Why the Sacramento Delta area differs from other parts of the great valley: Numerical modeling of thermal structure and thermal subsidence of forearc basins

Abstract

Data on present-day heat flow, subsidence history, and paleotemperature for the Sacramento Delta region, California, have
been employed to constrain a numerical model of tectonic subsidence and thermal evolution of forearc basins. The model assumes
an oceanic basement with an initial thermal profile dependent on its age subjected to refrigeration caused by a subducting
slab. Subsidence in the Sacramento Delta region appears to be close to that expected for a forearc basin underlain by normal
oceanic lithosphere of age 150 Ma, demonstrating that effects from both the initial thermal profile and the subduction process
are necessary and sufficient. Subsidence at the eastern and northern borders of the Sacramento Valley is considerably less,
approximating subsidence expected from the dynamics of the subduction zone alone. These results, together with other geophysical
data, show that Sacramento Delta lithosphere, being thinner and having undergone deeper subsidence, must differ from lithosphere
of the transitional type under other parts of the Sacramento Valley. Thermal modeling allows evaluation of the rheological
properties of the lithosphere. Strength diagrams based on our thermal model show that, even under relatively slow deformation
(10−17 s−1), the upper part of the delta crystalline crust (down to 20–22 km) can fail in brittle fashion, which is in agreement with
deeper earthquake occurrence. Hypocentral depths of earthquakes under the Sacramento Delta region extend to nearly 20 km,
whereas, in the Coast Ranges to the west, depths are typically less than 12–15 km. The greater width of the seismogenic zone
in this area raises the possibility that, for fault segments of comparable length, earthquakes of somewhat greater magnitude
might occur than in the Coast Ranges to the west.

Full text

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ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2007, Vol. 43, No. 1, pp. 75–90. © Pleiades Publishing, Ltd., 2007.

1

1. INTRODUCTION
While many excellent studies have been devoted to
thermal modeling of subduction zones (e.g., [McKen-
zie, 1969, 1970; Hsui and Toksöz, 1979; van der Beukel
and Wortel, 1988; Honda, 1985; Dumitru, 1991; Spen-
cer, 1994; Davis, 1999; Devaux et al., 2000]), we are
unaware of any attempt to make quantitative compari-
sons of forearc basin subsidence with numerically pre-
dicted values. A possible explanation is that modeling
of forearc basin subsidence calls for the solution of a
number of different problems. First, the thermal struc-
ture of forearc lithosphere is strongly influenced by the
subduction process and depends on the rate and angle
of subduction and the age of the approaching slab
[Dumitru, 1991; Bohannon and Parsons, 1995]. In the
subduction zone to the west of the Great Valley, Cali-
fornia, these parameters have changed considerably
during the last 150 Myr [Engebretson et al., 1985]. Sec-
ond, forearc basins develop on top of an oceanic litho-
sphere, which continues subsiding in the course of its

1

The text was submitted by the authors in English.

cooling. Thus, an initial temperature should be assigned
based on the nonsteady-state temperature profile of an
oceanic lithosphere of corresponding age. Third, ther-
mal subsidence usually forms the long-term back-
ground for a number of short-term events, such as
regional compression, arrival of terrains, etc. Thus, sub-
sidence curves of forearc basins contain at least three
components: (1) subsidence or uplift as a result of the
dynamics of the subduction zone, including temporal
variations in subduction parameters; (2) thermal sub-
sidence of oceanic lithosphere, as the temperature dis-
tribution is in a nonsteady state at the beginning of sub-
duction; and (3) local tectonic events. Close to trenches,
subduction erosion (see, e.g., [Scholl et al., 1980]) and
variation in coupling along plate interfaces could also
be important.
In this paper, we consider a geodynamic model of
thermal evolution and tectonic subsidence of a forearc
basin and calibrate it using data from three oil and gas
exploratory wells in the Sacramento Valley and one
measured section across the Diablo Range to the south-
east of the valley. Model predictions compare favorably

