differentiate between the ridge parallel and ridge normal
subduction initiation scenarios in Fig. 1Din cases where the ridge-
transform fault segments in the right-hand scenario are very short.
The similarity in the geochemistry of the sole and overlying
ophiolite also supports subduction initiation close to a spreading
center, where the eventual upper plate and lower plate (meta-
morphic sole) are not expected to be geochemically different.
Our data show that oceanic crust was formed at 35.24 Ma and
was then underthrust/subducted almost immediately, reaching
27 km depth, metamorphosed to amphibolite, and subsequently
cooled to 400 °C by 34.25 Ma, a remarkably short interval be-
tween crust formation and subduction. Assuming a slab dip con-
trolled by an 30° dipping lithosphereasthenosphere boundary,
this underthrusting would require a convergence rate on the order
of 5 cm/y.
In cases where the detachment fault (Fig. 1C)islocatedvery
close to, or at the spreading center, the models shown in Fig. 1 C
and Dbecome indistinguishable using age data alone. The weak-
ness at which subduction initiated could have been a detachment
fault very near the spreading center axis or at the spreading center
axis itself. Although the exact nature of the weak zone could be
debated, the high-precision age data from our sample site tightly
- Metamorphic Sole
- Palawan ophiolite
Ti / 50
Ti / 50
V50 * Sm 5 * Sc
Sample / Chondrite
- Metamorphic Sole (dated samples)
- Metamorphic Sole
- Palawan ophiolite (dated sample)
- Palawan ophiolite PW-00-18
(34.1 Ma)
(35.25 Ma)
(35.862 Ma)
(35.242 Ma)
Fig. 4. Geochemical data on samples from the central Palawan ophiolite
and its metamorphic sole. Palawan ophiolite samples plotted in ACinclude
pillow lavas, mafic dikes, gabbroic intrusions, and (A) felsic intrusions (pla-
giogranite). Metamorphic sole samples plotted in ACinclude amphibolites,
epidote amphibolites, and garnet amphibolites. (A) Chondrite normalized
REE concentrations in the samples. Samples that were selected for U-Pb
zircon geochronology are symbolized by diamonds and their ages are in-
dicated next to the data. The majority of samples have REE patterns re-
sembling MORB and possible differentiates of MORB. Two samples (PL-14-06
and PL-14-05) display positive Eu anomalies indicating cumulate plagioclase
in the samples. The geochemistry of the plagiogranite (PW-00-18) is consis-
tent with simple fractional crystallization from the MORB-like basaltic
magmas. (B) Ti-V-Sm and (C) Ti-V-Sc tectonic discrimination diagrams (38).
Basaltic samples from the ophiolite and sole are similar and plot as MORB or
transitional MORB-IAB.
Keenan et al. PNAS Early Edition
constrains subduction initiation at, or very close to the spreading
axis. If a detachment fault was the weakness, it would have to have
been located very close to the ridge where nascent oceanic crust
was generated and then underthrust and metamorphosed at a
depth of 27 km less than 1 My later.
The much younger age of the Palawan ophiolite compared with
the Cretaceous proto-SCSB suggests that subduction was initiated
within a young marginal oceanic basin hosted in older Cretaceous
proto-SCSB (Fig. 5). Because of the positive buoyancy of young
oceanic lithosphere, subduction initiation must have been forced
in this case. Following forced underthrusting of the young, buoy-
ant lithosphere, the old, cold, and dense Cretaceous lithosphere of
the proto-SCSB would have eventually entered the trench and
enabled the transition to self-sustained subduction until the NPCT
collided with the trench, caused obduction of the Palawan
ophiolite and its sole, and terminated subduction.
Regionally, the origin of the force that converted a divergent
boundary to a convergent boundary was probably the collision of
India with Asia. Tapponnier et al. (49) proposed that this collision
resulted in the extrusion of fairly rigid continental lithospheric
blocks from the southeast margin of Asia along large strike-slip
faults, such as the Red River shear zone (Fig. 2). The timing of the
onset of strike-slip movement along the Red River shear zone
has been estimated by U-Pb ages on monazite included in shear-
related rotated garnets at 34 Ma (48). This age coincides well
with the timing of the initiation of subduction in Palawan. Seafloor
spreading in the South China Sea, which accompanied the south-
ward motion of the NPCT, began around 3230 Ma (30), a time
also compatible with initiation of subduction in Palawan south of
the NPCT (18) and southward convergence of the NPCT with the
Palawan subduction zoneCagayan arc.
If the reconstruction of Fig. 5 is correct, the initiation of sub-
duction at the Palawan spreading center, forced by the extrusion of
Indochina, supports the results of numerical modeling (1), which
predict that the thin lithosphere at oceanic spreading centers re-
quires less force to converge comparedwithareasof thicker lith-
osphere. Several potential weak zones existed in the area: (i)the
southeast China passive margin, (ii) the contact between the older
Cretaceous proto-SCSB lithosphere and the much younger Pala-
wan marginal basin, and (iii) the spreading center of the Palawan
ophiolite. Despite the presence of older and denser oceanic litho-
sphere, our results indicate that subduction nucleated within the
young, buoyant, but weaker Palawan marginal basin. Presumably,
after the eventual underthrusting of the older Cretaceous litho-
sphere and an additional critical convergence of 100130 km,
subduction became self-sustaining (1).
There is currently no known modern analog for subduction
initiation exploiting an active oceanic spreading center. Intra-
oceanic subduction appears to be initiating along several sections
of the AustralianPacific plate boundary south of New Zealand,
exploiting weaknesses associated with extinct spreading centers
and/or fracture zones undergoing transpressional deformation (50
52). However, unlike the case in Palawan where deep un-
derthrusting occurred within 1 My of oceanic crust formation,
spreading at the boundary south of New Zealand had ceased and
was followed by strike-slip deformation several million years before
Cretaceous oceanic lithosphere
SE China
Spreading Center
Detachment of Palawan ophiolite at
ridge and initiation of subduction
Onset of Red River
shear zone (~34 Ma)
oceanic lithosphere
Break-up rifting.
