Download a slab window vs. stalled slab

Earth and Planetary Science Letters 186 (2001) 175^186
www.elsevier.com/locate/epsl
Three-dimensional thermal modeling of the California
upper mantle: a slab window vs. stalled slab
J.W. van Wijk a; *, R. Govers b , K.P. Furlong c
a
c
Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
b
Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands
Department of Geosciences, Deike Building, The Pennsylvania State University, University Park, PA 16802 USA
Received 28 August 2000; received in revised form 9 January 2001; accepted 11 January 2001
Abstract
In order to gain a better understanding of the behavior of microplates after their subduction, we studied two endmember scenarios for the post-subduction history of two offshore California microplates. In the first scenario,
Monterey and Arguello microplate remnants are present today below the North America Plate, while in the second
scenario subducted microplate remnants are absent. 3-D numerical modeling of the thermal evolution implied by these
scenarios results in two different present-day thermal structures of the central and southern California upper mantle. By
comparing the model-predicted surface heat flow values and seismic velocities to heat flow data and tomography, we
find that we cannot discriminate between the two scenarios as they both are consistent with the data. This result means
that the present-day upper mantle temperature field is relatively insensitive to the assumed microplate scenarios. A
slabless window is not needed for the generation of partial melt either, which is consistent with earlier 2-D studies for
this region. ß 2001 Elsevier Science B.V. All rights reserved.
Keywords: microplates; California; 3-D numerical model; thermal history; Mendocino fracture zone; triple junction
1. Introduction
The convergence of a spreading ridge with a
subduction zone has implications for both the
overriding plate and the subducting plate, as
well as for their joint boundary. When the Farallon^Paci¢c spreading ridge approached the subduction zone of western North America in the
* Corresponding author. Fax: +31-20-6462457;
E-mail: [email protected]
Oligocene, the intervening Farallon Plate fragmented into several smaller microplates (e.g.,
[1,2]), while the western part of the North America Plate deformed (e.g., [3^6]). The convergent
plate boundary changed into a transform regime.
The magnetic record preserved o¡shore western
North America has provided the input for numerous plate reconstruction studies of this area (e.g.,
[1,2,7^9]). The geometrical considerations resulting from these plate reconstructions, and the observations of higher heat £ow values [10] and
Cenozoic volcanism [11^13], led to the idea of
the formation of slabless windows, both in the
wake of the migrating Mendocino Triple Junction
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 2 4 3 - 6
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J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186
and to the south, with reference to the origin and
cessation of subduction of the Monterey and Arguello microplates [9,11,14^17]. When such a slabless window opens, the asthenosphere is assumed
to rise and ¢ll the space formerly occupied by the
slab.
More recently, it has been suggested [18,19]
that the geological observations of high heat
£ow and volcanism can also be explained by thermal re-equilibration of the stalled subducted microplates along the coast of central California.
Subduction of very young oceanic microplates
can lead to partial melting of the slab [19]. Seismic
sections across central and northern California
indicate the presence beneath extended parts of
the margin of a slab of partially subducted oceanic lithosphere [18,20^23], while in some regions
the presence of subducted oceanic lithosphere is
not demonstrated (e.g., the San Francisco Bay
area).
In this paper we focus on the tectonic history of
the Monterey and Arguello microplates in central
and southern California. The temporal existence,
precise location, and dimensions of the contingent
slabless windows in the Monterey^Arguello area
is rather uncertain [9,16]. A 3-D ¢nite di¡erence
model is constructed in which the lithospheric
plates can move with respect to one another,
and the thermal conduction^advection equation
is solved. With this thermal kinematic model we
test whether substantial thermal di¡erences can be
expected today as a consequence of the two endmember scenarios that are modeled : the slab window scenario and the stalled slab scenario. Here
we focus in particular on the question whether a
slabless window is needed to explain the observed
high heat £ow values and magmatism. Along
coastal California, incorporating the third dimension in the modeling can be decisive, because in
this time-dependent simulation the relative plate
motion between the North America Plate and the
Paci¢c Plate is considerable. With this 3-D model
we will test whether the conclusions drawn by,
e.g., ten Brink et al. [19] concerning the microplates can be con¢rmed. Furthermore, synthetic
tomographic images are calculated from the obtained thermal structures, and compared with Pwave velocities of this area.