Why the Sacramento Delta Area Differs from Other Parts
of the Great Valley: Numerical Modeling of Thermal Structure
and Thermal Subsidence of Forearc Basins
V. O. Mikhailov, T. Parsons, R. W. Simpson
a
Schmidt Institute of Physics of the Earth, Russian Academy of Sciences,
Bol’shaya Gruzinskaya ul. 10, Moscow, 123995 Russia
b
US Geological Survey, Menlo Park, CA USA
Received June 8, 2006

1




a


b


b

, E. P. Timoshkina

a

, and C. Williams

b






Abstract
region, California, have been employed to constrain a numerical model of tectonic subsidence and thermal evo-
lution of forearc basins. The model assumes an oceanic basement with an initial thermal profile dependent on
its age subjected to refrigeration caused by a subducting slab. Subsidence in the Sacramento Delta region
appears to be close to that expected for a forearc basin underlain by normal oceanic lithosphere of age 150 Ma,
demonstrating that effects from both the initial thermal profile and the subduction process are necessary and
sufficient. Subsidence at the eastern and northern borders of the Sacramento Valley is considerably less, approx-
imating subsidence expected from the dynamics of the subduction zone alone. These results, together with other
geophysical data, show that Sacramento Delta lithosphere, being thinner and having undergone deeper subsid-
ence, must differ from lithosphere of the transitional type under other parts of the Sacramento Valley. Thermal
modeling allows evaluation of the rheological properties of the lithosphere. Strength diagrams based on our
thermal model show that, even under relatively slow deformation (10
talline crust (down to 20–22 km) can fail in brittle fashion, which is in agreement with deeper earthquake occur-
rence. Hypocentral depths of earthquakes under the Sacramento Delta region extend to nearly 20 km, whereas,
in the Coast Ranges to the west, depths are typically less than 12–15 km. The greater width of the seismogenic
zone in this area raises the possibility that, for fault segments of comparable length, earthquakes of somewhat
greater magnitude might occur than in the Coast Ranges to the west.
PACS numbers:
91.45.Hc
DOI:
10.1134/S1069351307010089

—Data on present-day heat flow, subsidence history, and paleotemperature for the Sacramento Delta

–17



s

–1

), the upper part of the delta crys-

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MIKHAILOV et al.

0 100 km
122°120°
36°
38°
120° 122°
40°
38°
36°

PACIFIC
OCEAN

San Francisco
Coast Ranges
Sacramento
Valley
# Sacramento
Stockton
arch

1
2
3
4

Coast Ranges
San Joaquin Valley

Earthquakes
deeper than
20 km
Region of deep
quakes under
Sacramento
Delta
Boundary of
Cenozoic
sediments in
Great Valley
Major faults
Heat flow observations
<60 mW/m

2

>60 mW/m

2

Wells in study
Outcrop section

Fig. 1.
difference method [Ellsworth et al., 2000], with depths greater than 20 km. The deep cluster of earthquakes under the Sacramento
Delta is surrounded by the dashed line. Deep earthquakes in the northern part of the Sacramento Valley are related to the subducting
Juan de Fuca Plate (e.g., [Benz et al., 1992]). The locations of the three logged wells (
(
4
) used to define subsidence curves are also indicated. Heat flow values are indicated by circles filled with black or gray. Heat flow
values in the delta region are new preliminary values.

Map of the Great Valley of California and surrounding areas. Small black dots are earthquakes, relocated using the double-

1, 2, 3

) and the Mt. Diablo sedimentary section



with available data on the thermal and subsidence his-
tory of the Sacramento Valley as well as paleotempera-
ture observations. Last, we use the temperature profile
predicted by the model to estimate the present-day
strength of the lithosphere under the Sacramento Delta
region in order to better understand the origin of deep
earthquakes in this region. Indeed, an anomalous clus-
ter of earthquakes under the Sacramento Delta region