Seafloor spreading starts in
South China Sea at 32-30 Ma
spreading center
Detachment of Palawan ophiolite
at the spreading center and forced
initiation of subduction (~35-34 Ma)
due to extrusion of Indochina
Passive margin Extinct fault or
‘intrusive’ boundary
Palawan ophiolite
Fig. 5. Subduction initiation (3435 Ma) at the spreading center that generated the Palawan ophiolite. (A) Schematic map view of the area showing the
timing of initial strike-slip movement on the Red River shear zone (48), seafloor spreading in the South China Sea (30), and initiation of subduction atthe
Palawan ophiolite spreading center. Palawan ophiolite (yet to be obducted onto the rifted continental crust) shown in cross-hatched pattern. (B) Cross-
sectional schematic view of the area along A-Aduring the onset of subduction. Weak zones where subduction had the potential to initiate, but did not, are
also shown.
| Keenan et al.
incipient subduction. It should be noted that there are only a few
locations where subduction initiation might be happening (ref. 1
and references therein), and therefore it is unlikely that these
represent the full range of potential mechanisms for intraoceanic
subduction initiation. The process of subduction initiation may last
on the order of only 23 My (assuming a convergence rate of
5 cm/y) (1), a small fraction of the lifetime of a subduction zone,
and is therefore unlikely to be captured by present day observa-
tions. Ophiolites that have similar crystallization ages to the
metamorphic age of their sole, like the Palawan ophiolite, are not
uncommon (19, 25, 26). These other examples, however, lack dates
for the protolith of the sole. Dating these protoliths may show that
subduction initiation at spreading centers has been more pervasive
than currently recognized.
The subduction zone that existed in Palawan and the nascent
Puysegur trench (50) are possibly the only well-constrained geo-
logic examples that address the dynamics of subduction initiation.
In both of these examples, a forced initiation has been inferred.
Although spontaneous subduction has been simulated by numer-
ical models (9), corresponding geologic tests have been questioned
(53). Our study suggests that subduction initiation in the modern
day plate tectonic regime requires extant subduction, the main
driver of plate motion, to force new subduction. Furthermore, it
appears that forces generated in the interior of major continental
collisions zones can be transmitted out to the oceanic realm,
rapidly causing diverging plates at a spreading center to converge
and lead to incipient subduction in less than 1My.
Geochemical Analysis. Whole rock powders were analyzed by a combination of
a lithium metaborate/tetraborate fusion followed by inductively coupled
plasma (ICP) methods for major elements abundances and inductively coupled
plasma MS(ICP-MS) methods fortrace elements abundances.The analyses were
done at Activation Laboratories (Ontario, Canada) (ACTLAB). A detailed de-
scription of sample preparation methods is given on the ACTLAB Website
U-Pb Zircon Geochronology. All reported ages are
U error-weighted
mean ages, because the uncertainty of
Pb ages are pronouncedly large
due to the very young and low
U content of these zircons. They are cor-
rected for initial Th/U disequilibrium, and errors are reported as 2σ.
Twelve zircon grains from the epidote amphibolite pods (PL-14-05, PL-14-06,
and PL-14-07) were analyzed by chemical abrasion thermal ionization MS
(CA-TIMS) at the radiogenic isotope laboratory at Massachusetts Institute of
Technology. Samples PL-14-05 and PL-14-06 yielded weighted mean ages of
35.242 ±0.062 and 35.862 ±0.048 Ma, respectively, whereas sample PL-14-07
displays three distinct ages of 37.00 ±0.16, 35.25 ±0.15, and 35.97 ±0.11 Ma.
For sample PL-14-06, the youngest cluster of zircon ages was used for the
weighted mean age because this best approximates the timing of magma
crystallization (54).
Three zircon fractions from the plagiogranite (PW-00-18) were analyzed by
conventional TIMS at the University of California, Santa Barbara. One fraction
was analyzed as is, whereas the other two were air abraded to remove any
exterior zones with possible Pb-loss. The unabraded fraction is only slightly
younger thanthe two abraded fractions that give a mean age of 34.1 ±0.1 Ma.
Additional zircon fractions were analyzed using the stepwise chemical abra-
sion technique (55), and all analyzed fractions after the first few steps yielded
identical 34.1 ±0.1 Ma ages.
ACKNOWLEDGMENTS. We thank John Spray and two anonymous reviewers
for helpful and constructive comments. Freddie Dela Cruz provided excellent
boatmanship in Ulugan Bay. This work was supported, in part, by Saint Louis
University and the Geological Society of America. The geochronology
laboratories at University of California Santa Barbara and Massachusetts
Institute of Technology are supported by the National Science Foundation.
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Supporting Information
Keenan et al. 10.1073/pnas.1609999113
0.00554 0.00556 0.00560 0.00562
0.0356 0.0358 0.0360
0.0335 0.0362
0.0345 0.0364
0.0355 0.0366
weighted mean 206Pb/238U date
35.242 ± 0.046/0.050/0.062 Ma
MSWD = 0.72, n=4
weighted mean 206Pb/238U date
35.862 ± 0.024/0.029/0.048 Ma
MSWD = 0.86, n=3
z1 z2
Date [Ma]
z4 z6
Date [Ma]
0.034 0.035 0.036 0.037 0.038 0.039 0.0400.033
0.0054 0.0055 0.0057 0.00580.0056
Fig. S1. U-Pb Concordia diagram for samples from the metamorphic sole. (A) Data from PL-14-05. Weighted mean age is reported as 35.242 ±0.062 Ma.
(B) Data from PL-14-06. Weighted mean age is reported as 35.862 ±0.048 Ma. (C) Data from PL-14-07. Three externally discordant, but internally concordant,
ages are 37.00 ±0.16, 35.97 ±0.11, and 35.25 ±0.15 Ma.
