Thermal-rheologic evolution of the upper mantle and the development of the San Andreas fault system

Tectonophysics, 223 (1993) 149-164 Elsevier Science Publishers B.V.. Amsterdam

149

Thermal-rheologic evolution of the upper mantle and the development of the San Andreas fault system Kevin P. Furlong apartment

of Geosciences, T&ePen~~~an~ State Uni~~~~, &&em& Park, PA 16802, USA (Received April 8, 1993; revised version accepted May 5,1993)

ABSTRACT Furlong, K.P., 1993. Thermal-rheologic evolution of the upper mantle and the development of the San Andreas fault system. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth’s Surface. Tectonophysics, 223: 149-164. The evolution of the San Andreas fault system differs from that of many other major fault zones in that it can be directly linked to processes in and properties of the underlying mantle. This fault system serves as the plate boundary between the North American and Pacific plates. It has progressively formed since - 30 Ma in response to a fundamental change in plate boundary structure: subduction replaced by transform motion with the northward migration of a triple junction. As a result of triple junction migration, major adjustments to lithospheri~ structure occur and cause the growth and maturation of the fault system. The geodynamic processes which have driven the development of the San Andreas system are primarily associated with the thermal and rheologic evolution of the uppermost mantle in the vicinity of the plate boundary. The emplacement of asthenospheric mantle at shallow levels beneath the North America crust after triple junction passage has led to crustal partial melting and volcanism, development of a well-defined plate-bounding mantle shear zone, and a sequence of events which produced the observed pattern of crustal faults and terranes. As a result of a complex three-dimensional thermal structure, plate boundary deformation (within the lithospheric mantle) is localized to a narrow zone. High strain rates and cooling-induced strengthening of the plate boundary zone lead to changes in grain size and ultimately to changes in deformation processes. The overall result of this is the development of a well-defined and relatively narrow plate boundary within the mantle lithosphere which is initially offset from the crustal fault zone. The mismatch between crustal and mantle parts of the plate boundary leads to the development of additional faults in the system, within the North American plate, which eventually mature to become the primary plate boundary structure in the crust. This is seen in a discrete jump in the location of the crustai plate boundary. Ah aspects of the evolution of the crustal plate boundary can be linked to the rheologic character of the underlying mantle lithosphere which in turn is largely a consequence of the plate tectonic evofution and the conversion of asthenosphere to lithosphere at shallow levels along the plate boundary.

Introduction The San Andreas fault system through California serves as perhaps the preeminent example of a transform plate boundary for continental lithosphere. The San Andreas not only separates the North American and Pacific plates, but because of its location through heavily populated regions of California, earthquake hazards associated with it have a direct impact on society. Although the San Andreas system has been the object of intense research, particularly since the earthquake of 1906, it is only recently that we have begun to 0040-1951/93/$06.00

understand the coupling between plate-tectonicscale processes acting on the entire lithosphere and into the upper mantle and the response of the seismically active crustal faults which have the dramatic impact on everyday life in the region. The San Andreas is an excellent example of the way in which processes occurring on plate tectonic scales and involving both the crust and the mantle lead to observed patterns of crustal deformation. Key to this coupling between mantle processes and crustal consequences is the direct link between plate tectonics and the development of the

0 1993 - Elsevier Science Publishers B.V. All rights reserved

San Andreas plate boundary. In evaluating this linkage we must consider not only the seismically active part of the plate boundary, but we also the full three-dimensional geometry of the plate interactions. Of particular importance to the San Andreas evolution are the processes which transform the western margin of the North American plate from part of an active subduction zone to a transform plate boundary. Most studies of the evolution of the San Andreas and other major plate boundary zones have focused on the seismogenic middle and upper crust. Here we take a different approach putting our attention on the entire lithosphere with emphasis on the upper mantle. By evaluating the thermal and rheologic evolution of the upper mantle we can focus on the processes which serve as the framework which controls the processes in the middle and upper crust. For the San Andreas, the plate geometries associated with the change from convergence to transform motions produce thermal consequences in the uppermost mantle which lead inexorably to the present patterns of crustal deformation, volcanic activity, and earthquakes. The case can be made for the San Andreas (and perhaps it applies to other major fault zones) that all of the crustal tectonics of the region ‘can only be understood by studying the