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WHY THE SACRAMENTO DELTA AREA DIFFERS FROM OTHER PARTS 77

extends to a depth of ~20 km (Fig. 1), whereas hypo-
central depths in the Coast Ranges to the west are typi-
cally shallower than 12–15 km [Wong et al., 1988;
Oppenheimer and Macgregor-Scott, 1992]. The unusu-
ally deep (~20 km) seismicity in this region may be a
manifestation of higher strength of the lower crust in
comparison to other parts of the Sacramento Valley.
This can result from a different thermal state and/or
from a different composition of crustal rocks.
First, we briefly review the data on the structure and
evolution of the Great Valley, in particular, for its north-
ern part, the Sacramento basin, and present tectonic
subsidence curves at the locations of four sedimentary
sections. Next, we combine two thermal models: one
quantifying the thermal subsidence of a forearc basin,
taking into account changes in subduction rate and age
of the approaching plate, and the other quantifying the
thermal subsidence of cooling oceanic lithosphere, tak-
ing into account the blanketing effect of sediments and
the crystallization of basalt melt. Last, we compare our
thermal model to available geothermal data for the Sac-
ramento Delta region, including present-day heat flow
and estimates of paleotemperature, and calculate
strength diagrams based on the inferred temperature
distribution in order to explain the anomalous deep
cluster of earthquakes in this area.
2. ORIGIN, STRUCTURE, AND EVOLUTION
OF THE GREAT VALLEY
2.1. Data on the Origin and Evolution
of the Great Valley
The structure of California’s Great Valley has been
studied by various methods, including seismic refrac-
tion and reflection studies, numerous logged boreholes,
and gravity and magnetic surveys. Many events in its
development, revealed in the variation in thickness and
facies composition of sedimentary layers, timing of
volcanism, and phases of folding and faulting, correlate
with changes in the rate and direction of subduction to
the west (e.g., [Dickinson and Snyder, 1979; Page and
Engebretson, 1984; Moxon and Graham, 1987]).
The Great Valley of California is a Mesozoic–Ceno-
zoic forearc basin formed to the west of the Sierra
Nevada volcanic arc. Sedimentary fill of the valley
forms an elongate prism that thickens rapidly to the
west, where the basement depth probably exceeds
14 km [Wentworth et al., 1995]. On the west side, a
thick Upper Jurassic to Upper Cretaceous (Campanian
to Maastrichtian) turbidite sequence is exposed in the
eastern Coast Ranges deposited upon Upper Jurassic
(153–165 Ma) ophiolites [Hopson et al., 1981]. In the
eastern part of the valley, numerous oil and gas explor-
atory wells have never found strata older than Late Cre-
taceous. Seismic data fail to provide strong constraints
on the eastern limit of the Upper Jurassic–Lower Creta-
ceous layers; probably this limit coincides with a pro-
nounced magnetic anomaly stretching along the central


part of the Great Valley [Beyer, 1988]. The E–W-trend-
ing Stockton arch separates the valley into two basins:
the Sacramento basin to the north and the San Joaquin
basin to the south (Fig. 1).
A forearc basin became established in the region by
the Late Jurassic (e.g., [Hey et al., 1988]). Dickinson
et al. [1987] suggested that the Nevadan orogeny and
establishment of the Great Valley as a forearc sedimen-
tary basin was the result of a mid-Jurassic collision with
a migratory oceanic arc. From the Tithonian through
the Maastrichtian, the Great Valley was a deep-marine
basin, bounded on the west by the subduction zone now
represented by rocks of the Franciscan complex. Pale-
ocurrent indicators in outcrops of Upper Jurassic and
Lower Cretaceous turbidites on the west side of the val-
ley show evidence of flow to the south parallel to the
continental margin, suggesting that by this time the
Franciscan accretionary wedge had achieved sufficient
bathymetric relief to maintain the Great Valley as a
starved deep-marine basin [Ingersol, 1979].
Near the end of the Cretaceous, the character of the
deposystems changed significantly. In the Maastrich-
tian, the Sacramento basin fan facies were replaced by
periodically prograding deltaic deposits [Graham,
1987]. This shoaling may simply reflect infilling of the
bathymetric relief, or, alternatively, it could be the
result of a proposed Late Cretaceous and Early Tertiary
decrease in the angle of subduction [Dickinson et al.,
1987]. It is worth noting that a change in the subduction
angle inferred from a shift of volcanism is one of sev-
eral explanations [Bohannon and Parsons, 1995].
Emplacement of the granitic terrain of the Salinian
block along the proto-San Andreas fault occurred in the
Late Paleocene, at approximately 55 Ma [Graham,
1987]. This event forced a major reorganization of the
Great Valley basin, in particular, its separation into the
Sacramento and San Joaquin subbasins by the trans-
verse Stockton arch. Another explanation of the Late
Cretaceous–Early Tertiary events is oblique subduction
with a northerly to northeasterly convergence direction
[Engebretson et al., 1985]. During the Paleogene, the
Sacramento basin was a shelved forearc basin subjected
to episodic transgressions and regressions. At the end of
the Paleogene, a depositional hiatus related to wide-
spread regression extended through the Great Valley
basins. Neogene to Recent sedimentation in the Sacra-
mento Valley was entirely nonmarine except in parts of
the San Francisco Bay area [Beyer, 1988].
Many tectonic events in the period of the last 30 Myr
correlate well with the interaction between the East
Pacific Rise spreading center and North America. This
interaction resulted in the propagation of the San
Andreas transform fault system and the northwestward
migration of the Mendocino triple junction. (For
detailed consideration of this period, see [Beyer and
Bartow, 1987; Beyer, 1988; Bohannon and Parsons,
1995].) The very complicated history of the last 30 Myr
is not under discussion here because, in a first-order