Keenan et al. 1of3
Table S1. Zircon U-Pb analytical results from plagiogranite (PW-00-18) of Central Palawan Ophiolite
Zircon size
weight (mg)
Pb (ppm)
Pb (pg)
U (ppm)
Isotopic ratios Ages
100140 abr 1 0.2796 13.54 60.19 0.20051 0.05624 1362.4 0.005302 34.09
70100 abr 0.5 0.2575 6.66 55.41 0.20742 0.0575 1137.7 0.005301 34.08
70100 uabr 4 0.5382 65.00 117.4 0.19526 0.0528 2409.6 0.005255 33.79
Zircon sizes are in micrometers. Uncertainties on the
U ages are ±0.2% (2σ). Analyses were done by thermal ionization MS at the University of
California, Santa Barbara. abr, abraded; uabr, unabraded.
Keenan et al. 2of3
Table S2. Zircon U-Pb analytical results from epidote-amphibolite pods (PL-14-05 and PL-14-06) and amphibolite pod (PL-14-07) of the metamorphic sole
Composition Isotopic ratios Dates (Ma)
Z2 1.16 0.3 15.81 817.4 0.373 0.005474 0.79 0.03570 2.05 0.047316 1.817 35.19 0.28 35.61 0.72 64 43 0.47
Z4 1.16 0.2 13.86 719.8 0.371 0.005485 0.21 0.03561 2.07 0.047109 1.986 35.262 0.074 35.53 0.72 54 47 0.44
Z5 0.65 0.3 17.19 1001.2 0.209 0.005476 0.21 0.03532 1.76 0.046801 1.629 35.205 0.072 35.25 0.61 38 39 0.69
Z6 0.90 0.6 6.95 391.4 0.290 0.005488 0.29 0.03573 3.75 0.047234 3.648 35.28 0.10 35.6 1.3 60 87 0.39
Z1 0.23 0.4 35.9 2321.8 0.074 0.005577 0.12 0.03591 0.65 0.046712 0.618 35.855 0.042 35.82 0.23 33 15 0.36
Z2 0.25 0.3 25.52 1645.5 0.081 0.005582 0.11 0.03605 0.89 0.046867 0.858 35.881 0.038 35.96 0.32 41 21 0.38
Z3 0.20 0.3 35.5 2317.6 0.064 0.005602 0.15 0.03613 0.72 0.046800 0.666 36.014 0.055 36.04 0.25 38 16 0.42
Z4 0.22 0.3 22.84 1490.0 0.069 0.005576 0.13 0.03605 1.11 0.046918 1.064 35.844 0.047 35.96 0.39 44 25 0.38
Z5 0.25 0.7 25.86 1667.5 0.080 0.005592 0.14 0.03633 0.96 0.047134 0.894 35.950 0.050 36.23 0.34 55 21 0.51
Z2 0.53 0.4 4.80 301.6 0.169 0.005756 0.44 0.03726 5.44 0.046969 5.229 37.00 0.16 37.1 2.0 47 125 0.50
Z3 0.44 0.5 3.90 254.0 0.142 0.005483 0.43 0.03543 5.63 0.046884 5.487 35.25 0.15 35.4 2.0 42 131 0.37
Z4 0.41 0.4 6.28 400.7 0.132 0.005596 0.30 0.03650 3.59 0.047328 3.466 35.97 0.11 36.4 1.3 65 83 0.43
Blank composition:
Pb =18.15 ±0.48;
Pb =15.30 ±0.29;
Pb =37.11 ±0.88; Mass fractionation correction of 0.25%/amu ±0.02%/amu (atomic mass unit) was applied to all single-
collector Daly analyses.
Th contents calculated from radiogenic
Pb and the
Pb date of the sample, assuming concordance between U-Th and Pb systems.
Total mass of common Pb.
Ratio of radiogenic Pb (including
Pb) to common Pb.
Measured ratio corrected for fractionation and spike contribution only.
Measured ratios corrected for fractionation, tracer, and blank. All common Pb was assumed to be procedural blank. Total procedural blank for U was less than 0.1 pg.
Corrected for initial Th/U disequilibrium using radiogenic
Pb and Th/U [magma] =2.8.
**Isotopic dates calculated using the decay constants of λ
=1.55125E-10 y
=9.8485E-10 y
(42), and for the
U=137.818 ±0.045 (43).
Keenan et al. 3of3
... The subduction of the Proto-South China Sea toward the Borneo and Palawan Islands occurred in Oligocene-early Miocene time (Briais et al., 1993;Hall, 1996). This subduction created a series of volcanic arcs from Cagayan Ridge to Sabah (Hall, 2013;Hutchison et al., 2000;Keenan et al., 2016;Rangin et al., 1990). The subsequent collision of the Dangerous Grounds terrane with the Sabah-Palawan Islands was thought to have played an important role in terminating seafloor spreading in the South China Sea (Briais et al., 1993;Clift et al., 2008;Hall, 1996;Sun et al., 2006). ...
... The collision history of the southern South China Sea was extensively studied using volcanic records from the Borneo and Palawan Islands (e.g., Cottam et al., 2013;Encarnación et al., 1995;Hutchison et al., 2000;Lai et al., 2021;Soeria-Atmadja et al., 1999;Tsikouras et al., 2021). Age results from the central and southern Palawan ophiolites indicate the initiation of southward subduction of the Proto-South China Sea at ca. 34 Ma (Encarnación et al., 1995;Keenan et al., 2016). However, tectonomagmatic activities associated with the collision between the Dangerous Grounds terrane and Sabah-Palawan Islands during the Miocene are poorly constrained and have been debated due to their chronological overlap with magmatism caused by the northward subduction of the Celebes Sea (Cottam et al., 2013;Hutchison et al., 2000;Lai et al., 2021;Rangin et al., 1990;Tsikouras et al., 2021). ...
... The Palawan continental terrane includes the Reed Bank, south Mindoro Island, and west Panay Island according to seismic, gravity, and magnetic analyses by Liu et al. (2014). The north Palawan Island is part of the Palawan continental terrane, but the south Palawan Island (Fig. 1) is thought to have been in the forearc region of the Cagayan Ridge during subduction of the Proto-South China Sea (Keenan et al., 2016). The North-West Borneo Trough located to the southeast is thought to be the fossil trench of the Proto-South China Sea or a foredeep produced by accretion of the Borneo and Palawan Islands to the Dangerous Grounds terrane (Hutchison, 2004). ...