Gorda d 1

Fig. 1. Plate tectonic setting of the San Andreas plate boundary in California. The Mendocino Triple Junction bW7) migrates with the Pacific plate relative to North America.

processes in the lithospheric and underlying asthenospheric mantle. The San Andreas is a dynamic boundan, continually increasing in length and undergoing modifications which produce distinctive responses in the crust. The San Andreas also provides an excellent natural laboratory to study processes within the mantle such as melt generation and migration, and also to evaluate the earthquake hazards of a plate boundary. The northward migration of the Mendocino Triple Junction (Fig. 1) has occurred since approximately 25-30 Ma, producing an age progression in the tectonic rcsponse which is youngest to the north and oldest in southern California. This age progression allows us to compare the tectonism during the early stages of plate boundary evolution (north) with the response of the crust along more mature segments (south) of the fault system. Plate tectonic framework The formation of the San Andreas plate boundary can be linked directly to the northward migration of the Mendocino Triple Junction (MTJ) (e.g., Atwater, 1970; Dickinson and Snyder, 1979; Zandt and Furlong, 1982; Furlong et al., 1989). The geometry of the triple junction requires that it migrates northward at the rate of North American-Pacific relative motions (Fig. 1). The migration of the MTJ produces a transform boundary in place of the previous convergent margin. It is this change from convergence to transform interaction which not only produces strike-slip crustal faults, but also dramatically changes the three-dimensional lithospheric structure. This plate boundary geometry is shown schematically in Figure 2. The exposure of the thin western margin of North America to emplaced asthenospheric mantle leads to volcanism, large spatial and temporal variations in lithospheric temperature structure, and the associated large variations in mechanical behavior of the lithosphere. The patterns of crustal fault behavior also provide insight into the underlying plate boundary processes. In the vicinity of the MTJ, it is very difficult to identify the principal crustal fault.

DEVELOPMENT

OF THE SAN ANDREAS

FAULT

151

SYSTEM

Rather there is a relatively broad zone of active faults, although none apparently indicate large amounts of fault offset (Kelsey and Cashman, 1983). This is to be expected since this represents the youngest portion of the developing fault system. Further south, along the plate boundary in the region north of San Francisco, a well-developed San Andreas fault marks the location of primary strain accommodation in the upper crust. In the vicinity of the San Francisco, this active San Andreas has been joined by a series a faults 30-50 km to the east which also accommodate a significant fraction of plate motion (Lisowski et al., 1991). This region represents a part of the fault system which has been active 7-10 million years. South of the San Francisco Bay region, there is again a single fault strand which accommodates the plate motions. By focusing on two regions along the plate boundary near the MTJ

and in the San Francisco Bay region, where major changes are occurring, we get good insight into the processes responsible for plate boundary evolution. The large-scale plate boundary geometry in the MTJ region (Fig. 2) shows the normal subduction configuration in the region north of the MTJ where the Gorda plate (relict of the Farallon plate) slowly subducts beneath western North America producing arc volcanism (Cascade Mountains) and evidence of convergent tectonism. The passage of the MTJ produces cessation of subduction, but, since the subducted Gorda slab travels with the Gorda plate, MTJ passage results in the exposure of the base of a thin North American lithosphere (previously underlain by Gorda slab) to asthenospheric mantle. This region of asthenospheric upwelling is referred to as the slab window beneath North America. It is the

North American Plate Pacific Plate

Mendocino Triple Junction

I Plate I

1

A

Pl ate Boundary// Structure -7 San Andyxis Hayyard

- Rcdgers Creek

Fig. 2. Simplified view of the three-dimensional lithospheric structure in the vicinity of the Mendocino Triple Junction. Inset cross sections show schematic representations of Pacific and North American lithosphere evolution after passage of the triple junction.