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consideration of the thermal structure of the Great Val-
ley, one can neglect effects that took place relatively far
to the west in the region of the San Andreas fault (this
is especially true for the Sacramento basin, where these
events occurred considerably later). Results of Furlong
[1984] support this assumption: when modeling the
thermal effect of the northward-migrating Mendocino
triple junction, he did not find an increase in calculated
heat flow in the Great Valley area.

2.2. Crustal Structure and Seismicity
The crustal structure of the southern part of the Sac-
ramento basin and Sacramento Delta depocenter has
been studied by a seismic refraction profile [Eaton,
1963] running from San Francisco through the Sacra-
mento Delta to the Sierra Nevada and by the BASIX
seismic survey (e.g., [Hole et al., 2000]). Eaton [1963]
estimated the Moho depth in the Sacramento basin to be
as shallow as 20 km. Prodehl [1979] reanalyzed this
profile and estimated the Moho depth as 22–23 km.
BASIX data overlap only the western edge of the Sac-
ramento Delta region, suggesting the same range for the
crustal thickness, and more than 12 km of low-velocity
sediments in the western part of the Sacramento Delta.
Even though the crustal structure of the deepest parts of
the Sacramento basin remains unclear, it is obvious that
it differs considerably from the other parts of the Great
Valley. Indeed, the thickness of the sediments in the
Sacramento Delta area exceeds 14 km [Wentworth
et al., 1995] or even 16–18 km [Beyer, 1988], whereas
the depth to the Moho is estimated as 22–23 km. Thus,
the crystalline crust in this area ranges in thickness
from 5 to 10 km, which is similar to the average thick-
ness of most oceanic crust. Several seismic profiles,
characterizing mainly the San Joaquin basin and the
northern part of the Sacramento basin, yield a thickness
of the crystalline crust of 21–26 km (e.g., [Holbrook
and Mooney, 1987; Wentworth et al., 1987; Mooney
and Weaver, 1989]). This thickness could correspond
either to oceanic crust stacked on top of Sierran volca-
nic-arc crust [Wentworth et al., 1987; Godfrey et al.,
1997], to thickened sheared/intruded crust [Page and
Brocher, 1993; Holbrook et al., 1996], or to Great Val-
ley ophiolite crust [Jachens et al., 1995].
The Sacramento Valley appears to have undergone a
moderate level of crustal deformation, at least in the
Quaternary [Wong, 1987]. Figure 1 shows the distribu-
tion of double-difference relocated earthquakes deeper
than 20 km in a part of Northern California [Ellsworth
et al., 2000] and outlines the region of deeper events in
the Sacramento Delta area. Historic earthquakes with
magnitudes of 6–6.5 have occurred along the western
margin of the Great Valley to the north and south of this
cluster, and an
M