2. Two scenarios from the o¡shore central and
southern California tectonic history
O¡shore central and southern California, the
remnants of several Miocene microplates are preserved [1,2,7] as well as remnants of paleosubduction zones [13] (Fig. 1A). Detailed studies of the
magnetic anomaly pattern of the northeast Paci¢c
[1,2,7,8] have revealed the gist of the plate tectonic
history of this area.
Cretaceous and early Cenozoic times are characterized by fast and continuous Paci¢c^Farallon
spreading [1]. This changes in the middle and late
Cenozoic, when the Paci¢c^Farallon spreading
center approaches the subduction zone of North
America. Probably as a consequence of this approach, the Farallon Plate starts to fragment, ¢rst
into two parts (the Vancouver Plate in the north
breaks o¡ the Farallon Plate at V55 Ma [24,25])
and from 30 Ma onwards, into smaller pieces.
At about 30 Ma, the Monterey and Arguello
microplates break away from the South Farallon
Plate [16] (Fig. 1B,C). Meanwhile, the plate fragment between the Pioneer and Mendocino Fracture Zones (Fig. 1C) continues to subduct. South
of the Murray Fracture Zone, the Farallon Plate
fragments further, resulting in the origin of the
Guadalupe Plate (V25 Ma [26]). Subduction continues, and the ¢rst Paci¢c^North America contact between the Mendocino and Pioneer Fracture
Zones is established about 28.5 Ma (Fig. 1B).
This transition from a subduction zone into a
transform plate boundary is frequently suggested
to be accompanied by the opening of a slabless
window.
The Monterey^Paci¢c ridge ceases spreading at
about 19.5 Ma, and the Arguello^Paci¢c ridge is
extinguished at about 17.5 Ma (Fig. 1B). After the
cessation of spreading, the subduction zone develops into a transform fault at these latitudes as
well. This conversion from a convergent plate
boundary to a transform plate boundary is concurrent with several tectonic events on (the
boundary of) the North America Plate, like rotation of the western Transverse Ranges and intraplate extension (e.g., [3,27]).
The observed magnetic anomalies no longer
give clues to the history of the microplates after
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J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186
177
Fig. 1. Schematic plate tectonic history of o¡shore California, at (A) the present day, (B) V20 Ma, (C) V30 Ma. B and C are
not to scale. (A) Present-day Paci¢c^North America Plate boundary. Remnants of the Monterey and Arguello microplates (e.g.,
paleo spreading ridges) are found o¡shore southern California. (B) A slabless window (dark gray, with very uncertain position
and edges) has opened after subduction of a ridge segment. The Monterey and Arguello microplates probably cease to subduct
at about this time. The light gray rectangle indicates the model domain of this study. (C) The Monterey and Arguello microplates break o¡ the Farallon Plate. The ridge segment between the Pioneer and Mendocino Fracture Zones continues to drift
east. After [1,7].
their spreading ridges had ceased. One of the uncertainties that still exist concerns the subducted
parts of the microplates. There are two end-member possibilities on what happened to these subducted parts: either they are still present and attached to the microplate^Paci¢c Plate, the socalled stalled slab scenario, or they are no longer
present, the so-called slab window scenario (Fig.
2A,B). These two scenarios are distinctly di¡erent,
and have di¡erent implications for the overriding
plate.