15’

evolution of this emplaced asthenospheric mantle which controls the further evolution of the plate boundary. The schematic 3-D lithosphere structure shown in Figure 2 leads to a localization of strain within the slab window (offset to the east from the eastern edge of the Pacific plate) which favors activation of faults within the thin region of North America lithosphere. The offset between deep strain and surface structures produces the lithospheric configuration in the San Francisco Bay region (Fig. 3) where two major fault systems accommodate plate motions in the crust, while a localized region within the lower crust and upper mantle serves as the plate boundary. This complex plate boundary structure continues to evolve, finally reaching a stable configuration along the boundary in central California where the eastern fault system dominates. The San Francisco Bay region sits in the transition in plate boundary structure from the “two-fault” to “one-fault” segments. Much of the tectonism in the San Francisco Bay area can be directly linked to this transition and its associated fault structure. In particular the development of the Santa Cruz Mountains (site of the 1989 Loma Prieta earthquake), the slip characteristics of the Loma Prieta earthquake, and the development and subsidence of San Francisco Bay itself is a direct conse-

quence of the plate boundary structure thcrc and. therefore, is a consequence of the processes begun with the passage of the MTJ. This coupling between the mantle processes and the crustal response and its tectonic consequences, as shown in Figures 2 and 3? becomes clear when the thermal and rheologic evolution of the region are evaluated. Without the insight provided by such studies, the plate boundary evolution described above provides an interesting tale of plate tectonic processes, but is a difficult story to defend. In the following sections, the underlying evidence for this plate boundary evolution will be described. With this plate tectonic framework confirmed, details of the crustal response can then be outlined. Thermal-rheologic

evolution

The dramatic change in the thermal structure of the plate boundary from the cool subduction structure of Gorda-North American plate convergence to the shallow emplacement of hot asthenospheric mantle into the vacated slab window provides the key to plate boundary evolution. Although the temperature structure in the region can be predicted by considering the 3-D plate kinematics with MTJ passage, there is convincing geophysical and geological observations

San Andreas

Plate Boundary Fig. 3. Three-dimensional view of plate boundary structure in the vicinity of San Francisco. The San Andreas fault in this region is connected to the East Bay faults (Hayward/Calaveras) and the deeper plate boundary via a lower crustal detachment surface.

DEVELOPMENT

OF THE

SAN

ANDREAS

FAULT

SYSTEM

Fig. 4. Extent of seismic low-velocity region in the uppermost mantle (30-70 km depth) as imaged by seismic tomography (Benz et al., 1992). Shaded region shows region with velocities > 2% slow relative to average velocities in that depth range. The position of the west edge of slab window (eastern extent of the Pacific plate) is primarily a consequence of thermal processes in the mantle.

1.53

which provide support for the thermal model. Several seismic tomographic studies (Zandt, 1981; Zandt and Furlong, 1982; Benz et al., 1992) indicate that in the depth range of 30-90 km seismic velocity (p-wave) varies laterally with significant low-velocity anomalies associated with the slab window. This region in northern California is shown in Figure 4 (Benz et al., 1992). The geometry of the slab window as imaged by seismic tomography is more complicated than the simple geometries of Figures 2 and 3, but the complex geometry of the seismic velocity anomalies provides an important constraint on the evolution of the coupled crust-mantle plate tectonic system. The influx of hot asthenospheric mantle to such shallow levels might be expected to generate volcanism in the region. Figure 5 is a map of volcanic provinces within coastal California which have erupted since the passage of the MTJ (Liu and Furlong, 1992). An analysis of the timing, volume and composition of these volcanics indicates that they are the expected consequence of MTJ passage and slab window development. It is

Fig, 5. Relation between position of post MTJ volcanics and the evolution of the plate boundary system (from Liu and Furlong, 1992). Region indicated (?) north of the Clear Lake volcanic field is region of extreme low velocity in the upper crust as imaged by seismic tomography (Benz et al., 19921, and may represent an incipient volcanic center.