≤
6.0 event in 1889 may have
occurred within the cluster [Bakun, 1999]. Earthquakes
in the cluster have been attributed to activity on the
near-vertical Pittsburg/Kirby Hills and Kirker faults
[McCarthy et al., 1994], although complex nonvertical





structures, including blind thrusts, tectonic wedges, and
mid-crustal decollements, have been identified along
the eastern margin of the Sacramento Valley (e.g.,
[Weber-Band et al., 1997; Unruh and Lettis, 1998;
O’Connell et al., 2001]), some of which may separate
near-surface structures from the deep seismogenic
faults within the basement. Hypocenters from the
Northern California Seismic Net relocated using dou-
ble-difference techniques [Ellsworth et al., 2000] do
not define any single, clear, through-going structure in
this cluster, although several local planes of limited
extent, both vertical and dipping, are defined by align-
ments of hypocenters. Simultaneous inversion of earth-
quake and seismic refraction traveltimes yielded depths
for earthquakes in the cluster that were shallower than
double-difference depths (mostly less than 20 km, com-
pared with 23 km) [Hole et al., 2000, Plates 1, 2],
although the cluster still stands out by virtue of its
greater depth extent.

2.3. Heat Flow and Paleotemperature
Heat flow in the Great Valley is lower in the western
part than in the central and eastern parts, but every-
where is lower than in the Coastal Ranges [Sass et al.,
1971]. Published heat flow measurements [Sass et al.,
1971, 1982, 1997; Lachenbruch and Sass, 1980; Wang
and Monroe, 1982; Williams et al., 1994] are shown in
Fig. 1, along with preliminary values from new mea-
surements in the Sacramento Delta region. The impor-
tant feature of heat flow in the region is a rapid decrease
in values from the Coast Ranges to the Great Valley.
Heat flow in the Great Valley is typically less than
45 mW/m
and averages 30–35 mW/m
Using corrected well log temperatures from more
than 3000 wells, Lico and Kharaka [1983] concluded
that the temperature gradient in Sacramento basin sed-
imentary rocks ranges from 18 to 25
different geochemical data, Zieglar and Spotts [1978]
estimated thermal gradients in the Great Valley sedi-
ments for the period from the Cretaceous to the present
to be in the range 22–35
°
C/km. Dumitru [1989] arrived
at considerably lower thermal gradients (9
Tertiary and 15
°
C/km in the Cretaceous) using results
of fission track analysis. He may have overestimated
burial depth by (1) assuming that the thickness of the
Upper Jurassic–Lower Cretaceous layers in the eastern
part of the Coast Ranges was not increased tectonically
by later deformations (cf. [Wentworth and Zoback,
1989]) and (2) increasing burial depth by adding the
Upper Cretaceous–Lower Paleocene deposits, which
are now present only in the eastern part of the Great
Valley.


2


2

.

°

C/km. Based on



°

C/km in the


3. SUBSIDENCE HISTORY
Different types of lithosphere should possess differ-
ent thermal structures and, as a consequence, should
exhibit different long-term subsidence histories. Thus,