The tectonic evolution of the South China Sea is closely associated with multiple subduction-collision processes in Southeast Asia. When the collision of the Dangerous Grounds terrane with Sabah-Palawan Islands terminated is debated due to poor age constraints at the southern margin of South China Sea. A deep well drilled on Meiji Atoll penetrates Cenozoic carbonate strata in central Dangerous Grounds. Robust strontium isotope ages and laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dates provide critical chronological constraints on the Cenozoic evolution of the southern South China Sea. A middle Miocene hiatus spanning 9 m.y. on Meiji Atoll is thought to be mainly caused by tectonic uplift in the central Dangerous Grounds. The uplift in the central Dangerous Grounds was accompanied by under-thrusting beneath the southern Palawan margin and orogenic uplifting in north Borneo during the middle Miocene. Data interpretation indicates an active collision in the southern South China Sea during the middle Miocene. The regrowth of the Meiji Atoll above the middle Miocene hiatus represents the end of this collision event in the southern South China Sea at ca. 11 Ma, after the cessation of seafloor spreading, which occurred at ca. 15 Ma.
... Collectively, these domains occupy a large part of the ''Sunda Plate" (Fig. 2b), as defined by GPS data showing significant relative motions between SE Asia and Eurasia (Bird, 2003;Simons et al., 2007;Tingay et al., 2010). Steinshouer et al. (1999) using regional studies (Aurelio et al., 2014;Balaguru and Nichols, 2004;Galin et al., 2017;Keenan et al., 2016;Shao et al., 2017;Witts et al., 2012). Circles show dredged and drilled bedrock geology using same colour scheme as terrestrial geology Yan et al., 2006). ...
... Rifting at 35 Ma in a marginal ocean basin generated the MORB (Mid-Ocean Ridge Basalt) to transitional MORB-IAT (Island Arc Tholeiite) ophiolitic magmatism of central Palawan (Keenan et al., 2016). $ 1 km of metamorphosed mafic and sedimentary rocks are exposed at the ophiolite's northern extent, adjacent to mantle peridotite, and previously interpreted as a ''metamorphic sole" (Keenan et al., 2016). ...
... Rifting at 35 Ma in a marginal ocean basin generated the MORB (Mid-Ocean Ridge Basalt) to transitional MORB-IAT (Island Arc Tholeiite) ophiolitic magmatism of central Palawan (Keenan et al., 2016). $ 1 km of metamorphosed mafic and sedimentary rocks are exposed at the ophiolite's northern extent, adjacent to mantle peridotite, and previously interpreted as a ''metamorphic sole" (Keenan et al., 2016). The metamorphic units have identical ages to the ophiolite (34-35 Ma from hornblende and mica Ar-Ar, and U-Pb zircon chronology; Encarnación et al., 1995;Keenan et al., 2016). ...
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We present a new extensional tectonic model for the Cenozoic history of SE Asia and the opening of the South China Sea (SCS), proposing a feedback mechanism by which intracontinental rifts initiate and propagate without invoking mantle plumes. Four principal tectonic models have been proposed for SCS opening: 1) Slab pull from subduction of a Proto South China Sea (PSCS); 2) Extrusion tectonics from the India-Asia collision; 3) Basal drag from a mantle plume; and 4) Backarc rifting. Each model was developed around different particular data, and all tend to perpetuate independently through selective data prioritisation. We present a new GPlates model, showing that the geological and geophysical correlations between the opposing SCS conjugate margins best agrees with a common initial development on the South China Margin, and that regional development via protracted extension since the Mesozoic is in agreement with available paleomagnetic data for Borneo. The geodynamic mechanism for protracted lithospheric extension in SE Asia is via the development of progressive feedback processes, initiated by Mesozoic slab rollback and migration of the subduction zone beneath South China, leading to intracontinental thinning and extension. This in turn drove passive asthenospheric upwelling, increasing heat flow and crustal ductility, and enhancing further extension as a wide rift rather than narrow crustal neck. Subsequently, following sufficient continental extension, SCS oceanic spreading occurred. This feedback mechanism (involving shallow, not deep mantle processes) may enhance and enable intracontinental rifting elsewhere.
... Although the subduction has been debated in the last decade due to the different rotation results from paleomagnetism analyses (Figure 2-32). The subduction initiated around 35 to 40 Ma, Late Eocene times having been dated in the metamorphic in Palawan and Eocene conglomerate formation engulfing ophiolite blocks in Sabah (Omang and Barber, 1996;Keenan et al., 2016). ...
... The context may still be reconstructed in a southward subduction scenario (Figure 6-2). The initiation of subduction is Paleocene and it developed during Eocene times based on geochronologic and stratigraphic studies although the precise age is still in debate (Tan, 1979;Pieters et al., 1987;Bladon and Supriatna, 1989;Omang and Barber, 1996;Keenan et al., 2016;Hennig-Breitfeld et al., 2019). We have some information from literature concerning the age that a cluster of Paleocene and Eocene volcanism with arc-like magmatic characteristics is located in the Sarawak and Kalimantan (NE Indonesian Borneo) regions (Figure 6-2). ...
... We have some information from literature concerning the age that a cluster of Paleocene and Eocene volcanism with arc-like magmatic characteristics is located in the Sarawak and Kalimantan (NE Indonesian Borneo) regions (Figure 6-2). A few samples of metamorphic sole dated at 34 Ma underlay the ophiolite in Palawan, possibly indicating the incipient subduction at a mid-ocean ridge (Keenan et al., 2016). On the stratigraphic aspect, an extensive hiatus reflects the Sarawak Orogeny (or Rajang) Unconformity and is distributed extensively onshore Borneo at c. 37 Ma (Cullen, 2010;Hennig-Breitfeld et al., 2019). ...