154

important to note that the volcanics are limited to the region substantially east of the San Andreas in northern California, in agreement with the region of low seismic velocity delineated in the tomographic studies. Additional conclusions of the Liu and Furlong (19921 study are that (1) the geometry of the slab window precludes significant thermal convection implying a dominantly conductive regime for thermal evolution, and (2) the volume of erupted material can be coupled to the rates of relative Pacific-North American plate motion, which have varied by a factor of 2-3 since 30 Ma. Plate boundary defoma tion Although geophysical and geological observations help identify the location and character of the slab window-plate boundary region, in order to fully understand how the boundary evolves, we need to unravel the deformation processes which occur along the boundary. Evaluating the deformational history is made easier since the region is part of a plate boundary. As such, the total plate velocity between the Pacific and North American plates (strain rate integrated across the plate boundary region) must be accommodated within the plate boundary zone. In the ductile deformation regime below the seismically active crustal faults, the local stress/strain regime will adapt to

satisfy this external constraint of plate motion. The seismic velocity anomalies indicated by the tomography can be related to a temperature variations and this indicates a significant temperature variation within the plate boundary region. The transient thermal regime produced by the simple plate geometries produced by MTJ passage (Fig. 2) can be modeled (e.g., Furlong, 1984; Furlong et al., 1989; Liu and Furlong, 1992) and produce temperature structures in agreement with both the seismic topographic results and petrologic observations (Liu and Furlong, 1992). As seen in Figure 6, highest temperatures in the plate boundary region are within the initial slab window. Since the slab window can cool by heat transfer both vertically and laterally, there are substantial horizontal temperature variations, with temperature maxima east of the initial limit of the Pacific plate below the thin North American plate. Since deformation within the lithospheric mantle is dominated by temperature-dependent creep processes we expect significant strain localization within the interior of the slab window. Many deformation mechanisms may occur within the temperature-pressure-stress-strain rate regime of a plate boundary such as the San Andreas, it is convenient to consider two main classes of deformation mechanisms for the region - grain-size-independent (e.g., dislocation creep)

50

100

150

Distance(km) Fig. 6. Modeled temperature history for slab window after M’FJ passage. Cooling occurs both vertically and laterally producing localization of maximum temperatures within the slab window.

DEVELOPMENT

and grain-size-dependent (e.g., diffusion creep). As discussed by Rutter and Brodie (1992), such a categorization is reasonably inclusive of the dominant deformation mechanisms, and allows us to focus on the important parameters of temperature, strain rate, and grain size in evaluating the rheologic history. Any deformation model for a plate boundary such as the San Andreas must include the consequences of spatial patterns of rheologic properties and the possibility of spatial and temporal variations in deformation mechanism. The high strain rates (10-12-10-‘4 s-l) appropriate for plate boundaries and the large amounts of total strain make them ideal locations for mineralogical changes which modify the rheology. In particular we would expect stress-driven grain size reduction to occur and allow the transition from grain-size-independent (GSI) dislocation creep to grain-size-dependent (GSD) mechanisms such as diffusion creep (Karat0 et al., 1986; Rutter and Brodie, 1988; Handy, 1989). In such cases the deformation picture becomes complex and the strength of the plate boundary varies as a function of strain rate, temperature, and strain history. The effects of strain history will be most pronounced in the relatively older/colder parts of the San Andreas, where lateral temperature variations are minimized but lateral variations in strain-induced mineralogical effects (e.g., grain size) may dominate in the deformation response. To evaluate this effect, which becomes a dominant mantle process controlling crustal deformation, we can evaluate the rheologic history of the slab window. The general behavior of ductile deformation processes for mantle materials is given by: i = Aand”

155

OF THE SAN ANDREAS FAULT SYSTEM

exp( - Q/RT)

where i (s- ‘> is strain rate, A is an empirical constant, CTis flow stress (MPa), d is characteristic grain size, Q is activation energy, R is the gas constant and T is absolute temperature. The exponent it describes the power law nature of the stress-strain relation, while m describes the power law dependence on grain size. For GSI creep in the mantle m is zero and n is typically in the range of 2.5-3.5. For GSD creep m is of

