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589
Revista de la Asociación Geológica Argentina 61 (4): 589-603 (2006)
REFOLDING OF THIN-SKINNED THRUST SHEETS BY
ACTIVE BASEMENT-INVOLVED THRUST FAULTS IN THE
EASTERN PRECORDILLERA OF WESTERN ARGENTINA
Andrew MEIGS1, William C KRUGH2, Celia SCHIFFMAN1, Jaume VERGÉS3and Victor A. RAMOS4
¹ Department of Geosciences, Oregon State University, Corvallis, OR, 97331, U.S.A.
2Earth Science, Institute of Geology, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland.
3Institute of Earth Sciences "Jaume Almera", CSIC, Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain.
4Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina
ABSTRACT
Devastating earthquakes like the 1944 San Juan earthquake reflect active deformation in western Argentina. Although the earthquake cau-
sed considerable damage to San Juan, the source of the earthquake remains uncertain. Potential source faults occur in the thin-skinned
fold-and-thrust belt Precordillera province and in the thick-skinned Sierras Pampeanas province, to the west and east, respectively of Sierra
de Villicum, a thrust sheet in the eastern Precordillera northwest of San Juan. Sierra de Villicum is a west-vergent thrust sheet bound on
the northwest by the Villicum thrust, which juxtaposes a southeast dipping panel of Cambro-Ordovician and Neogene strata in the han-
ging wall with Neogene red beds in the footwall. A series of Late Pleistocene fluvial terraces developed across the Villicum thrust show
no evidence of active fold or fault deformation. Terraces are deformed by active folds and faults in the middle of the southeastern flank
of the Sierra de Villicum thrust sheet. A southeast-facing, southwest-plunging monocline characterizes the Neogene red beds in the region
of active folding. Co- and post-seismic surface rupture along roughly 6 km of the La Laja fault in 1944 occurred in the limb of the mono-
cline. Evidence that surface deformation in the 1944 earthquake was dominated by folding includes terrace´s fold geometry, which is con-
sistent with kink-band models for fold growth, and bedding-fault relationships that indicate that the La Laja fault is a flexural slip fault. A
blind basement reverse fault model for the earthquake source and for active deformation reconciles the zone of terrace deformation,
coseismic surface rupture on the La Laja fault, refolding of the Villicum thrust sheet, a basement arch between the Precordillera and eas-
tern Precordillera, and microseismicity that extends northwestward from a depth of ~5 km beneath Sierra de Villicum to ~35 km depth.
Maximum horizontal shortening rate is estimated to be ~3.0 mmyr-1 from the terrace fold model and correlation of the terraces with
dated terraces located to the southwest of the study area. Basement rocks beneath Cerro Salinas, another eastern Precordillera thrust sheet
to the southwest, are also characterized by an east-facing monoclinal geometry, which suggests that blind thrust faulting on east-vergent
basement faults represents a significant, underappreciated seismic hazard in western Argentina.
Keywords: Neotectonics, San Juan, La Laja, Villicum, flexural slip fault, 1944 earthquake.
RESUMEN: Replegamiento de láminas de corrimiento epidérmicas mediante fallas inversas de basamento activas en la Precordillera Oriental del oeste de Argentina.
La deformación activa en el oeste de Argentina está reflejada por terremotos devastadores como el sismo de San Juan en 1944. Aunque el terremo-
to causó un daño considerable a San Juan, la fuente del terremoto permanecía incierta. Fuentes potenciales de falla ocurren en las fajas plegadas y
corridas adyacentes, la epidérmica de la Precordillera o la faja de basamento de Sierras Pampeanas, al oeste y este respectivamente de la sierra de
Villicum, una lámina de corrimiento de la Precordillera Oriental, ubicada al noroeste de la provincia de San Juan. La sierra de Villicum es una lámi-
na de corrimiento limitada al oeste por el corrimiento de Villicum, que yustapone un panel de estratos cambro-ordovícicos y neógenos de la pared
colgante, con estratos rojos neógenos en la pared yaciente. Una serie de terrazas fluviales desarrolladas a través del corrimiento de Villicum no tie-
nen evidencia activa de plegamiento o falla. Las terrazas están deformadas por fallas y pliegues activos en el medio del flanco sudeste de la lámina de
corrimiento de la sierra de Villicum. La región de replegamiento activo está caracterizada por un monoclinal buzante al sudoeste de estratos neóge-
nos que inclinan al sudeste. Rupturas superficiales cosísmicas y postsísmicas de la falla de La Laja ocurrieron en 1944 a lo largo de aproximadamen-
te 6 km en el limbo del monoclinal. La evidencia de deformación superficial en el terremoto de 1944 estuvo dominada por plegamiento, incluido el
de terrazas con geometría de pliegues, las que son consistentes con modelos de kink-bands para pliegues de crecimiento y con relaciones de fallas de
estratificación que indican que la falla de La Laja es una falla por flexo deslizamiento. Un modelo de falla inversa ciega en el basamento para la fuen-
te del terremoto y para la deformación activa, reconcilia la deformación en la zona de terrazas, la ruptura superficial cosísmica en la falla de La Laja,
el repliegue de la lámina de corrimiento de Villicum, el arqueamiento del basamento entre la Precordillera y la Precordillera Oriental y la sismicidad
que se extiende en profundidad hacia el noroeste desde ~5 km a unos ~35 km por debajo de la sierra de Villicum. La tasa de acortamiento horizon-
tal máximo es estimada en unos ~3,0 mm por año a partir del modelo de plegamiento de terraza y su correlación con las terrazas datadas ubicadas
al sudoeste del área estudiada. Rocas de basamento por debajo del cerro Salinas, una lámina de corrimiento de las Sierras Pampeanas ubicada al sud-
oeste de la región estudiada, también está caracterizada por una geometría monoclinal inclinada al este, que sugiere que fallas inversas ciegas en fallas
de basamento con vergencia oriental representan un riesgo sísmico significativo, pero subvalorado en el oeste de Argentina.
Palabras clave: Neotectónica, San Juan, La Laja, Villicum, falla flexo-deslizante, terremoto de 1944.
0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina
590 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
INTRODUCTION
On January 15th, 1944 San Juan, Argentina
experienced a devastating Ms 7.4 earthqua-
ke. Whereas the precise location, depth, and
focal mechanism of the earthquake are
poorly known, post-earthquake field sur-
veys revealed surface deformation on the
eastern flank of Sierra de Villicum. (Bastías
et al. 1985, Groeber 1944, Harrington 1944,
Paredes and Uliarte 1988). Harrington
(1944) documented 30 cm of vertical co-
seismic and 30 cm of postseismic displace-
ment along ~6km of the La Laja fault.
Sierra de Villicum is the hanging wall of
one of a series of thrust sheets comprising
the eastern Precordillera structural provin-
ce, which sits between the thin-skinned
Precordillera fold-and-thrust belt on the
west and the thick-skinned Sierras Pampea-
nas structural province on the east (Gon-
zález Bonorino 1950) (Fig. 1). Three faults
have been suggested as the seismogenic
source of the earthquake. Surface rupture
on the La Laja fault identifies it as a candi-
date structure (Alvarado and Beck 2006,
Bastias et al. 1985, Perucca and Paredes
2000, 2002). A second potential candidate is
the southeast-dipping thrust, the Villicum
thrust, which bounds the northwestern
flank of Sierra de Villicum (Siame et al.
2002), which juxtaposes Cambrian carbona-
tes with Neogene red beds (Ragona et al.