This study focuses on the tectonic configuration of a rare example of the subduction-related opening on the downgoing plate of a subduction zone. The birth, the stages, and the cessation, of activity of the South China Sea (SCS) basin in Cenozoic seem to be related with the decay of the Proto-South China Sea (PSCS), by subduction in Borneo and Palawan. However, the mechanisms are still poorly constrained in terms of leading parameter and fundamental mechanism. Being narrow, the SCS margin offers the advantage of easily feasible correlation of sedimentary infill, structural styles and magmatic activities. The comprehensive and regional tectono-stratigraphic studies on subsurface seismic data and field observation could allow me to understand the genetic relation between the ocean opening and the adjacent subduction zone.The SCS continental crust stretching and thinning by ubiquitous large extensional detachment faults were expressed in a diachronous rifting and breakup, initiated in the east while rifting continued in the west until the propagator’s arrival. To illustrate the pre-breakup geometries of the southwestern SCS margins, we restore two conjugate sections near the first oceanic magnetic anomaly. The COT configuration, not only illustrates local asymmetrical hyper-extension, but also appears in map view to have a rhomb-shape controlled by N-S abrupt segments and E-W hyper-extended ones. The spatial variation of the crustal structures records an initial N-S extension simultaneous with the first phase of seafloor spreading in the eastern SCS, and a significant change of extensional direction shortly later (circa 23Ma) to NW-SE. During the post-spreading period, in the ocean floor, few key seismic images show that the local uplift in diameter of few kilometers goes with important magma production, interestingly within an episodic subsidence period in regional scale. This large global subsidence is anomalous compared to the theoretical thermal subsidence of post-rifting basins. Meanwhile, collision initiated since Late Oligocene in the PSCS, and mélange and shale-prone delta deposits characterize the shallow part of the accretionary wedge. This unit was occasionally brought to the surface and the overlying strata were deformed as circular synclines and anticlines. I, therefore, suspect that the mechanism of exposing the mélange involved a diapiric process that pushed this low viscosity material upward. Accordingly, the entire process illustrates a tectonic evolution involving shale tectonics from the end of the collision to the post-collision setting of northern Borneo, delineating an age of termination of orogeny when the seafloor spreading also ceased at ca. 16 Ma.The evolution of the rifting, spreading and cessation of rifting in the South China Sea seems to be controlled by the effect of subduction in the PSCS margin. The closure of the PSCS undergo a longer period of shortening in front of Palawan and Sabah where the oceanic domain is wide – none or very limited continental crust - whereas a limited shortening to the tip of the basin in the Sarawak region. The lithospheric heterogeneity of lower plate (thick continental crust in the south and thin one to oceanic in the north) induced a counterclockwise rotation off the subduction zone, and simultaneously, the contiguous SCS margin that change the extensional direction from N-S to NW-SE by 23Ma. The slab ultimately broke off in the Late Miocene to induce large plutons in the PSCS margin, and dramatic subsidence as well as local magmatic intrusions in the SCS during the post-spreading period. This suggests a slab detachment which provoked deep mantle upwelling to the surface. This dual tectonic evolution on two opposite margins therefore illustrates how far-field subduction process may impact the regional tectonics.
... Many supra-subduction zone (SSZ) ophiolites, such as the Oman Ophiolite where N-MORBlike tholeiites underlie the slightly younger boninitic rocks, may represent on-land forearc fragments (i.e., forearc-type ophiolites) (Reagan et al., 2010;Whattam and Stern, 2011). However, a recent analysis of the forearc-type Central Palawan-Amnay ophiolites (West Philippines), which were related to a rapid conversion of an active spreading center to a subduction zone (Keenan et al., 2016) and were found to have voluminous E-MORBs but a lack of boninitic rocks (Yu et al., 2020), suggested that no single rule exists for the chemical dynamics of subduction initiation magmatism. The forearc E-MORBs are products of flux melting of mantle wedge by Nb-enriched slab melts that invoke further melting of an eclogitic residuum of subducted slab (Yu et al., 2020). ...
... (2) synchronous NTS [17] Nb-enriched rocks* [17] (3) AFSM + FMER [17] Related reference: [1] Stern and Bloomer (1992); Whattam and Stern (2011);[13] Rollinson (2009Rollinson ( , 2015; [14] Guilmette et al. (2018); [15] this study; [16] Keenan et al. (2016); [17] Yu et al. (2020 (Yu et al., 2020). Furthermore, the ubiquitous (but age restricted) boninites in the IBM forearc (Stern and Bloomer, 1992;Shervais et al., 2021) are indicators of extensive slab melting in the amphibolite facies and mantle metasomatism. ...
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To better understand the establishment of subduction zones, considerable effort is required regarding various factors, such as the time of subduction initiation and the age of the slab's lithosphere. In this study, adakitic dikes (ca. 17.3 Ma: εNd of 7.4-7.9 and 206 Pb/ 204 Pb of 18.27-18.42) were recognized to crystallize along with the normal and enriched mid-ocean ridge basalt (N-and E-MORB)-like oceanic crust (ca. 17.8-14.1 Ma: εNd of 8.8-13.3 and 206 Pb/ 204 Pb of 17.71-18.22) in the East Taiwan Ophiolite (ETO). Given that the outcropping in ultramafic sequence, the distinct Nd-Pb isotopes, and the slight precedence than corresponding arc volcanism (≤ ca. 14.1 Ma), the origins of the ETO adakitic dikes are related to amphibolite-facies melting of the subducted slab beneath the Luzon forearc spreading center during subduction initiation. In this respect, we conclude that the sparsely exposed but isotopically heterogeneous E-MORB-type rocks (εNd of 8.8-10.2 and 206 Pb/ 204 Pb of 18.06-18.22) are Nb-enriched basalts and relate to the Nb-enriched slab melts derived from further melting of the hot eclogitic residuum of the young South China Sea subducted slab. In addition, the isotopic diversity of N-MORB-like samples can be attributed to the diking of adakitic and Nb-enriched slab melts beneath the Luzon forearc spreading center. Combined with the previously reported early Miocene uplift of the Zambales Ophiolite in the western Luzon, we infer an early Miocene induced-subduction initiation of the South China Sea and the delayed near-trench spreading of the Luzon forearc by <5 Myr. Compared to the absence of Nb-enriched slab melts and >20 Myr subducted slab in the genesis of the in-situ Izu-Bonin-Mariana (IBM) forearc and the voluminous Nb-enriched rocks and ca. 0 Myr subducted slab of the Central Palawan-Amnay ophiolites, the sparse Nb-enriched rocks and <15 Myr subducted slab of the ETO sequence invoke the governing factor of slab's lithosphere age during subduction initiation magmatism. We propose that the lack of Nb-enriched slab melts in the forearc chemostratigraphy of the Oman Ophiolite may indicate the subduction initiation of an old and cold oceanic plate.