2.1. Stalled slab scenario
The stalled slab scenario is based on geological
observations, thermal and rheological modeling,
and re£ection and refraction seismics. In the
stalled slab scenario, the subducted parts of the
Monterey and Arguello microplates, together with
the unsubducted parts, become attached to the
Paci¢c Plate (i.e., plate capture) after cessation
of spreading (Fig. 2A). Nicholson et al. [3] proposed the plate capture hypothesis to explain rotation of the western Transverse Ranges. The
western Transverse Ranges block started to rotate
at about the same time as subduction of the Monterey microplate ceased. The same conclusion was
drawn by Bohannon and Parsons [5], who stated
that much of the Late Oligocene and Miocene
continental deformation was caused by the traction imposed on the deep part of the continental
crust by the subducted microplate lithosphere.
Bohannon and Parsons [5] showed by thermal
and rheological modeling that coastal California
was at this time a strong zone at all depths. The
partially subducted Monterey and Arguello microplates thus were allowed to be ¢rmly linked
to the Paci¢c Plate. Seismic sections [18,20^23]
furthermore show the presence of subducted microplates under extensive parts of the California
margin.
For the stalled slab model tested in this study
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Fig. 2. Sketch of stalled slab and slab window scenarios, situation of 19 Ma, and model con¢gurations. Nam = North America
Plate, PAC = Paci¢c Plate. Note the corresponding dashed rectangles of A^C and B^D. (A) Stalled slab scenario. Slab remains
attached. The kinematically required gap in the slab is at sub-lithospheric levels. (B) Slab window scenario. Slab is no longer
present. The space left behind is ¢lled with asthenospheric material. (C and D) The 3-D model con¢gurations as used in the
modeling, not to scale. See also Fig. 1B for comparison, and text for explanation. Front of the model domain (dashed rectangle)
corresponds to Murray Fracture Zone.
we follow the ideas of Nicholson et al. [3] and
Bohannon and Parsons [5] as closely as possible.
In this stalled slab scenario, the kinematically required gap [5] in the slab develops at sub-lithospheric levels. This implies that at lithospheric
levels, subducted material of the microplates has
been present over the past 19 Myr. As the adjacent Guadalupe Plate south of the Arguello Plate
continues to subduct until 12.5 Ma [1], the Arguello Plate and its subducted parts either must
have been torn o¡ the Guadalupe Plate, or their
connection (the Murray Fracture Zone) acted as
an active transform fault. As the plates in this
numerical model are rigid and cannot deform, it
is kinematically necessary to assume that the subducted parts of the microplates are not attached
to the overriding North America Plate, but only
to the Paci¢c Plate.
2.2. Slab window scenario
From geometrical considerations [9,11,17], heat
£ow data [10,12,13], and observed Miocene volcanism, another possible scenario for the subducted parts of the microplates follows: the slab
window scenario (Fig. 2B). The heat £ow in Cal-
ifornia is unusually high, with values between 50
and 90 mW/m2 [10]. Dickinson [13] reconstructed
the positions of mid-Tertiary, mid-Miocene, and
post-mid-Miocene volcanic ¢elds in coastal California. He related these pulses of volcanism to the
subduction of the Monterey and Arguello microplates and of the Vancouver^Farallon plates. According to this interpretation, upwelling of asthenospheric material following detachment of the
microplate slab caused the decompression melting.
From calculations and reconstructions by Severinghaus and Atwater [16] it follows that in
the Monterey^Arguello area, the last subducted
lithosphere before microplate subduction ceased
was both very young and very warm. Atwater
and Stock [9] concluded from a careful study of
the sea £oor isochron patterns that breaks must
have occurred within the subducting plates. Assuming that the previously subducted parts of
the Farallon Plate continued to subduct, gaps
had to appear somewhere in the downgoing slab
after the Monterey and Arguello microplates were
born. Atwater and Stock [9] supposed that the
break in the slab occurred about 50 km inboard
of the paleotrench. They furthermore assumed
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that the break took place just after the microplates were born. The exact latitudes and times
of the gap, however, remain uncertain.
For the slab window model tested in this study
we deviate from these ideas to some extent in the
timing and positions of the slab gap. This allows
us to maximize the thermal di¡erences between
the two tested hypotheses. In this slab window
scenario, there is no or hardly any subducted microplate slab present at lithospheric levels (Fig.