1995). Microseismicity beneath the range
suggests that a third possibility is a base-
ment fault zone that strikes northeast, dips
northwest, and extends from 5 to 35 km
depth (Smalley et al. 1993). Thus, alternati-
ve models for the source of the 1944 event
include surface rupturing thin-skinned
faults (La Laja or the Villicum fault) or a
blind thick-skinned thrust.
In this paper, we merge new field mapping
of fluvial terraces and bedrock structure,
structural models of fold growth, and pu-
blished seismicity in a new model for the
crustal structure of the transition between
the thin-skinned fold-and-thrust belt and
the thick-skinned thrust province in the
foothills between the Andes and the Sierras
Pampeanas in western Argentina. Field re-
lationships indicate that the fault system
along the northwestern range front of
Sierra de Villicum does not cut a suite of
fluvial terraces; equivalent terraces, howe-
ver, are deformed by an east-facing mono-
cline and surficial thrust faults on the sou-
theast flank of the range. Within this region
of active folding and faulting, flexural slip
faulting characterizes some of the active
faults, including the La Laja fault, which
indicates these are secondary faults related
to folding. When this observation is com-
bined with field data indicating the southe-
astern flank of Sierra de Villicum defines a
broad, southeast-facing anticlinorium, these
data support the inference that the seismo-
genic source of the 1944 event occurred on
a blind, northwest-dipping thrust in the
basement. Moreover, comparison with o-
ther active structures along strike suggests
that active deformation in the basement
refolds earlier emplaced thin-skinned thrust
sheets. If correct, this interpretation has
local implications for the tectonic develop-
ment of the eastern Precordillera structural
Figure 1: a) Regional tectonic map of Argentine Andes between 31° and 32° south. Four structu-
ral provinces comprise the crustal structural at this latitude, including the Frontal Cordillera
(beige), the Precordillera (pale green), an east vergent thin-skinned fold-and-thrust belt, the eas-
tern Precordillera (pink), a zone of west vergent thrust sheets, and the thick-skinned Sierras Pam-
peanas (turquoise) structural provinces in the vicinity of San Juan, Argentina (modified from
Ragona et al. 1995). LL marks the La Laja fault and TVS indicates the Tulum Valley Syncline. A
box marks the location of the map in figure 2. Yellow star marks the location of terraces dated by
Siame et al. (2002). Cross-section location in figures 11 and 13 are indicated. b) Shaded relief map
of the San Juan region showing focal mechanisms of historical seismicity (1977 to the present) for
shallow crustal events (<50km) from the Harvard Centroid Moment Tensor Catalog. Depth is gi-
ven below beach balls. White boxes are 1944 earthquake epicentral locations summarized in Alva-
rado and Beck (2006).
591Refolding of thin-skinned thrust sheets by active basement-involved ...
transition region, seismic hazard for large
cities along the eastern foothills of the A-
ndes in Argentina, and global implications
for understanding the development of
thick-skin thrust structural provinces and
for recognition of blind thrust fault seismic
hazards.
THRUST ARCHITECTURE BETWE-
EN THE PRECORDILLERA AND
SIERRAS PAMPEANAS
Outcrops in the Precordillera, eastern Pre-
cordillera, and Sierras Pampeanas constrain
the stratigraphic location and depth of the
principal thrust systems. Ordovician and
younger rocks are exposed in the Precor-
dillera (Ragona et al. 1995, von Gosen 1992)
(Fig. 1), which forms the basis for the inter-
pretation that the Precordillera is an east-
vergent thin-skinned thrust belt with a basal
décollement in Orodovician-Lower Devo-
nian sediments (Cristallini and Ramos 2000,
von Gosen 1992). Precambrian and early
Paleozoic basement rocks are exposed in
Pie de Palo and other ranges in the Sierras
Pampeanas province farther east (González
Bonorino 1950). Historic earthquakes, struc-
tural geometry, and active faults indicate
that east-dipping, west-vergent basement-
involved faults are the dominant structure
of the Sierras Pampeanas province (Costa et
al. 1999, Costa and Vita-Finzi 1996, Jordan
and Allmendinger 1986, Jordan et al. 1983a,
b, Kadinsky-Cade et al. 1985, Ramos et al.
2002, Zapata and Allmendinger 1996b).
Bedrock geology indicates that many of the
basement faults bounding the Sierras Pam-
peanas ranges are reactivated Paleozoic and
Figure 2: a) Bedrock geologic map of the southwestern end of Sierra de Villicum. Brown shading of Neogene redbeds highlights repetition of Cambro-
Ordovician-Neogene red bed thrust imbricates. Grey color used to show Neogene red bed distribution on southeast flank. b)Terrace map. The loca-
tions of figures 3 and 5 are shown. Structural data are omitted from (b) to highlight distribution of terraces. Note that thrust faults at southwestern end
of Sierra de Villicum do not extend southwestward into Neogene red beds.
592 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
older sutures and terrane boundaries (Ra-
mos et al. 2002). Eastern Precordillera
thrust sheets define a narrow belt between
east-directed thrust sheets of the Precor-
dillera and west-directed basement reverse
faults of the Sierras Pampeanas (Fig. 1).
Strata exposed at the base of east-dipping
thrust sheets are restricted to Cambro-Or-
dovician carbonates (Fig. 2). Thus, the eas-
tern Precordillera has characteristics in
common with the Precordillera, in that co-
ver sedimentary rocks are exposed in thrust
sheets at the surface and characteristics in
common with Pampean range thrusts, in
that faults dip east. Whether the eastern
Precordillera has a thin-skinned structural
style, in which upper plate faults root to a
décollement at the base of Cambrian strata
or into the basement is unresolved
(Alvarado and Beck 2006, Ramos et al.
2002, Siame et al. 2002, von Gosen 1992).
Sierra de Villicum is a N-S trending range
located in the eastern Precordillera (Fig. 1).
Bedrock structure exposed in the core of
the range reveals a series of west-vergent
imbricate thrust sheets involving an Upper
Cambrian through Middle Miocene strati-
graphic sequence (Fielding and Jordan
1988, von Gosen 1992). An angular
unconformity marks the contact between
steeply-tilted Cambro-Ordovician rocks
and overlying moderately-tilted Neogene
red beds (Figs. 2a and 3) (von Gosen 1992).
A range front fault system, the Villicum
thrust fault (Siame et al. 2002, von Gosen
1992), marks the contact between folded
Neogene red beds in the footwall and the
Cambro-Or-dovician sequence in the han-
ging wall along the northwestern flank of
the range (Figs 2a and 3). On the southeas-
tern flank of Sierra de Villicum, an angular
unconformity separates the Cambro-
Ordovician rocks from shallowly dipping
Neogene conglomerates, sandstones, and
siltstones (Krugh 2003). A second uncon-
formity marks the contact between the
Neogene red beds and the overlying Mogna
Formation, a conglomerate characterized
by abundant volcanic and plutonic clasts
(Fig. 2a).
Five terraces were identified around the
periphery of Sierra de Villicum (Fig. 2b).
An individual terrace comprises an erosio-
nal surface on bedrock, a strath, and an
overlying capping gravel, typically 1-3
meters thick (Krugh 2003). Clast composi-
tion of the capping gravels is dominated by
limestone, dolostone, and chert. Gravel in
terraces on the southwest end of Sierra de
Villicum includes volcanic and plutonic
clasts as well. The relative ages of the terra-
ces were determined by their distribution,
strath elevation above modern channels,
and differences in surface morphology and
composition. Sequentially younger terraces
are inset into older terraces and occupy
lower elevations with respect to modern
channels. Older terraces are more dissected,
have lower surface relief, and have a higher
concentration of varnished chert clasts.