TABLE 1 Rheologic parameters of olivine (dry) Deformation mechanism

A

Q

(MPa-” s-l)

(kJ moleC’)

Dislocation creep ’ Diffusion creep ’

3.4 x lo4 4.2 x 10’

544 240

II

m

3.5 1

-3

0

’ Zeuch (1983). ’ Karat0 et al. (1986).

order -3 and n is normally 1 (Karat0 et al., 1986; Rutter and Brodie, 1988). Values for these parameters are given in Table 1. As implied above, grain size is not a static property of actively deforming regions, and an empirical grainsize vs. stress relation has been determined for olivine (Karat0 et al., 1982, 1986). This relationship is given approximately by:

(2) The experiments of Karat0 et al. (1982, 1986) and previous studies discussed in those papers determined this relationship between flow stress and dynamic grain size using “dry” olivine materials. Although the relationships between P and CT[e.g., eq. (01 depend on experimentally determined parameters which vary with the hydrous condition of the material, until recently it has not been clear whether the grain size-stress relation of equation (2) also depended on the wet or dry condition of the material. In the case of the shallow portions of the mantle along the San Andreas because of the long period of convergence/subduction which preceded formation of the San Andreas, the possibility of hydrous conditions must be considered. Recent work by van der Waal et al. (1992) indicates that equation (2) holds under natural hydrous conditions, although the rates of grain size reduction/growth may vary with water content. The accomplishment of recrystallization and grain size reduction requires substantial ductile strain, a condition easily met in transform regimes. Although the specifics of grain size reduction versus total strain are not pinned down, the work of Rutter and Brodie (1988) indicates that an equilibrium grain size is reached within natural strains of 0.4. Although such strains are large by

Deformation

Fig. 7. Temperature-stress relationships for ductile creep processes at upper mantle temperature conditions. Strain rates range from lo-r6 to IO-‘” s-r in each case. A characteristic grain size of 100 m was used in calculating the diffusion creep results.

intraplate standards, they are easily accomplished along transfo~ boundaries. Along the transform system we would expect grain size to maintain an equilibrium with applied stress (until secular cooling “freezes in” a grain size). The coupling of stress-dependent grain size with GSI and GSD defo~ation mechanisms leads to a complex deformation behavior within the high-strain regions of a transform boundary. As examples of the potential effects of the switch to GSD creep (diffusion), the relationships among temperature, stress, and strain rate for olivine (dry) under GSD and GSI mechanisms are shown in Figure 7 for a range of strain rates between lo-l2 and lo-l6 s-i (using the values of Table 1). In the temperature range typical of the upper parts of the lithospheric mantle along the San Andreas ~~-~~), stress levels of w lo’-10’ MPa can drive plate boundary strain rates under GSD, but substantially higher stress levels are needed for GSI mechanisms of plate boundary deformation. As the region cools, this effect becomes even more critical to the stress regime along the plate boundary. If strain rate is held fixed iat lo-l3 s-l), the relations among stress, grain size, and temperature can be evaluated. The change from GSI dislocation creep to GSD diffusion creep will occur at the grain size-temperature condition where one mechanism is favored over the other (i.e. deformation occurs at lower stress levels). This approach is shown in

Map

Pig. 8. Deformation map for olivinc ~1 upper mantle contlitions and strain rate of lo- ” ?I ‘. Stresr is indepcndcnt of grain size in dislocation creep field but strongly deprndent on grain size in diffusion creep field. Dashed line shows equilibrium stress-grain size relationship of Karat0 et al. li982, l98h).

Figure 8, in conjunction with grain size-stress relationship of equation (2). If sufficient strain has occurred the temperature-stress-grain size conditions should reach a dynamic equilibrium.