... 40 Ar-39 Ar, U-Pb, Lu-Hf) show that the metamorphic sole rocks commonly form at the same time or slightly earlier/later compared to the ophiolite formation (e.g. Hacker, 1991Hacker, , 1994Jones et al. 1991;Hacker et al. 1996;Wakabayashi and Dilek 2000;Çelik 2008;Guilmette et al. 2009Guilmette et al. , 2018Parlak et al. 2013aParlak et al. , 2019Keenan et al. 2016;Rioux et al. 2016;Pourteau et al. 2019). It is commonly considered that the SSZ ophiolites together with metamorphic sole rocks were generated ≤ 10 Ma (Stern and Bloomer 1992;Robertson 2002;Furnes 2008, 2011;Pearce 2003;Wakabayashi et al. 2010). ...
The Late Cretaceous accretionary complexes along the İzmir-Ankara-Erzincan (IAE) Neo-Tethyan suture zone in northern Turkey record the subduction–accretion processes of the oceanic lithosphere ranging in age from the Late Triassic to the Late Cretaceous. These accretionary complexes contain fragments of Early and Middle Jurassic metamorphic and non-metamorphic ophiolites. Here, we report new geochemical and geochronological data from the metamorphic and non-metamorphic ophiolitic rocks, which are observed in the Tekelidağ mélange (northern Sivas) of the IAE suture zone. Geochemical characteristics of these rocks point to formation in a subduction-related tectonic setting. Igneous zircons from meta-plagiogranite injected into the meta-ophiolitic rocks yielded zircon U–Pb age of 188 ± 4 Ma (2σ, Early Jurassic), and those from a non-metamorphic plagiogranite crosscutting the non-metamorphic ophiolitic rocks gave an age value of 168 ± 2 Ma (2σ, Middle Jurassic). The igneous crystallization age of the non-metamorphic plagiogranite is identical with the metamorphic age of meta-ophiolitic rocks, which has been dated as Middle Jurassic (166.7 ± 2 Ma, 2σ) by the 40Ar–39Ar method. These age data indicate that (i) the supra-subduction zone ophiolite formation lasted about 20 Ma, (ii) the supra-subduction zone ophiolite and the meta-ophiolitic rocks formed simultaneously in the Middle Jurassic, and (iii) the meta-ophiolitic rocks are remnants of the metamorphosed equivalents of the Early Jurassic supra-subduction zone oceanic crust. The supra-subduction zone ophiolite formation probably occurred over an extended period of time in the Jurassic Neo-Tethys.
... D espite over fifty years of studies into the workings of plate tectonics, the phenomenon of subduction initiation is still not well understood [1][2][3] . How slabs start to subduct, whether via shallow convergence 4 or vertical foundering 5,6 , to the point that they begin to interact with the mantle continues to be debated. ...
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How subduction-related magmatism starts at convergent plate margins is still poorly understood. Here we show that boron isotope variations in early-formed boninites from the Izu-Bonin arc, combined with radiogenic isotopes and elemental ratios document rapid (~0.5 to 1 Myr) changes in the sources and makeup of slab inputs as subduction begins. Heterogeneous hornblende-granulite facies melts from ocean crust gabbros ± basalts fluxed early melting to generate low silica boninites. Hydrous fluids from slab sediments and basalts later fluxed the low silica boninites mantle source to produce high silica boninites. Our results suggest that initially the uppermost parts of the slab were accreted near the nascent trench, perhaps related to early low-angle subduction. The rapid changes in slab inputs recorded in the boninites entail a steepening subduction angle and cooling of the plate interface, allowing for subduction of slab sediment and basalt, and generating hydrous fluids at lower slab temperatures. The geochemical record of subduction initiation is still not well understood, despite >50 years of study. Here, the authors use boron isotopes in Izu-Bonin boninites to document rapid changes in slab inputs to melting at the start of subduction, related to the steepening and cooling of the downgoing Pacific plate.
The initiation of subduction is widely recognized as a critical process associated with the evolution of the Meso-Tethys Ocean. Here, we present zircon U–Pb, Lu–Hf isotope, and whole-rock geochemical data for a suite of Triassic–Jurassic magmatic rocks from the central Tibetan Plateau, with the aim of gaining insights into the subduction initiation of the Bangong–Nujiang Tethys Ocean (BNTO). Early Triassic trachyandesite (248 Ma) from the South Qiangtang terrane resemble intraplate magma and probably resulted from magmatic activity at the passive continental margin during the BNTO opening. Late Triassic S-type granodiorite (220 Ma) and Middle Jurassic SSZ-type gabbro (∼165 Ma) from the South Qiangtang terrane may result from the northward subduction of the BNTO oceanic lithosphere. Jurassic granitoids (I and S type; 193–177 Ma) and metamorphic rocks (179–174 Ma) from the Amdo and Jiayuqiao subterranes are related to the northward subduction of the BNTO oceanic lithosphere. Middle–Late Jurassic diorites (164–162 Ma) from the Central Lhasa subterrane have geochemical features of continental margin arc magma with a sediment component, and these are derived from the southward subduction of the BNTO oceanic lithosphere. Finally, this Triassic–Jurassic magmatic suite of various geochemical features was combined with the results of previous studies, and we suggest that the initiation of northward BNTO subduction was probably diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). This initiation resulted both from collision and slab breakoff during the closure of the Longmuco–Shuanghu–Lancangjiang Paleo-Tethys Ocean and considerable density and rheology contrasts between the thick oceanic lithosphere and continental lithosphere. The southward subduction was initiated no later than the Middle Triassic and was attributed to the closure of the Sumdo Paleo-Tethys Ocean and the spreading of the Indus–Yarlung Zangbo Neo-Tethys Ocean.