Based on these observations, the terraces
were classified T1 = oldest to T5 = youn-
gest. Four of the terraces (T1-4) are regio-
nally extensive, well preserved around the
periphery of Sierra de Villicum, and can be
correlated throughout the study area.
METHODOLOGY
Geologic and geomorphic mapping and
detailed topographic surveying of terrace
surfaces was conducted along the flanks of
Sierra de Villicum. Elevation and form of
modern channels were used to constrain
the magnitude and style of terrace defor-
mation. Terraces were physically correlated
with a suite of dated surfaces to the south-
west of the study area (Siame et al. 2002).
Deformation rates were calculated by assig-
ning ages to the terraces of this study on
the basis of the correlation with the dated
surfaces to the southwest (Siame et al.
2002). A structural model at the crustal
scale was developed from field relationships
Figure 3: Detailed bedrock and geomorphic map of southwestern end of Sierra de Villicum along
the northwestern range front. 1, 2, and 3 mark the three thrust imbricates that duplicate the se-
quence of Cambro-Ordovician carbonates unconformably overlain by Neogene red beds on sou-
theast-dipping reverse faults. Photos in 4a and b are from imbricate 1 and figures 4c and d are
from along the Villicum thrust (VT) along the northwestern margin of imbricate 2. Neither the
Villicum thrust nor the thrust on the northwest margin of imbricate 3 continues southwest in the
red beds
593
Refolding of thin-skinned thrust sheets by active basement-involved ...
and published microseismicity (Smalley et
al. 1993). Together, the crustal model and
patterns of Quaternary and younger defor-
mation were used to propose a model for
the seismogenic source of the 1944 earth-
quake, to characterize the regional structu-
ral style, and to relate the rate and style of
crustal deformation to GPS and other data
(Brooks et al. 2000, Kadinsky-Cade et al.
1985, Ramos et al. 2002, Smalley et al. 1993).
BEDROCK AND TERRACE
DEFORMATION, NORTHWEST
RANGE FRONT, SIERRA DE
VILLICUM
A prominent topographic escarpment
marks the northwest range front of Sierra
de Villicum. Whereas the range front has
been considered a simple thrust contact be-
tween Cambro- Ordovician carbonates and
Neogene red beds in the hanging wall and
footwall of the Villicum thrust, respectively
(Siame et al. 2002, von Gosen 1992), several
thrust imbricates involving the Cambro-
Ordovician and Neogene red beds are ex-
posed with both faulted and folded con-
tacts at the range front (Fig. 3). Northwest-
dipping carbonates are unconformably
overlain by less-steeply tilted red beds in the
northwestern-most exposures in the study
area (imbricate #1, Figs. 3 and 4a). To the
southeast, the carbonates and red beds dip
southeast (Fig. 4b). An anticline with a ste-
eply dipping northwest limb and a modera-
tely dipping southeast limb are defined by
the contact trace and bed dips (#1, Fig. 3).
Red beds in the southeast limb of the anti-
cline are cut by the Villicum thrust, an obli-
que slip reverse fault (imbricate #2, Figs. 3,
4b, and 4c). The Villicum thrust can be tra-
ced in outcrop along strike to the south-
west, but disappears into the subsurface
and the range front is characterized at the
surface by northwest dipping Cambro-Or-
dovician rocks unconformably overlain by
northwest-dipping red beds (Figs. 3 and
4d). This contact traces around the north-
west end of Sierra de Villicum, which defi-
nes a southwest plunging anticline, the axial
trace of which continues to the southwest.
Terraces T1-3 are well preserved along the
northwest range front (Fig. 3). Both T1 and
T2 are continuous across the axial trace of
Figure 4: Views of the range-front fault system on the northwest flank of Sierra de Villicum. a) Western-most outcrop is the northwest limb of a
fold marked by an unconformable northwest-dipping depositional contact between Cambro-Ordovician strata and Neogene red beds (ball-arrow
symbols give dip direction). Terrace gravels of T2 depositionally overlie the contact and are not deformed. b) Cambro-Ordovician and Neogene stra-
ta dip southeast in the southeastern limb of the fold. Red beds are truncated by the Villicum reverse fault on the southeast (trace marked by white
arrows). c) Dip of Villicum fault varies between 45° and ~50° southeast and has right-oblique shear sense striations on the fault plane. Cambro-
Ordovician rocks in the hanging wall are juxtaposed with Neogene rocks in footwall. d) Reverse fault continues at surface to yellow arrow on the
southeast (Fig. 3). Southwestward from yellow arrow to the southwestern end of Sierra de Villicum the range-front structure is a fold characterized
by northwest-dipping Cambrian unconformably overlain by Neogene red beds. Whereas the fault is well exposed in deeply incised washes along stri-
ke, neither scarps nor steps occur in terraces where they overlie the fault trace.
594 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
the northwestern folded unconformity bet-
ween the Cambro-Ordovician and the Neo-
gene (Figs. 3 and 4a). The terraces extend
across the fold axial trace. To the southeast,
the Villicum thrust occurs midslope above
the elevation of the terraces along the range
front (Fig. 4d). Because the Villicum thrust
looses elevation to the southwest, however,
terrace relationships with faults and
bedrock contacts can be resolved (Fig. 3).
On the southwest, terraces T2 and T4 con-
tinue without scarps across the Villicum
fault trace and the northwest-tilted uncon-
formity between the Cambro-Ordovician
and Neogene strata. Whereas the Villicum
thrust has been interpreted to cut surficial
deposits (Siame et al. 2002), these field ob-
servations indicate that although the range
front is characterized by both folded and
faulted Cambro-Ordovician and Neogene
red beds, neither the folds nor the Villicum
fault affect the terraces along the northwes-
tern range front. These observations sug-
gest that the Villicum thrust and range
front folds are either inactive or growing
imperceptibly on the timescale of develop-
ment of the terraces. Moreover they reaf-
firm Whitney's (1991) conclusion, on the
basis of detailed mapping on aerial photo-
graphs and field mapping, that the fault
system along the northwestern range front
does not cut terraces of any age.
BEDROCK AND TERRACE DEFOR-
MATION, SOUTHEAST FLANK,
SIERRA DE VILLICUM
A 10 km-wide panel of shallowly- to mode-
rately-inclined southeast-dipping red beds
characterizes the southeast flank of Sierra
de Villicum (Fig. 2a). A ~20° angular un-
conformity marks the contact between the
Cambro-Ordovician sediments and the
Neogene red beds. Two structural domains
can be differentiated within the red beds
exposed on the southeastern flank (Fig. 5).
Shallow southeast bed dip (<15°) and no
faults or small-scale folds characterize the
western domain. A laterally continuous
southeast dipping thrust fault marks the
boundary between the western and eastern
domains. Small displacement thrust faults
and a southeast-facing monocline are deve-
loped throughout the eastern domain (Figs.
2 and 5). Bedding dip changes from 10°-15°
to 35°-45° from northwest to southeast,
respectively, across the eastern domain.
Mogna Formation, the youngest stratigra-
phically differentiable bedrock unit, is well
exposed in three outcrop belts. On the nor-
theast edge of the area the Mogna For-
mation dips moderately southeast. On the
southwest end of the area, in contrast, the
Mogna formation dips < 15° and faces
south (Fig. 2a). Bedding orientation in the
underlying Neogene red beds also changes
from southeast facing in the northeast to
south facing along strike to the southwest
(Fig. 2a).