2B). It is assumed that detachment of the slab
takes place very close to the paleotrench, not
when the microplates are formed (28 Ma) but
later, when they cease to subduct (at V19 Ma).
A shallow slab window develops, and the already
subducted parts of the microplate continue to
subduct.
3. Modeling approach
These two hypotheses are tested with a numerical model. In order to implement the stalled slab
and slab window scenarios in a numerical procedure, further interpretation of these scenarios is
required.
3.1. Numerical procedure
Lithospheric temperatures are calculated by
solving the 3-D thermal conduction^advection
equation, using the code developed by Goes et
al. [28]:
dT
ˆ c WO_ ‡ k9 2 T ‡ b H
b cp
…1†
dt
in which T = temperature, t = time, k = thermal
conductivity, b = density, cp = heat capacity,
H = heat production per unit mass, c = stress tensor, O_ = strain rate tensor, and cWO_ = the frictional
heat, taken as zero in this study. This implies that
no frictional heat is supposed to be released at the
San Andreas transform fault, which is in agreement with the ideas of Lachenbruch and Sass [10].
A ¢nite di¡erence method [29^31] is used to solve
the conductive part of this equation numerically.
The advective part of the equation is taken into
179
account by moving each point in the grid at the
corresponding plate velocity at speci¢ed time
steps, and subsequently interpolating temperatures back to the grid on which the conductive
equation is solved.
3.2. Geometry
A schematic box representation of the initial
plate con¢guration of both scenarios is shown in
Fig. 2C,D. As we aim to derive ¢rst order constraints on the present-day thermal structure of
the central and southern California upper mantle,
only the most important aspects of the plate con¢guration, i.e., the large-scale features, are taken
into account in de¢ning the geometry of the models. The in£uence of the presence or absence of
the microplate slab on the temperature structure
is studied. Therefore, other lithospheric processes,
like the rotation of crustal blocks and intraplate
extension that have taken place since the conversion to the transform plate boundary, have been
left out of consideration here.
The dip of the plate boundary between the
North America Plate and the underlying slab window or slab is assumed to be 15³ based on the fact
that very young lithosphere is being subducted
[16]. Continental crustal thicknesses are in accordance with seismic data [18,23], thickening towards the east. The spreading ridge between the
microplates and the Paci¢c Plate drifts east, but
does not actually contact the subduction zone at
all latitudes; interpretations of the magnetic record show abandoned spreading ridges lying o¡shore the California coast [2] (Fig. 1A). The ridge
stalls just before the trench in our modeling. The
newly formed transform plate boundary between
the Paci¢c Plate and the North America Plate is
formed along the paleotrench in this study [3].
The distance between the ridge and the trench is
25 km in this simpli¢ed geometry.
3.3. Kinematics
The starting time of both model simulations is
19 Ma, when the shallow slab window in the slab
window scenario had just opened and subduction
and spreading had ceased (Fig. 2A,B). The frame
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Table 1
Material parameter values
Thermal expansion coe¤cient (oceanic) [1/K]
DensityUspeci¢c heat [J/m3 /K]
Thermal conductivity [W/m/K]
Continental surface heat production [W/m2 ]
Characteristic depth for exponential decay [km]
of reference is attached to the Paci¢c Plate, i.e.,
the microplates do not move after 19 Ma. Between 19 Ma and 12 Ma, the mean North America^Paci¢c plate velocity is about 10 mm/year [7].
From 12 Ma until the present, the present-day
relative velocity of 55.4 mm/year is assumed
[32]. More recent, small changes in velocity [33]
are not taken into account. In the stalled slab
model, the slab is attached to the Paci¢c Plate.
Velocities in the slab window assume Paci¢c values. It is supposed that the microplates have
moved with the Paci¢c Plate [4] since 19 Ma.
3.4. Initial and boundary conditions
When the slab window opens, it ¢lls instantaneously with upwelling asthenospheric material.