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WHY THE SACRAMENTO DELTA AREA DIFFERS FROM OTHER PARTS 79

different thermal models of the deep basin structures
can be tested against available lithologic and seismic
data on the structure and subsidence history of the
Great Valley sedimentary basin. To investigate the sub-
sidence history of the Great Valley, we compiled bore-
hole data collected by Moxon [1990] with data from a
USGS borehole database [Brabb et al., 2001]. We used
eustatic sea level curves of Haq et al. [1987], correcting
for both long-term and short-term sea level changes and
porosity–depth dependences suggested by Stam et al.
[1987]. Williams [1997] tested the sensitivity of Great
Valley subsidence curves to errors in the input data and
demonstrated a low sensitivity to all input parameters
except paleobathymetry estimates. Our resulting sub-
sidence curves differ in certain details from those of
Moxon [1990], although the main features are the
same.
Below, we consider four water-unloaded (i.e., with-
out the load of water filling depressions up to sea level)
tectonic subsidence curves corresponding to three wells
and one outcrop site situated in different parts of the
Sacramento basin (Fig. 2; for locations, see Fig. 1).
Curve
1
is from Standard’s Dodge Land #1 well; curve
from Chevron’s Peter Cook #13 well; curve
Arco’s Mantelly #1 well; and curve
Diablo outcrop section. Details can be found in
[Moxon, 1990; Brabb et al., 2001].
Subsidence curve
1
characterizes the northern
edge of the basin, curves
2
ence of the Sacramento Delta region, and curve
characterizes the southeastern edge close to the
Stockton arch. The shaded periods in Fig. 2 corre-
spond to the onset (75–80 Ma) and cessation (40–
45 Ma) of the Laramide orogeny [Moxon and Gra-
ham, 1987].
We restricted our analysis to the interval from the
Late Cretaceous through the Neogene, thus ignoring
earlier periods represented by turbidites, because the
depth of turbidite formation is poorly constrained. For
example, Dickinson et al. [1987] estimate its upper and
lower limits as 1500 and 3000 m. This interval is too
broad to constrain any subsidence scenario (uncertainty
limits are given in Fig. 2 by dashed lines). In addition,
it is unclear whether the thicknesses of Upper Jurassic–
Lower Cretaceous strata measured across the Diablo
Range outcrops represent the real vertical depositional
thickness or a composite section resulting from en ech-
elon stacking of deposits of different ages and possibly
from different depths (e.g., [Moxon, 1990]). Many sec-
tions measured across the outcrops indicate a total com-
posite stratigraphic thickness of Lower Jurassic–Upper
Cretaceous strata in excess of 10–15 km, whereas
nearby boreholes penetrating to the basement traverse
less than 4–6 km of sedimentary rocks [Williams,
1997]. As a result, uncertainty limits for the period
before 65–70 Ma (see, e.g., Fig. 2, curves
enough to accommodate either a horizontal line, corre-
sponding to passive sedimentary infilling of an initial



2

,

3

, from

4

; from a Mt.




and

4

describe the subsid-

3


3

,

4

) are wide
topography [Graham, 1987; Williams, 1997], or a
descending curve, corresponding to tectonic control of
subsidence [Moxon and Graham, 1987].
Let us consider the main features of Great Valley
subsidence revealed by the subsidence curves. It is
worth noting that the subsidence curves show the aver-
age subsidence over the deposition interval of each
stratigraphic section. Because stratigraphic subdivi-
sions in the wells are different (Fig. 2, cf. curves
with curves
2
,
4
), some events registered by one curve
can be smoothed and not seen on the others. Subsidence
curves
2
and
4
for the Sacramento depocenter area are
similar for the last 80 Myr. They display a number of
short-term tectonic events taking place on the back-
ground of a total long-term subsidence that started
before 80 Ma. In particular, the uplift in curve
between 80 and 60 Ma can be attributed to an uplift
of the western edge of the Sacramento basin that
started in the Late Cretaceous [Zieglar and Spotts,
1978; Dickinson et al., 1987]. This uplift was fol-
lowed by several relatively fast subsidence events
that occurred during the Laramide orogeny. One of
these events occurred about 55 Ma and coincides
with the arrival of the Salinian block followed by a
relatively deep and long subsidence period between
50 and 42 Ma.

1

,

3










4

We are particularly interested in the long-term com-
ponent of tectonic subsidence, which is likely linked to
the temporal and spatial thermal changes within the
lithosphere and underlying mantle. To estimate the
influence of the various factors that might determine the
long-term subsidence of forearc lithosphere, we con-
structed a numerical thermal model, which is described
in the next section.
4. THERMAL MODEL OF FOREARC
LITHOSPHERE

4.1. Statement of the Problem

The temperature and subsidence of forearc basins
are controlled by changes in rate and age (thickness,
temperature profile) of the subducting plate; by non-
steady-state temperature in the overriding plate at the
beginning of subduction; and by the blanketing effect
of sediments, which can exceed 15 km in forearc
basins. In our modeling, we used thermal conduc-
tive/advective equations with a preassigned velocity
field. The heat transfer equation is linear, so, by choos-
ing the boundary conditions properly, the problem can
be subdivided into two parts: thermal subsidence of the
forearc lithosphere refrigerated by the subducting plate
(taking into account changes over time in the rate and
age of the approaching plate) and thermal age-depen-
dent subsidence of the upper plate. Let us now consider
both thermal problems.