A low relief topographic surface comprised
of a series of fluvial terraces etched into
Neogene red beds typifies the southeast
flank of the range (Fig. 5). Individual terra-
ces consist of a low-relief bedrock erosio-
Figure 5: Map of terrace surfaces T1-T3 on the eastern flank of Sierra de Villicum, San Juan,
Argentina. Faults and folds that deform terraces define an eastern domain, which contrasts with
the western domain where terraces are undeformed. The La Laja fault, which ruptured in the
1944 earthquake (Fig. 7a), and a regionally extensive active axial surface (red dashed line) mark the
southeastern edge of the eastern domain. Figures 7b, 7c and 8 depict structural relationships in
the wash indicated by the red box near 'Calle La Laja'.
Figure 6: Profile of terraces and modern wash across eastern domain. Faults are steep due to 5x
vertical exaggeration (VE) (upper profile; lower profile has no VE). Equivalent horizontal shorte-
ning across faults for the T2 surface (Ht2) is calculated from vertical separation and a 40° fault
angle. Box indicates structural location of photos in figure 7 and structural data summary of
figure 8. LL marks the La Laja fault.
595
Refolding of thin-skinned thrust sheets by active basement-involved ...
nal surface (a strath), which is overlain by 1-
3 meters of poorly sorted limestone-and-
dolomite-clast gravels. Map relationships
and terrace profiles indicate that the terra-
ces are undeformed in the western domain.
Both terrace surfaces and modern channel
profiles are concave up and have a nearly
constant gradient of ~ 3° across the wes-
tern domain. In contrast, folds and faults of
the eastern domain affect terraces of nearly
all ages (Fig. 6). Displacement of terraces
across faults varies as a function of age and
does not exceed ~10 m for T1. Roughly 17
m of horizontal shortening is deduced
from the fault dip and separation of T2
straths across faults in the eastern Domain
(Fig. 6).
The most pronounced deformation of te-
rraces is localized in a narrow region near
the La Laja fault scarp on the southeastern
edge of the eastern domain (Figs. 2, 5 and
7). In this region, the shallowly dipping
Neogene strata are tilted to the southeast ~
40° (Fig. 7). Photographs taken after the
1944 event show that surface rupture on
the La Laja fault is approximately parallel to
bedding (Fig. 7a) (Harrington 1944). Rou-
ghly 60 cm of uplift was measured from a
road warped by surface rupture on the
fault. Total displacement of individual te-
rrace surfaces is larger than that observed
from the 1944 earthquake (Fig. 7b), sugges-
ting that the terrace surfaces provide a com-
Figure 7: Photos of deformation style on southeastern limit of the eastern domain, southeast flank Sierra de Villicum. (A) Surface rupture on the La
Laja fault in 1944 (from Harrington 1944)). Resistant beds are inclined at a similar angle to the surface rupture trace. (B) Exposure of La Laja fault
southwest of La Laja road. Bed and fault dip (ball-ended arrow) are similar. Terraces 2 and 4 occur in the hanging wall and footwall, respectively. (C)
(D) Views to the southwest and northeast of deformed terraces in a wash ~100 m southwest of La Laja road (red dot, Fig. 5). Solid lines mark strath
surfaces at the base of terraces; dashed lines mark the abandoned surface. Monoclinal fold hinges within individual terraces do not fold older or
younger terraces or underlying Neogene bedrock. Fold hinges in structurally lower and geomorphically lower terraces are located systematically to the
southeast of hinges in older, higher surfaces. (E) Synclinal hinge controlling the southeastern break in slope exposed in Neogene bedrock (red dashed
line, Fig. 5).
596 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
posite record of earthquakes prior to and
including the 1944 earthquake. Dip of the
red bed sequence and the La Laja fault are
the same suggesting the fault result from
bed-parallel slip during the earthquake (Fig.
7b) (Krugh 2003). Therefore, the La Laja
fault is interpreted to be a flexural slip fault
(Krugh 2003), which are secondary faults
related to folding (Yeats 1986). Folded te-
rraces in the hanging wall to the southeast
of the La Laja fault also record deforma-
tion related to folding (Figs. 7c-e).
Geometric relationships between individual
terraces and underlying bedrock are well
exposed in southeast-draining, transverse
stream channels (Figs. 5 and 7). Southeast-
facing monoclines typified by a change in
dip from ~ 3°SE to ~15°-20°SE are pre-
sent in T1-3 (Figs. 7c and d). Hinges preser-
ved in individual terraces are unique to a
given terrace and affect neither younger nor
older surfaces. Hinge location change in
space systematically as a function of age;
the hinge in T3 is located to southeast of
T2 (Fig. 7d), which is to the southeast of
the T1 hinge (Fig. 7c). Underlying Neogene
bedrock is not folded about any of the hin-
ges, in contrast, and has a uniform dip of
~40°SE beneath each of the terraces. A
prominent break in slope marks the southe-
astern limit of the tilted terraces. Terrace
T5 is undeformed and inset into older te-
rraces in transverse channels but onlap ol-
der surfaces on interfluves. Thus the break
in slope is a depositional contact between
older, tilted terraces to the northwest and
the youngest undeformed terraces on the
southeast. Along strike to the northeast,
however, a folded unconformity within the
Neogene red beds reveals the nature of the
same break in slope in terms of bedrock
structure. Dip of strata above the uncon-
formity shallows from ~30° to ~10° and
from ~60° to ~40° below the unconfor-
mity from northwest to southeast, respecti-
vely (Fig. 7e). The break in slope is therefo-
re interpreted as the trace of a fold hinge
separating more steeply from shallowly
southeast dipping strata in the Neogene
bedrock. At the structural and stratigraphic
level of the terraces, the hinge separates til-
ted terraces on the northwest from equiva-
lent undeformed sediments on the southe-
ast. The youngest terrace deposits are a thin
veneer deposited across the hinge.
TERRACE FOLD MODEL
The observed geometric relationships bet-
ween bedrock structure, folded terraces,
and topography constrain the style of fold
growth and shortening rate across the sou-
theastern flank of Sierra de Villicum. Any
model for fold growth must account for the
principal structural and geomorphic obser-
vations along the southeastern edge of the
eastern domain, as summarized in figure 8.
At the structurally lowest level in the
bedrock, bedding changes from shallow to
steeper dip across a synclinal hinge that
coincides with the topographic break in
slope at the southeastern edge of the eas-
tern domain (Figs. 5 and 7e). Bedding to the
northwest of the hinge has invariant dip,
despite the complex folding of terraces at
shallower structural levels. The La Laja fault
is a flexural slip fault parallel to bedding
(Figs. 7a and b). Terrace folds have a mono-
clinal geometry marked by a hinge unique
to each terrace (Figs. 7c and d). Hinges
developed in a terrace of a given age do not
affect bedrock or older or younger terraces.
Hinge position in space increases with dis-
tance northwest from the synclinal hinge as
a function of age. A model of fold growth
via kink-band migration best reconciles
these features (Fig. 9).
Kink-band migration is a mechanism of
folding whereby beds change dip due to
rotation about axial surface(s) (Suppe 1983,
Suppe et al. 1997). The region between two
corresponding axial surfaces is termed a
"kink-band". Fold growth results from mi-
gration of one or both axial surfaces defi-
ning a kink band, which translates material
into the fold limb. In growing structures,
one or both of these axial surfaces may
move relative to bedrock (Suppe et al. 1992).
An active axial surface refers to a hinge that
moves with respect to rock whereas a fixed
axial surface is one that is stationary with
respect to rock. As an active axial surface
migrates, rock is instantaneously rotated
about the axis and translated onto the fold
limb. Fold growth rate can be determined
when the position of the fixed and active
axial surface can be determined in time
(Mueller and Suppe 1997, Shaw and Suppe
Figure 8: Summary sketch (to scale) of
structural relationships in region of
wash southwest of La Laja road (red
box, Fig. 5; photos in Figs. 7b and c).