4.0U1035
3.5U106 (oceanic), 2.8U106 (continental)
3.1 (oceanic), 3.35 (continental)
2.65U1036
10
Here, small-scale convection is neglected. It is
not known whether small-scale convection could
occur within this time span and window geometry
[34,35]. The initial temperatures of the oceanic
lithosphere of both the Paci¢c and the microplates
are calculated using a boundary layer model with
a constant basal heat £ux [36]. The ages used in
this calculation are derived from the magnetic
anomaly pattern [1]. The initial slab window temperature in the slab window scenario is set at
1300³C. For the North America Plate, the initial
temperatures follow a geotherm with a heat production exponentially decaying with depth [37].
Parameters used are shown in Table 1. The initial
temperatures of the slab window, north of the
Pioneer Fracture Zone (Figs. 1B and Fig. 2C,D)
that had been cooling since 28.5 Ma, are calcu-
Fig. 3. Initial oceanic and continental geotherms for di¡erent positions in the model domain, and thermal boundary condition
for incoming North America Plate material. A^D are oceanic geotherms (dependent on the age of the lithosphere), examples
shown are for 2, 8.3, 8 and 11 Myr old oceanic lithosphere respectively. E^G are continental plus slab (G2) or slab window (G1)
geotherms. The depth at which the continental geotherm switches to the slab window (G1) or slab (G2) is dependent on the
thickness of the continent at that location. E is the boundary condition on the northern side of the domain. It consists of the
geotherm prescribed to the incoming North America Plate (shallow part) and the northern slab window (deeper part). F is the initial geotherm for the northern part of the domain; below the continent the continental geotherm switches to the geotherm of
the cooling northern slabless window.
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J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186
lated using the boundary layer model for cooling
lithosphere.
The boundary conditions include a zero heat
£ow through the oceanic part and a constant
( = mantle) temperature in the slab window part
at the north side (Mendocino Fracture Zone) of
the model domain. Through the east, west, and
south sides of the box, the heat £ow is zero. This
implies that no major variation in the temperature
structure beyond these sides of the model domain
is expected. For the top and bottom of the domain the temperatures are set to 0³C and 1300³C
respectively. As the temperature remains constant
(1300³C) over a large depth interval in the deepest
part of the domain during the model evolution, a
constant temperature assumption at the bottom
side of the box is justi¢ed. In this model the
North America Plate entering at the northern domain boundary is warmed by the slab window
that develops in the wake of the NE moving
Juan de Fuca Plate (see Fig. 1B, dark gray slabless window), although the existence of this slabless window has been questioned [18,19].
The thermal initial and boundary conditions
are summarized in Fig. 3. The northern 250 km
of the model domain serve as an extended boundary condition for the southern part, i.e., where the
microplates are present. In this way care is taken
that the North America Plate entering the interesting zone of the microplates has a geotherm that
is adjusted to the northern geological environment. We believe that this yields more realistic
`boundary conditions' for the southern part of
the domain.
4. Central and southern California thermal model
The thermal evolution implied by the slab window scenario is shown in Fig. 4. 19 Ma is the
starting point of the model simulation, and 0 Ma
is the present-day situation. The in£uence of
the third dimension is considerable. The eastern
(North America) part of the domain is in£uenced
by the heat input due to opening of the slab window in the wake of the northward moving Juan
the Fuca slab. This slab window warms the North
America Plate, which in turn moves southwards
181
with respect to the Paci¢c Plate. Upon entering
the southern part of the model domain, the southward migrating North America Plate has had
enough time to be in£uenced by the material below; the vertical heat di¡usion time scale is
slightly smaller than the horizontal advection
time scale.
The (young) oceanic lithosphere in the western
part of the domain shows overall cooling. In the
stalled slab evolution the same process of cooling
of the young oceanic lithosphere o¡shore takes
place, as well as an `onshore' area which is in£uenced by the North America Plate heat content.
Note that only the southern part of the model
domain is shown in Figs. 4, 5, and 7.
4.1. Present-day thermal models
In Fig. 5 the predicted present-day thermal
structures implied by the two scenarios are shown.