RETROGRADE: PATH

2 1.5 ;@ 0.5 0 2

2.5

3 3.5 Leg D (~1

4

4.5

Fig. 9. Part of deformation map of Fig. 7, showing the rheological evolution of the slab window region during cooling. Individual points represent the history if a series of temperature steps occurred during cooling, shaded arrow represents the more likely continues path taken by the rock of the plate boundary. Dashed arrow shows rheoiogic path of slab window material away from the high-deformation zone which does not undergo grain size reduction, but rather increases in strength by orders of magnitude during cooling.

DEVELOPMENT

OF THE

SAN

ANDREAS

FAULT

157

SYSTEM

shown in Figure 9 is along a series of discrete temperature reduction steps; the rheologic history for a specific part of the plate boundary will result from a continuous temperature decrease more like the path shown by the shaded arrow. Since grain-size reduction occurs in response to strain at elevated stress, it will be spatially limited to areas of relatively high strain. Early in the history of the San Andreas, there is signifi-

The route to such an equilibrium is shown schematically in Figure 9 for the retrograde (cooling) thermal history appropriate for the slab window region of the San Andreas. As the system cools, the flow stress needed to produce the strain rates required by the magnitude of relative plate velocities increases. This stress increase can be moderated by the effects of grain size reduction and change to GSD creep mechanisms. The path

I Surface

-

NORTH

RTH

AME RICAN

PLATE

AYE

PLATE

RICAN

Gulf of California

R TH

AME

RICAN

PLATE

Gulf of California

Fig. 10. Map views of plate boundary position at the surface and 30 km depth during the evolution of plate boundary. Ruled region is hot asthenospheric material which progressively cools and accretes to either the North American of Pacific plates. Plate boundary configuration predicted on thermal-rheologic grounds is in good agreement with results of seismic tomography.

15X

cant lateral variation in temperature within the slab window, allowing large variations in strain rate (Figs. 6,7). The region of concentrated strain will, as the system cools, become the primary location for grain size reduction and the change in deformation mechanism. This has the effect of concentrating deformation into a relatively narrow zone. This narrow zone is both the region of highest temperature and smallest grain size and thus will deform at significantly lower stress levels. Modeling of the consequences of this plate boundary configuration (Verdonck and Furlong, 1992) indicates that such a system has a feedback to it which will continue to localize strain. Upon further cooling, grain size may not be able to grow sufficiently even in the absence of high strain, preserving the plate boundary as a relict zone of continued weakness long after it is part of the active transform. That part of the slab window in which strain rates are low (cooler regions at each depth) will not undergo grain size reduction. As shown by the dashed path in Figure 9, during the secular cooling of the slab window after MTJ passage, those regions will continue to strengthen further localizing the plate boundary.

Plate accretion and plate boundary geometry

The region of concentrated strain defines the boundary between the Pacific and North American plates. Concurrent with the localization of strain is the overall cooling of the slab window resulting in the conversion of asthenospheric mantle to lithospheric mantle. In this way the extent of the Pacific and North American plates changes in time after passage of the MTJ. An additional factor which affects the final plate boundary geometry is produced by the differences in effective age across the San Andreas. What is meant here by effective age of the plate boundary, is how long that part of the boundary has been associated with the San Andreas plate boundary system. For the North American side of the boundary, assigning this effective age is simple. It is simply the time since the MTJ passed that point of the boundary, since prior to MTJ passage that point would have been associated