Topographic profile, T1, T2, and bed-
ding are from ridge on southwest side
of wash. T4-5 are from outlet of
wash on southeast. T3 projected on
to profile from ridge on northeast side
wash (Fig. 7c). T2-4 in footwall of the
La Laja fault are projected from wash
adjacent to the La Laja road (Fig. 5).
Bed form lines in Neogene bedrock
schematically illustrate bedding.
Relationships above ~610 m elevation
are observed in the field. Structure
below 610 m is pro-jected on the basis
of surface data.
597Refolding of thin-skinned thrust sheets by active basement-involved ...
1996, Suppe et al.1992).
Strata deposited coevally during fold
growth are referred to as syntectonic or
growth strata (Riba 1976). Growth strata
geometry depends on geometrical changes
of fold limbs during folding (DeCelles et al.
1991, Ford et al. 1997, Suppe et al. 1997,
Vergés et al. 1996). Hinge migration relative
to rock and to other hinges dictates growth
strata geometry for folds formed due to
kink band migration (Suppe et al. 1992).
For the case of motion of an active axial
surface relative to a fixed axial surface, de-
position across the fixed hinge results in
formation of a new hinge, a growth axial
surface, which is fixed with respect to the
accumulating growth strata (Suppe et al.
1992). At the moment after a bed is deposi-
ted, but prior to its incorporation into the
fold, the growth and active axial surfaces
intersect at a point at the surface. Conti-
nued migration of the active axial surface
relative to the growth axial surface creates a
growth triangle, the size of which is pro-
portional to the total amount of syndeposi-
tional fold growth (Suppe et al. 1997).
Growth axial surfaces, active axial surfaces,
and growth triangles thus preserve a record
of fold growth because the displacement of
the active axial surface relative to the
growth axis is represented by their offset in
space. Because older growth strata have
endured more folding, growth triangle
width progressively decreases in younger
growth strata.
When sedimentation is unsteady relative to
fold growth, such as during punctuated
events like earthquake induced folding or
climatically variable deposition or erosion,
discrete growth axial surfaces form in sedi-
ments deposited between growth events
(Mueller and Suppe 1997). To illustrate a
sequence of alternating erosion/deposition
and uplift events and the consequent affect
on terraces and bedrock, a simple model of
migration of a single active hinge was cons-
tructed (Fig. 9). A channel forms a strath as
it bevels an erosional surface into titled
bedrock and deposits a gravel cap in step 1
(Fig. 9a). Migration of the active hinge by
some arbitrary horizontal amount (H1) ro-
tates rock and the terrace as they are drawn
into the fold limb, although the amount of
rotation is a function of initial dip (Fig. 9b).
Incremental folding results in formation of
a growth hinge offset a distance H1 with
respect to the active hinge. Uplift of the
strath at the base of the terrace (V1) de-
pends on H1 and tilt of the bedrock outsi-
de of the kink band. Renewed down cutting
of the channel forms a new strath/ gravel
cap pair comprising a younger terrace (Fig.
9c). A second folding event de-forms the
newly formed terrace, creates a growth
hinge in the new surface, and the incre-
mental fold growth is given by horizontal
offset of the growth hinge with respect to
the active hinge and the uplift of the strath
relative to the modern channel, H2 and
V2, respectively (Fig. 9d). Whereas the
cumulative growth is given by the sum of
the vertical and horizontal motions, all
structures on the side of the hinge opposi-
te to the direction of active hinge migra-
tion experience uplift, but no additional til-
ting. Thus, tilted bedrock and terraces and
earlier-formed growth axes are not tilted
further during subsequent folding events.
The position of a growth axial surface is
therefore dependent on the migration rate
of the active axial surface relative the time
required to form the strath/gravel cap pair
of a terrace. Discrete earthquake events
are recorded by growth strata formed by
kink band migration only under special
conditions that include depositional events
with greater frequency than folding events,
fold growth in scale that exceeds the scale
of interseismic sediment accumulation, and
structural tilting that is greater than primary
depositional dip (of bedding and bed
forms).
ACTIVE DEFORMATION AND
SHORTENING RATES CROSS THE
EASTERN PRECORDILLERA
Active deformation is localized on the sou-
theast flank of Sierra de Villicum (Figs. 2, 3,
and 5). No evidence of folding or thrust
faulting of terraces is observed along the
Villicum thrust fault or folds cropping out
along the northwestern range front (Figs. 3
and 4d). In contrast, abundant evidence for
active folding is represented on the southe-
ast edge of the southeast flank of Sierra de
Villicum (Figs. 5, 6, and 7). The La Laja
fault, for example, is a flexural slip fault
that ruptured in the 1944 earthquake
(Harrington, 1944) and offsets all of the
terraces, older terraces have greater vertical
separation than younger surfaces (Fig. 8).
This later observation argues for the long-
term flexural slip faulting accompanying
Figure 9: Terrace fold growth model. (A) Fluvial incision cuts a strath terrace into folded bed-
rock. A thin (1-3 m-thick) gravel layer overlies the strath (grey bar). (B) A fold event deforms the
terrace due to a migration of a hinge (dash) through the rock. Strata to the right of the hinge in
(A) uplift but do not tilt, whereas strata to the left uplift and tilt, which forms a hinge (solid)
fixed within the terrace. Incremental shortening (H1) and uplift (V1) are given by the offset of
the hinges and uplift of the strath, respectively (C) Erosion into the fold results in formation of a
second terrace, represented by the strath and gravel cap (dotted bar). (D) A second fold event
forms a hinge restricted to the second terrace. Total shortening and uplift are equal to the sum of
the incremental horizontal (Hn) and vertical (Vn) motion. Compare (D) with figures 6b, 6c, and 7.
598
fold growth in earthquakes. Terraces of all
ages are folded as well. A kink-band migra-
tion model in which uplift, erosion, and
deposition are out of phase fits the geo-
metry of deformed terraces on the southe-
ast flank well (compare Figs. 6, 7 and 8d).
Determination of shortening rates across
the zone of active folding thus requires
absolute age assignments for the various
surfaces. To date, no age constraints, unfor-
tunately, are available from the terraces in
the study region.
Six kilometers southwest of Sierra de Villi-
cum, however, a suite of three terraces has
been dated using cosmogenic radionuclides
(Siame et al. 2002). Siame et al. (2002) map-
ped terrace surfaces A3, A2, and A1 that
are elevated 11±1 m, 6±2 m, and ~0.6 m
above the modern channel, respectively.
Radionuclide concentrations from A3, A2,
and A1 indicate ages of 18.7 - 18.0 ka, 6.9 -
5.3 ka and 1.9 - 1.1 ka, respectively. The
terraces are cut into Neogene red beds that
are structurally contiguous with the southe-
astern flank (Fig. 1). Because the terraces of
this study and those of Siame et al. (2002)
occur within the same structural domain, a
physical correlation between the two areas
was performed to provide estimates on the
ages of the terraces in this study. Terraces
T1-4 were physical correlated by following
terraces up and down channels and across
adjacent drainages from the northeast cor-
ner of the study area on the southeast flank
across strike to the southwestern end of
Sierra de Villicum and around to the north-
west range front (Fig. 2b). Strath and gravel
cap elevation with respect to modern chan-
nels form the basis of the correlation.