At shallow depths, the higher temperatures associated with the ancient spreading ridge are still
visible in the oceanic parts of the panels. Both
the slab window and stalled slab panels show a
cold anomaly in the region of the former slab
window or slab. These relatively cold thermal regions are fully explained by the kinematics during
the model evolution, i.e., relatively warmer North
America Plate material enters the model domain
in the north, see also the evolution in Fig. 4. Especially at greater depths, signi¢cant temperature
di¡erences between the slab window and the
stalled slab scenarios are visible, increasing to
up to V150³C at 100 km depth.
4.2. Comparison with observations
4.2.1. Heat £ow
The surface heat £ow provides a direct way to
study the thermal state of the Earth's crust and
mantle below. A synthetic surface heat £ow map
was calculated from the obtained present-day
temperature structures. The synthetic surface
heat £ow values from the stalled slab model are
shown together with the observed data [10] in Fig.
6. The heat £ow values obtained by the modeling
study are within the range of the observed values.
From Fig. 5 it follows that the thermal di¡er-
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J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186
Fig. 4. Thermal evolution of the slab window scenario, at a depth of 40 km. Temperature in ³C.
Fig. 5. Present-day temperature structures. Note that the
temperature scales change with depth.
Fig. 7. Synthetic tomography at a depth of 70 km. Temperatures were converted to P-wave velocities using the method
developed by Goes et al. [38]. The same parameters were
used as in Goes et al. [38]. NAm = North America Plate.
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Fig. 6. Observed surface heat £ow values [10] (dots) and surface heat £ow values resulting from the stalled slab model
(line). The Lachenbruch and Sass data are projected on a
line segment parallel to the coastline, the model results are
obtained from the same line segment in our model domain.
Note that the surface heat £ow data do not necessarily re£ect the thermal structure of the mantle that lies directly below today, as the plates have moved with respect to each
other since the early Miocene.
ences at shallow depths between the slab window
and the stalled slab scenarios are very small. This
is also re£ected in the surface heat £ow values of
both models: they are practically equal, which
means that no discrimination is possible between
the two scenarios based on the heat £ow values.
The average surface heat £ow values for the
stalled slab model and the slab window model
are 65 mW/m2 and 67 mW/m2 respectively. This
is lower than the average surface heat £ow values
measured by Lachenbruch and Sass [10] and Sass
et al. [38] (74 þ 4) for this area.
The large range in the observed heat £ow values clearly is not present in the results of the
modeling study. This can probably be explained
by (more recent) tectonic events that were not
incorporated in the modeling. Ten Brink et al.
[39] for example suggested that the high heat
£ow values at the Inner Continental Borderland
could be the result of extension in this area during
the early and middle Miocene. A slabless window
in the sense that is modeled in this study does not
give a better explanation for the observed high
heat £ow values than the stalled slab model.
4.2.2. Tomography
The thermal anomalies (relative to the average
183
temperature) were converted to P-wave velocities
according to Goes et al. [40]. The method developed by Goes et al. [40] takes into account the
decrease in seismic velocities when temperatures
are close to the melting temperature, and the effects of anelasticity. In this study the same parameters were used as in Goes et al. [40]. The velocity
anomalies thus obtained (shown in Fig. 7) vary
between 30.8 and 1.5%. The maximum expected
present-day di¡erence between the velocity
anomalies of the stalled slab and the slab window
scenarios is only 0.7%. This is not su¤cient to
express a preference for one of the two scenarios,
based on tomography.
The P-wave anomalies as obtained by Humphreys and Dueker [41] of the western USA are
considerably larger. Moreover, they show a pattern of slow and fast velocities that is not convincingly present in the synthetic results of this study;
although both models in Fig. 7 show a transition
from negative to positive velocity anomalies near
the present-day coastline, this transition is positioned closer to the coastline here than in Humphreys and Dueker's [41] observations, and weaker.