Metamorphic soles within ophiolite mélanges record key information on subduction initiation and evolution of paleo-oceans. Mafic granulite and amphibolite occur in metamorphic soles within the Saga and Bairang ophiolitic mélanges of the Yarlung Tsangbo suture zone, southern Tibet. In fresh mafic granulite, the early prograde assemblage (M1) is preserved as inclusions in the core of garnet. Minerals in matrix and garnet mantle define the peak metamorphic assemblage (M2). Near-isothermal decompression (M3) is recorded by the garnet rim and a symplectic corona. The mineral assemblage of the fresh amphibolite only records one major stage. Many samples were overprinted by strong metasomatism at sub-greenschist-facies conditions (M4). According to geochemical fingerprints, the soles originate from a mid-ocean-ridge environment with strong affinities to the non-metamorphic oceanic crust of the Yarlung Tsangbo ophiolites. P-T calculations of the granulites by pseudosection techniques, Zr-in-rutile and REE-based thermobarometry show ~690-760°C/9.5-12.5 kbar for the prograde (M1) stage, 900-970°C/14-16 kbar for the peak (M2) stage followed by 902-983°C/9.5-12.6 kbar for the retrograde (M3) stage. From the amphibolite P-T conditions of <700-880°C/7-12.5 kbar were derived. This results in a clockwise trajectory with peak-high pressure to ultrahigh-temperature conditions characterizing a “hot” subduction environment. U-Pb dating of magmatic zircon yields ~133-127 Ma as protolith ages of the soles, almost coeval with the igneous ages of the unmetamorphosed oceanic crust. U/Pb ages of metamorphic zircon show a similar range within uncertainties (~131-119 Ma) indicating that initiation of subduction subsequently followed the protolith crystallisation. The metamorphic soles were subducted to a depth of 50 km triggering partial melting in the mantle wedge by dehydration reactions and formation of supra-subduction-zone magma. The mantle wedge became cooler and more buoyant when the subduction zone matured, which caused exhumation of the metamorphic soles and surrounding mantle rocks as well as cooling. A new model for a “hot” subduction initiation close to a mid-ocean ridge near the Asian margin is proposed based on the data from this study and previous studies.The relict mid-ocean-ridge basalts after subduction initiation and the subsequent supra-subduction-zone basalts formed the YTSZ ophiolites during ~130-120 Ma.
The whole rock compositions of the blocks and the surrounding matrix of the Dalrymple Amphibolite are investigated in this study to determine the protolith of the blocks and the effect of mechanical mixing and fluid infiltration in the matrix of this fossil slab-mantle wedge interface. The major and trace element contents of the metamafic blocks indicate their mid-oceanic ridge basalt origin similar to the mafic lavas of the crustal section of central Palawan Ophiolite. Similarities in their rare earth and trace element patterns indicate the genetic relationship between the mafic lavas of the Palawan Ophiolite and the metamafic blocks of the Dalrymple Amphibolite. This confirms that the metamafic blocks represent the basalt to gabbro section of the oceanic lithosphere of the subducting slab. The matrix surrounding the blocks exhibit highly variable phase assemblages. In order to determine its petrogenesis, we distinguished groups of components/elements which behave similarly (Group 1 TiO2, Al2O3, Zr, Th and the light rare earth elements; Group 2 Cr, Ni and MgO) based on geostatistical (correlation coefficient) analyses. These groups indicate mixing of metasedimentary (Group 1) and metamafic (Group 2) end-members to form the matrix. The mixing proportions of the end-members were estimated by employing regression analysis wherein the measured concentration of fluid immobile elements (Cr, Ni, Zr, TiO2 and Al2O3) in the matrix samples were fitted against a modelled concentration by changing the end-member and their relative proportions. The end-members and mixing ratio with the highest regression value (r2) was selected to obtain the modelled composition of the matrix. The modelled and the measured matrix compositions were then used as the original (unmetasomatized) rock and the altered rock respectively in the isocon analysis, assuming that TiO2, Al2O3, Cr, Nd, Zr, and Hf are immobile. This assumption is supported by the prevalence of kyanite, ilmenite and zircon in the matrix mineral assemblage. This procedural workflow helped distinguish end-member components, estimate their mixing ratios, and determine the effects of infiltrating fluids. In particular, the whole rock composition of the matrix was controlled by mixing of a subordinate amount of metamafic blocks in a metasedimentary-dominated shear zone. This is supported by the Cr-Nb content of rutile grains included in the matrix samples which indicate mixed metamafic and metapelitic signatures. The metamafic-metasedimentary dominated matrix in the Dalrymple Amphibolite contrasts with other high-pressure/temperature (P/T) type metamorphic terranes which are dominated by low T minerals (serpentine, Mg-chlorite, and talc) derived from an ultramafic end-member, and could be reflective of conditions in warmer subduction zones. Mass balance calculations further revealed that an early fluid infiltration event likely occurred following the mixing process. This preferentially leached out elements which are either fluid-mobile (e.g. CaO and SiO2) or are not incorporated into the growing minerals in the matrix. The strong control of mineral assemblage of the matrix in its chemistry is exhibited by a number of samples which showed variable degrees of losses and gains in elements traditionally interpreted to be fluid immobile (e.g. heavy rare earth elements and Y). A later hydration event linked to retrograde metamorphic stage imprinted gains of K2O, Rb, and Ba in the matrix samples with the growth of replacement minerals (e.g. muscovite on kyanite). This later fluid infiltration event possibly masked the original loss of these fluid-mobile elements in the matrix samples during the earlier fluid-rock interaction.
