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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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 176 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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 178 J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186 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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186 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 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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 180 J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186 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. EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 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- EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 182 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. EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186 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 EPSL 5759 13-3-01 Cyaan Magenta Geel Zwart 184 J.W. van Wijk et al. / Earth and Planetary Science Letters 186 (2001) 175^186 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. 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