Furthermore, tomographic results (e.g., [42,43])
show the presence of an upper mantle high velocity anomaly below the Transverse Ranges. This
anomaly is thought to be caused by mantle
downwelling associated with oblique convergent
plate movements, where the San Andreas fault is
oriented more east^west. This bend in the San
Andreas fault was probably formed in the late
Miocene [44,45]. The high velocity anomaly there
thus has no relation to the presence of a slab in
the stalled slab model; it is the result of a more
recent tectonic event which is not included in our
model.
4.2.3. Partial melt
Miocene volcanic ¢elds are found in central
California [13] where the microplates were situated, although there is less volcanism here than
farther north. We used the empirical relations developed by McKenzie and Bickle [46] to determine whether partial melt can be expected in the
stalled slab or slab window model. The model
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results of 18 Ma are used to calculate the melt
fraction, as partial melt formation is expected to
occur at this time.
We ¢nd that both in the stalled slab model and
in the slab window model partial melting is possible. In the slab window model the generation of
partial melt is predicted at depths larger than 45
km near the coastline, and at depths larger than
70 km further (150 km) to the east. In the stalled
slab model the production of partial melt is expected at depths larger than 58 km near the coastline, increasing to depths larger than 80 km further east. This means that a slab window is not
needed in order to explain the mid-Miocene volcanism in California although melt is produced in
this model more easily and at shallower depths.
This is in agreement with ten Brink et al.'s [19]
results from 2-D thermal modeling.
5. Discussion and conclusions
3-D thermal kinematic modeling of two possible end-member scenarios on the post-subduction
history of the Monterey and Arguello microplates
was performed. In the stalled slab scenario subducted microplate material is present below the
North America Plate, while in the slab window
scenario the North America Plate overlies asthenospheric material, and subducted microplate
remnants are absent. In order to model these scenarios kinematically in 3-D, it was necessary to
simplify the geology and the tectonic processes.
Especially the assumption that the plate system
behaves rigidly and plates move independently
from each other is not in agreement with the
knowledge of plate boundaries in general and
these microplates in particular. Ten Brink et al.
[19], for example, argue that the Monterey Plate
was not quite rigid during the last stages of microplate capture. In this the stalled slab scenario is
not consistent with, e.g., Nicholson et al.'s [3]
plate capture model; in their plate capture model
the microplates are attached to both the Paci¢c
and North America plates. Nevertheless, the
present-day thermal structures predicted by the
two scenarios are fairly consistent with the observations.
An implication of the stalled slab scenario is
that the absence of subducted oceanic lithosphere
under some parts of California cannot be explained by it. According to ten Brink et al. [39],
there are no indications for the presence of subducted microplate material below the Inner Continental Borderland, while the stalled slab endmember scenario assumes microplate material to
be present everywhere below coastal California.
The slab window scenario in turn implies that
observations of subducted microplates below extended parts of coastal California cannot be explained. Furthermore, the rotation of the Transverse Ranges cannot be explained in the way
Nicholson et al. [3] proposed. Ten Brink et al.
[39] developed a model for the tectonic evolution
of the Inner Continental Borderland which is consistent with these observations. It is a combination of the end-member scenarios modeled here;
they suggested that the rotation and translation of
the Transverse Ranges after cessation of subduction was accompanied by a tear in the underlying
Monterey Plate, resulting in a slab gap below the
Inner Continental Borderland.
In this model the thermal structure of the
studied area is to a large extent determined by
the relative plate motions between the Paci¢c
Plate and the North America Plate. The maximum present-day temperature di¡erence that can
be expected between the two scenarios is V150³C
in the upper mantle. This is not su¤cient to state
that one of the models is more consistent with the
available data than the other. A slabless window
in this area is not needed to explain the observations, which is in agreement with former 2-D
studies for this area [19].
Acknowledgements
Dr. S. Goes is gratefully acknowledged for allowing us to use her FD code, as well as for
providing useful comments. Tom Brocher and
Uri ten Brink are thanked for their very useful
comments that greatly improved the manuscript.
This study is part of the research program of the
Vening Meinesz Research School of Geodynamics, Utrecht University.[SK]
EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart
J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186
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