Uncertainty in the correlation was introdu-
ced by correlation across regions with limi-
ted preservation of T1-3, correlation across
small-displacement faults, by variable eleva-
tion of modern channels, and by paleoto-
pography, paleocanyons in particular, pre-
served between strath surfaces and capping
gravels. Uncertainty in the age determina-
tions are introduced because inheritance
was not specifically considered in the sam-
pling or subsequent evaluation of the nucli-
de concentrations (Siame et al. 2002). Mo-
reover, inspection of the Siame sampling
sites suggests the possibility that one or
more of the surfaces are cut-in-fill terraces
and therefore have a more complex expo-
sure history than a simple abandoned fill
terrace. Either of these two sources of un-
certainty implies that the terrace ages may
be minimum ages, which means that struc-
tural rates based on correlations are maxi-
mum values.
Horizontal shortening rates are inverted
from the horizontal separation of growth
axial surfaces in T1, T2, and T3 relative to
the active axial surface to the southwest
(Figs. 8-10). On the basis of our terrace
mapping, older and younger correlations
with the terraces dated with cosmogenic
radionuclides of Siame et al. (2002) are per-
missible (Fig. 10). A 'younger' correlation
makes T1 equivalent with A3, T2 correlati-
ve with A2, and yields a maximum shorte-
ning rate of 5 mmyr-1 or greater (Fig. 10).
An 'older' correlation ties T2 to A3, T3 or
T4 to A2, and implies a minimum shorte-
ning rate of ~3 mmyr-1 (Fig. 10). These rate
approximations assume constant shorte-
ning since the formation of T2 or T1 for
the older and younger correlations, respec-
tively. Independent age determinations in
progress for the terraces of this study will
provide independent constraint on terrace
age and therefore structural rates.
CRUSTAL SCALE MODEL FOR
REFOLDING OF THIN SHINNED
THRUST SHEETS BY BASEMENT
DEFORMATION
A number of observations suggest that the
Sierra de Villicum thrust sheet is being acti-
vely refolded. The thrust sheet extends
from the Villicum thrust on the northwest
to the Tulum Valley syncline on the southe-
ast (Fig. 1). Dip data and stratigraphic con-
tact relationships reveal the presence of a
structural high in the middle of the southe-
ast flank of Sierra de Villicum. Dip in the
red beds defines a southeast facing mono-
cline characterized by a change from sha-
llowly southeast- to moderately southeast-
dipping within the western domain (Fig. 2).
Active folding is localized in the moderately
dipping limb of the monocline, which is
where the La Laja fault and folded terraces
are located (Figs. 5 and 8). One key active
structure in this region is the active axial
surface that crops out in both the terraces
and in the bedrock (Figs. 2 and 7). Strike of
the contact between the red beds and the
overlying Mogna Formation changes from
northeast to west along strike from northe-
ast to southwest. In the region where the
contact is west striking, both the Mogna
Formation and the red beds dip south (Fig.
2b). Together, these observations indicate
that the middle of the Sierra de Villicum
thrust sheet is marked by a structural high,
the structural high is a southeast-facing
monocline, and active folding is concentra-
ted on the southeast limb of the monocline
(Fig. 11).
A structural high between the Precordillera
on the west and the eastern Precordillera on
the east is required by structural and strati-
graphic relationships (Cristallini and Ramos
Figure 10: Horizontal shortening rate estimates based on alternative terrace correlations with Siame
et al. (2002) and horizontal displacement measurements of growth hinge relative to active hinge in
terraces and bedrock (Fig. 8). Constant maximum (long dash) and minimum (solid) rates (in millime-
ters per year (mm/yr)) depend on whether T1 ('younger' correlation, open diamonds) or T2 ('older'
correlation, closed diamonds) correlate with Siame's 18.7 ka terrace. Variable shortening rate in time
is implied by the 'younger' correlation (short dash). Age error is from Siame et al. (2002) (± 2.3 and
± 1.0 ka for 18.7 and 6.8 ka surfaces, respectively). Hinge position is assigned a ± 5 m error.
599
Refolding of thin-skinned thrust sheets by active basement-involved ...
2000, Ramos et al. 2002, von Gosen 1992).
The basal décollement of the Precordillera
dips west (Fig. 11). If the hanging wall of
the Villicum thrust fault is a hanging wall
flat, which is consistent with the dip of the
fault and hanging wall bedding (Figs. 3 and
4), the Tulum Valley syncline would repre-
sent the down dip branch line of the Villi-
cum thrust system with a regional décolle-
ment near the base of the Cambrian Strata
(von Gosen 1992). This geometry indicates
that the Ullum Valley to the northwest of
Sierra de Villicum sits above an arch in the
basement separating opposing regional dips
of the décollement beneath the Precordi-
llera and eastern Precordillera (Figs. 1 and
11). Microseismicity beneath this region is
suggestive of an active fault(s) between 5
and 35 km depth in the basement (Smalley
et al. 1993). The principal plane revealed by
these data is N45E° striking and dips 35° to
the northwest (Fig. 11). Focal mechanisms
and focal depths of current seismicity indi-
cate active reverse faulting concentrated be-
tween 20 and 30 km at depth (Fig. 1b)
(Kadinsky-Cade et al. 1985, Ramos et al.
2002, Siame et al. 2002, Smalley et al. 1993).
Thus, field relationships, the loci or active
deformation, historical seismicity, and mi-
croseismicity can be linked in a structural
model that relates active refolding of the
Sierra de Villicum thrust sheet to an active
northwest-dipping fault that extends to the
middle crust at depth (Fig. 11).
Four alternative models relate basement
faulting in the middle crust with accommo-
dation of slip on upper crustal structures
(Fig. 12). A simple fault-propagation fold
where the east-facing monocline and struc-
tural high between the Precordillera and
eastern Precordillera are the forelimb and
crest of the fold, respectively, growing abo-
ve fault tip below the cover sequence (Fig.
12a). Slip from the basement may be trans-
ferred to the detachment at the base of the
Sierra de Villicum thrust sheet and daylight
on the east side of the Tulum Valley syncli-
ne (Fig. 12b). Third and fourth alternatives
are represented by a wedge geometry whe-
reby east-directed slip on the basement
fault is accommodated in the cover sequen-
ce by west-directed thrusting (Figs. 12c, d).
Transfer of slip to the La Laja fault was first
proposed by Smalley et al. (1993) to account
for the fact that La Laja and basement
faults have opposing dips (Fig. 12c). Slip
transfer to the Villicum thrust system could
also absorb slip at depth on the basement
fault (Fig. 12d).
Neither the transfer of slip to a thrust on
the east side of the Tulum Valley syncline
nor to the La Laja fault is supported by field
observation (Figs. 12b, c). No active east
vergent faults are present between the west
flank of Pie de Palo and the hinge of the
Tulum Valley syncline (Fig. 1a) (Ragona et
al. 1995). Stratigraphic separation across the
La Laja fault would have to be proportiona-
te to slip on the basement thrust fault, in
the case of the latter alternative, yet field re-
lations suggest that separation is effectively
zero (Figs. 7 and 8). Whereas none of the
imbricates of the Villicum thrust system
apparently affects the terrace sequence, it is
possible that shortening on the fault system
is reflected at a longer wavelength. There-
fore, it is reasonable to conclude that slip
on the basement fault system is accommo-
dated by a combination of monoclinal fol-
ding above the fault tip, which is potentially
accompanied by displacement transfer to
one of the faults within the Villicum thrust
system (Figs. 12a, d).