This volume pays tribute to the great career and extensive and varied scientific accomplishments of Walter Alvarez, on the occasion of his 80th birthday in 2020, with a series of papers related to the many topics he covered in the past 60 years: Tectonics of microplates, structural geology, paleomagnetics, Apennine sedimentary sequences, geoarchaeology and Roman volcanics, Big History, and most famously the discovery of evidence for a large asteroidal impact event at the Cretaceous–Tertiary (now Cretaceous–Paleogene) boundary site in Gubbio, Italy, 40 years ago, which started a debate about the connection between meteorite impact and mass extinction. The manuscripts in this special volume were written by many of Walter’s close collaborators and friends, who have worked with him over the years and participated in many projects he carried out. The papers highlight specific aspects of the research and/or provide a summary of the current advances in the field.
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The Semail ophiolite in the Oman Mountains is the world's largest and best preserved thrust sheet of oceanic crust and upper mantle (>10 000 km2, ~550 km long, ~150 km wide); it was emplaced onto the Arabian continental margin during Late Cretaceous time. The ophiolite originated 96-94 Ma at a spreading center above a northeast-dipping subduction zone associated with initiation of immature island-arc tholeiitic lavas (Lasail arc) at the highest levels of the ophiolite. Simultaneous underthrusting of Triassic (and Jurassic[?]) mid-oceanic-ridge basalt and alkalic volcanic rocks beneath >12 km of upper mantle depleted harzburgites produced garnet + clinopyroxene amphibolites formed at temperatures of ~850 °C, dated as 95-93 Ma. Subduction cannot have been initiated at a mid-oceanic ridge, otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above. In the northern part of the Oman Mountains in the Bani Hamid area, United Arab Emirates, ~870 m of granulite facies rocks (enstatite + spinel ± diopside quartzites, garnet + diopside + wollastonite calc-silicate marbles, clinopyroxene-bearing amphibolites) were formed at temperatures similar to those of the garnet + diopside amphibolites of the Oman sole, 800-850 °C, but at slightly higher pressures, as much as 9 kbar. They are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out-of-sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction. Subduction of the Arabian continental crust beneath the obducting Semail ophiolite to ~78-90 km depth has been proven by thermobarometry of the As Sifah eclogites (to 20-23 kbar) in the eastern sector. In the United Arab Emirates the subducted continental crust began to partially melt, producing unusual biotite ± muscovite ± garnet ± tourmaline ± cordierite ± andalusite-bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite. We suggest that the entire leading (northeast) edge of the Arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks. Higher temperatures and pressures in the United Arab Emirates sector, possibly due to a thicker or double-thickness ophiolite section, led to blueschist, amphibolite, and granulite facies conditions in the metamorphic sole, and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite. A model for ophiolite obduction is presented, which accounts for all the structural and metamorphic conditions reported from the Oman Mountains.
A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and the applied to Late Jurassic ophiolotes of California. The model accounts for several key observations which require that the earliest stages of subduction involve rapid retreat of the trench; this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithosphere subduction. -from Authors
The central part of Palawan Island is composed predominantly of ophiolitic rocks stretching for approx 100 km in a SW-NE direction. This complex fills the requirements of a complete ophiolite. It is built up of a basal tectonized peridotite that consists mainly of foliated harzburgite and dunite; this grades upward via foliated troctolite to the gabbro unit that is generally foliated but exhibits massive, isotropic fabrics in its highest level. The uppermost unit of the ophiolite sequence is formed by pillow basalts with associated cherts. Microgabbro dykes cut the whole sequence, whereas pyroxenite and dunite dykes are restricted to the peridotite unit, and plagiogranites to the upper gabbro unit. A sheeted dyke complex is missing. Chromite stringers and bodies are associated with dunite. The ophiolite is overlain by flyschoid sediments, either deformed or undeformed, the palaeontological age of which ranges from late Cretaceous to Oligocene. They are covered by younger shallow water sediments. Intensely foliated rocks of mainly basaltic origin are associated with the peridotite contacts towards flyschoid sediments and pillow basalts with cherts. They are metamorphosed in amphibolite and greenschist facies. K/Ar determinations on amphibole and mica yielded isotopic ages of approx 40 m.y. (Authors' abstract)-A.W.H.
Although plate tectonics is well established, how a new subduction zone initiates remains controversial. Based on plate reconstruction and recent ocean drilling within the Izu-Bonin-Mariana, we advance a new geodynamic model of subduction initiation (SI). We argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone. The combination of thermal and compositional density contrasts between the overriding relic arc and the adjacent old Pacific oceanic plate promoted spontaneous SI. We suggest that thermal rejuvenation of the overriding plate just before 50 Ma causes a reduction in overriding plate strength and an increase in the age contrast (hence buoyancy) between the two plates, leading to SI. The computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation, while transform faults and passive margins are not.
The Island of Mindoro, the N part of Palawan Island, and the Reed Bank area (SW Philippines) together constitute a continental fragment, the N Palawan block, lying within an island arc-oceanic setting. The Permian to Paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the Philippine Archipelago. These rocks also suggest that the N Palawan block once occupied a pre-drift position contiguous with the S China mainland. A suite of palinspastic reconstructions has been prepared, which shows the evolution of the S China Sea area from the late Triassic to the Pliocene.-from Author
The 40Ar/39Ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in Earth's history, but the accuracy of this method is limited by the accuracy with which the ages of neutron-fluence monitors (dating standards) are known. The emerging astronomically calibrated geomagnetic polarity time scale (APTS) offers a means to calibrate the ages of 40Ar/39Ar dating standards that is independent of absolute isotopic abundance measurements. Seven published 40Ar/39 dates for polarity transitions, nominally ranging from 0.78 to 3.40 Ma, are based on the Fish Canyon sanidine standard and can be compared with APTS predictions. Solving the 40Ar/39Ar age equation for the age of the Fish Canyon sanidine that produces coincidence with the APTS age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27.78 to 28.09 Ma, with an inverse variance-weighted mean of 27.95 ± 0.18 Ma. -from Authors