We propose that the 1944 earthquake occu-
rred on the northwest-dipping basement
fault that extends into the mid-crust bene-
ath Sierra de Villicum. This model con-
trasts, however, with current models for the
1944 source. Those models propose that
the earthquake occurred to the east of Sie-
Figure 11: Crustal scale model relating basement seismicity and upper crustal structure beneath the eastern Precordillera. Whereas a thrust sheet
carrying Cambro-Ordovician sedimentary cover strata underlies Sierra de Villicum, active deformation is restricted to folding on the southeast
flank. Microseismicity between ~5 and 35 km depth defines a northwest-dipping fault plane (black dots) (Smalley et al. 1993), which is inferred to
be the source of the 1944 San Juan earthquake. Active folding of terraces and flexural slip faulting on the La Laja fault are secondary effects of
thrust sheet refolding. A basement high between the Precordillera and the eastern Precordillera is required by the opposing regional dips between
the two structural domains. Precordillera structure and stratigraphy are modified from von Gosen (1992). Eastern Precordillera structure is modi-
fied from Krugh (2003). Sierras Pampeanas structure and seismicity (light dots) modified from Ramos et al. (2002).
600 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
rra de Villicum on an east-dipping fault
system that extends into the basement
(Alvarado and Beck 2006, Perucca and Pa-
redes 2000, 2002, Siame et al. 2002). An
earthquake source on an east dipping fault
system is based on the inference that the
Villicum thrust system ruptured in the 1944
event and is therefore the up-dip continua-
tion of a fault that extends to ~20 km
depth beneath the Tulum Valley (Siame et
al. 2002). Others contend that because of
the surface rupture in the earthquake, the
east-dipping La Laja fault is the up-dip con-
tinuation of the seismogenic fault (Alva-
rado and Beck 2006, Perucca and Paredes
2000, 2002, Smalley et al. 1993). A source
on an east-dipping fault is consistent with
two of the three reported locations for the
epicenter of the 1944 earthquake, which lie
to the southeast of Sierra de Villicum (Fig.
1) (Alvarado and Beck 2006). The La Laja
fault can be connected with a hypocenter at
~11 km depth beneath the Tulum Valley,
which was determined via an inversion of
historic seismograms based on assuming
apriori that epicenter is to the east (Alva-
rado and Beck 2006). In this model the La
Laja fault is an imbricate splay off the fault
system proposed in the Siame et al. (2002)
model.
A number of observations can be used to
evaluate the competing structural models
for the eastern Precordillera and therefore
the potential seismic sources for the 1944
event. The key observations include the re-
lative activity and style of faulting on the
Villicum and La Laja faults, the location and
geometry of active folding and faulting,
alternative epicentral locations for the 1944
earthquake, and the regional distribution of
seismicity with depth. Field data indicate
that the Villicum thrust system is either
inactive or moving at a rate slower than can
be recorded by geomorphic markers, given
that none of the individual thrust imbrica-
tes of the fault systems cuts or folds terra-
ces of any age (Figs. 3 and 4). Although
there is no field evidence for active slip
along the fault trace, it is possible that some
slip on the thrust may accumulate as the
consequence of folding or slip transfer
from the basement fault (Fig. 12). Bedding-
fault relationships across the La Laja fault
are consistent with a flexural slip interpreta-
tion of the active faulting (Figs. 7a, 7b, and
8). Thus, surface rupture of the La Laja
fault reflects coseismic fold growth and not
surface rupture of the primary seismogenic
fault, which is consistent with the location
of the fault within the actively growing
monocline.
Bedrock and terrace geometries indicate
that the east-facing monocline is refolding
the Sierra de Villicum thrust sheet (Figs. 2
and 11). A model where the active defor-
mation at the surface is dominated folding
above a northwest dipping basement thrust
reconciles the fold-related flexural slip faul-
ting on the La Laja fault, bedrock and terra-
ce structural relationships, and the spatial
distribution of active structures. A fault
that extends westward to ~35 km depth, as
suggest by microseismicity (Smalley et al.
1993), is consistent with an epicentral loca-
tion to the west of Sierra de Villicum (Fig.
1), which is one of three proposed loca-
tions (Alvarado and Beck 2006). Moreover,
the majority of earthquakes occur between
20 and 30 km (Fig. 1b) (Smalley and Isacks
1990). If the hypocenter of the earthquake
is shallow (~11 km) and to the east of
Sierra de Villicum, as suggested by the epi-
central locations in the vicinity of San Juan
(Fig. 1) and the inversion of historic seis-
mograms (Alvarado and Beck 2006), the
west-dipping crustal fault proposed here
represents a second and previously unre-
cognized active seismic source in the boun-
dary region between the Precordillera and
Sierras Pampeanas.
Global positioning system (GPS) data indi-
cate that the rate of convergence of the
Precordillera with respect to the Sierras
Pampeanas across the eastern Precordillera
is ~4.5 mmyr-1 (Brooks et al. 2003), which
provides a framework for evaluating shorte-
ning rates determined from the deformed
terraces. Horizontal shortening rate across
the southeastern flank of Sierra de Villicum
is estimated to be no more than 3 mmyr-1
(Fig. 10). That rate is determined from the
sum of shortening of local folding of terra-
ces where the folding is being forced by a
local change in dip (Fig. 8) and by the dis-
placement of faults in the eastern domain
(Figs. 5 and 6) (Krugh 2003), of a regionally
extensive long-wavelength monocline (Fig.
11). If the fold is growing via kink band
migration, the horizontal component of
fold growth is controlled by the underl-
ying fault orientation and slip rate (Suppe
Figure 12: Alternative models for the
accommodation of slip in the cover
sedimentary rocks (grey shading) due
to fault displacement on the basement
reverse fault. A) Fault slip is accom-
modated by fault-propagation folding
above the fault tip. (B) Fault slip is
transfered to a fault system beneath
the Tullum Valley syncline (TVs). (C)
Fault slip is transfered to the La Laja
fault on the east flank of Sierra de
Villicum (SdV). (D) Fault slip is
transferred to the imbricate fan thrust
system bounding the west flank of
Sierra de Villicum. Depth of seismi-
city is indicated by cross-hatched pat-
tern (after Smalley and Isacks 1990).
601Refolding of thin-skinned thrust sheets by active basement-involved ...
et al. 1992).
Shortening at ~3 mmyr-1 across the eastern
Precordillera due basement thrust faulting
is consistent with seismicity and short- and
long-term shortening rates measured regio-
nally. Evidence of active folding and faul-
ting is present across Pie de Palo to the east
of Sierra de Villicum (Fig. 1a) (Ramos et al.
2002). Shortening rate across Pie de Palo is
of 4 mmyr-1 since 3 Ma (Ramos et al. 2002),
The sum of the rates across Pie de Palo and
Sierra de Villicum (~7 mmyr-1) are consis-
tent with, albeit higher, than the GPS-deter-
mined rate change between the Precor-
dillera and the Sierras Pampeanas (4.5
mmyr-1, Brooks et al. 2003). If the sum of
the two rates represents the long term
regional shortening rate, despite the fact
that rates are measured over different
timescales, it seems likely that the ~3 mmyr-
1horizontal rate across the eastern Precor-
dillera is a maximum estimate (Figs. 8 and
9). Independent age determinations for the
terraces in the study are will provide better
constraints on the deformation rates within
the eastern Precordillera. Shortening rate at
31° S north of the study area is ~5 mmyr-1
since 2.7 Ma (Zapata and Allmendinger
1996a). Seismicity is focused beneath the
eastern Precordillera and Sierras Pampea-
nas region (Alvarado and Beck 2006,
Kadinsky-Cade 1985, Kadinsky-Cade et al.
1985, Siame et al. 2005, Smalley and Isacks
1990, Smalley et al. 1993), which supports
the inference that active deformation is
concentrated deformation in eastern Pre-
cordillera-western Sierras Pampeanas region.
Fault dip is a key difference between the
crustal model proposed in this study and
the structural model used to interpret the
GPS velocity field. An elastic model of the
GPS data using a 10° west dipping plane
fits well the velocity gradient across the
Precordillera-Sierras Pampeanas transition
(Brooks et al. 2003). The rationale for this
geometry is a cross-section across the Ber-
mejo basin north of 31° S (Zapata and
Allmendinger 1996a). A key feature of this
model of the GPS velocity field is that the
model fault represents the basal décolle-
ment of the Precordillera (Brooks et al.
2003). At 32° S in the study area, however,
there is little evidence that the frontal thrust
of the Precordillera is active. Historical seis-
micity, recent earthquakes, and the crustal
scale model presented here suggest that
active structures involve the basement to
depths of ~35 km (Figs. 1 and 10) (Costa et
al. 2000, Costa et al. 1999, Kadinsky-Cade et
al. 1985, Ramos et al. 2002, Smalley et al.
1993). Active, strain-accumulating faults
that continue at moderate dips to depths of
~35 km are permissible from the perspecti-
ve of the GPS velocity field given that the
fact the data are consistent with a range of
model fault dips (Brooks et al. 2003).
Other thrust sheets in the eastern Precor-
dillera have similarities with the Villicum
thrust sheet. Cerro Salinas is an isolated
outcrop of Cambro-Ordovician carbonates
unconformably overlain by Neogene red
beds (Fig. 1) (Comínguez and Ramos 1991,
Vergés et al. 2002). An east-dipping thrust
fault marks the western boundary of the
thrust sheet and both long-term and active
folding are concentrated on the eastern,
down-dip end of the thrust sheet (Fig. 13).
A monoclinal geometry marks the style of
folding and the geometry of growth strata
in the red beds is consistent with fold
growth due to kink band migration. The
basement, be-low the basal décollement is
structurally higher in the footwall on the
west than it is beneath the undeformed
foreland on the east (Fig. 13). Cerro Salinas
is apparently underlain and folded by an
antiform that involves the basement.
Moreover, active deformation is concentra-
ted in the east, which is the forelimb of the
fold, rather than on the west along the sur-
face trace of the thin-skinned thrust sheet.
Thus, the model for basement faulting pro-
posed for Sierra de Villicum (Fig. 11) may
also apply to Cerro Salinas (Fig. 13), which
suggests that the structural style of active
deformation in the boundary region betwe-
en the thin- and thick-skinned Precordillera
and Sierras Pampeanas provinces involves
active basement-involved thrusting that is
overprinting earlier-formed thin-skinned
thrust sheets. The onset of this transition is
uncertain, but has implications for seismic
sources for large cities in western Argentina
that lie in the foothills of the Andes.
CONCLUSIONS
1. The principal structure in the eastern
Precordillera northwest of San Juan, Ar-
gentina is a southeast dipping thrust sheet
beneath Sierra de Villicum, which is bound
at the base on the northwest by the
Villicum reverse fault along the range front
and on the southeast by the Tulum Valley
syncline. Cambro-Ordovician carbonates
unconformably overlain by Neogene red
beds comprise the bedrock of the thrust
sheet.
2. A suite of 5 fluvial terraces beveled into
Neogene red bed bedrock provide markers
that allow the late Quaternary deformation
in the eastern Precordillera at ~32° to be
characterized.
Figure 13: Interpretation of seismic line 31017 from Repsol-YPF (from Vergés et al. 2002). The Las Peñas thrust marks the leading edge of the
Precordillera thin-skinned thrust belt and is active (Costa et al. 2000). Note that Cerro Salinas has an east-facing monoclinal geometry in the cen-
ter of the thrust sheet. The basement is shallower on the west than it is on the east. Growth strata document the long-term growth of the fold.
602 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A. RAMOS
3. The Villicum thrust system along the
northwestern range front of Sierra de Vi-
llicum juxtaposes thrust imbricates of fol-
ded Cambro-Ordovician carbonates against
Neogene red beds. All the field evidence
indicates that (A) none of the imbricate
thrusts extends southwest of the Paleozoic
rocks holding up Sierra de Villicum, (B) the
terraces overlie faults of the thrust system,
and (3) no fault scarps are preserved in te-
rraces of any age.
4. Active deformation in the eastern Pre-
cordillera at 32° is localized on the southe-
astern flank of Sierra de Villicum, in the
middle of the Villicum thrust sheet.
5. A number of active small displacement
active faults cut fluvial terraces. A narrow,
well-defined region on the southeast flank
of Sierra de Villicum contains evidence of
folding, which includes the La Laja fault
that ruptured in the 1944 San Juan earth-
quake.
6. Evidence for active folding includes fle-
xural slip faulting on the La Laja fault,
which is a bedding parallel fault in the
Neogene bedrock that cuts terraces of all
ages. The oldest three terraces (T1-3) are
folded and the fold geometry is characteri-
zed by monoclinal form. Individual terra-
ces are folded about hinges that are unique
to that terrace and affect neither older nor
younger terraces or bedrock. Hinges in
younger terraces are located to the southe-
ast with respect to hinges in older terraces.
7. A kink band model of fold growth satis-
fies all of the key field observations at the
structural level of the bedrock and the
terraces. From this model the shortening
since abandonment of the oldest terrace,
T1, can be estimated.
8. Correlation with dated terraces roughly
15 km to the southwest (18.7-18.0 ka. 6.8-
5.8 ka, and 1.9-1.1 ka) (Siame et al. 2002)
allows for older and younger age assign-
ments for terraces T1-3 of this study. Ma-
ximum shortening rate across the eastern
Precordillera is ~3 mmyr-1, as determined
from the integration of the fold model and
terrace correlation. These rates are consis-
tent with short-term rates determined from
GPS (Brooks et al. 2003) and with long-
term rates determined via cross-sections
and growth strata to the north of 31° S
(Zapata and Allmendinger 1996a).
9. The loci of active folding, dip data and
bedrock contacts suggest that the Sierra de
Villicum thrust sheet is being actively fol-
ded by a deeper structure. A crustal model
is proposed to reconcile the pattern of fol-
ding of the thrust sheet, a northwest-dip-
ping zone of microseismicity, and a structu-
ral high in the basement between the
Precordillera on the west and eastern
Precordillera on the east. In this model,
active deformation of the eastern Precor-
dillera is a function of folding of upper
crustal thin-skinned structures due to slip
on a northwest-dipping blind thrust fault
that extends to ~35 km depth.
10. Near surface structures record active
folding and coseismic faulting during the
1944 earthquake, which implies that the
basement reverse fault is the source of the
1944 San Juan earthquake.
11. Basement rocks beneath Cerro Salinas,
another eastern Precordillera thrust sheet
to the southwest, are also characterized by
an antiformal geometry, which suggests
that blind thrust faulting on east-vergent
basement faults represents a significant,
underappreciated seismic hazard in western
Argentina.
ACKNOWLEDGMENTS
Drs. Ernesto Cristallini and Matias Ghiglione
provided careful and thoughtful reviews
that improved the quality of this work.
Carlos Costa, Daniel Ragona, Robert Yeats,
and Emily Schultz are thanked for nume-
rous discussions of this paper and the seis-
motectonics of western Argentina. AJM
was supported by U.S NSF grants EAR-
0232603and EAR-0409443 and a Research
Equipment Reserve Fund grant from
Oregon State University. We thank Repsol-
YPF S.A. for providing us with the seismic
line and for the permission to publish it
(Fig. 13). J.V. was partially supported by
99AR0010 CSIC-CONICET project and
Grups de Recerca Consolidats (II Pla de
Recerca de Catalunya) Projects 1997 SGR
00020.
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Recibido: 30 de junio, 2006
Aceptado: 15 de noviembre, 2006