with the COrda/Parallon subduction rcgimc. The situation is less straightforward for the Pacific side of the plate boundary. The fact that the Pacific plate is bounded by two transform faults at the MTJ means that the northeast corner o! the Pacific plate at the MTJ has occupied that position since the initial formation of the triple junction 5 30 Ma. Thus the oldest segment of the Pacific side of the boundary is at the MTJ which represents the youngest part of the North American segment of the boundary. Further complicating the tectonic consequences of this effective age difference is the thermal effects of emplacing asthenospheric mantle at shallow levels in the vicinity of the MTJ. This will have the effect of retarding the cooling effect of the eastern edge of the Pacific plate. Taking this age effect into account, WC:can produce an estimate of the location of the boundary between the Pacific and North American plates through time. In Figure 10 such a view is shown both for the near surface and at a depth of approximately 30 km. At 20 Ma, the length of the San Andreas system was substantially shorted than today, but already the north east corner of the Pacific plate has grown to extend beneath the thin lithosphere of the North American plate. At 5 Ma, the picture is reached a condition similar to the present, where the Pacific plate extends east of its initial extent, and this is dominantly a consequence of lateral accretion to the lithosphere rather than from convergence. The eastern continuation of the Pacific plate in northern California as predicted by thermal modeling is equivalent to the eastern edge of the plate imaged by seismic tomography (Fig. 4), indicating at most a minor component of oblique convergence at work on lithospheric scale. This is not altogether surprising in that the plate boundary within the ductile regime has much more freedom to adjust to changes in plate motions than does the brittle upper crust. Thus, the mismatch between plate motion vectors and crustal faults results in compressional or extensional tectonics while the ductile plate boundary merely shifts its position slightly. As we shall see below, the combination of strain localization and the continuation of the Pacific plate beneath North America (both a con-

DEVELOPMENT

OF THE SAN ANDREAS

sequence of mantle processes) produces crustal tectonics we presently observe. Crust-mantle

159

FAULT SYSTEM

the

kinematics

The tectonic consequences of the localization of the plate boundary beneath the thin North American lithosphere migrate northward in the wake of the MTJ. At present these effects are best seen in the vicinity of San Francisco, where crustal deformation and seismic activity are both directly linked to the mantle regime. Figure 3 showed our best estimate of the current plate boundary structure in the San Francisco region. There the plate motions are accommodated in the upper crust by two fault systems - the San Andreas which runs along the San Francisco Peninsula and the East Bay faults (primarily the Hayward and Calaveras faults) which run on the east side of San Francisco Bay. Geodetic observations (Lisowski et al., 19911, seismic tomography, and our thermal rheologic modeling indicate that the plate boundary below the seismogenic layer is localized beneath the East Bay faults, requiring the Peninsular segment of the San Andreas to be connected to the plate boundary by a sub-horizontal shear zone or similar localized deforma-

CRUSTAL

tional regime. A recent seismic imaging experiment (BASIX) is attempting to image this structure. Early results from BASIX are consistent with this plate boundary configuration. The San Francisco region sits in a fundamental transition along the San Andreas. North of the region, the northward continuation of the Peninsular San Andreas is the primary crustal fault, while to the south, the San Andreas is essentially in line with the East Bay faults. Connecting these two segments of the San Andreas is the Santa Cruz segment on which ruptured during the Loma Prieta earthquake. In this region there is a difference between the kinematics of the crust and the underlying mantle lithosphere. The relevant kinematics for these two parts of the plate boundary are shown in Figure 11. The motions of the crust are defined by the position and strike of the crustal faults, while the mantle motions follow the large scale plate motions. South of San Francisco Bay, the motions of the crust deviate from the mantle requiring decoupling of the crustal block and allowing its westward abduction onto Pacific plate. As a result there is a complicated pattern of crustal blocks and coupling between the crust and mantle lithosphere. East of the mantle plate boundary, the North

MANTLE KINEMATICS

KINEMATICS

38”

38’

Pacific (Coupled)

1

37”

Pacific Lithosphere 36’

123” Fig.

11.

122”

121”

123”

122”

121°

Kinematics and locations of plate boundary structures in San Francisco region. Crustal blocks are constrained by faults to paths which deviate from the overall plate motions, requiring decoupling between the crust and lithospheric mantle.

American crust and mantle are well coupled. however the North American crust which overlies the Pacific plate (Bay Area Block of Fig. 11) is decoupled from the underlying Pacific lithosphere, and at best partially coupled on the east the North America. The Santa Cruz Block travels with the Pacific plate (it is west of both the crustal and mantle plate boundaries) but its trajectory differs from that of the Pacific. We expect partial decoupling, evidence of some compressional tectonism and the emplacement of this block onto oceanic lithosphere of the Pacific plate. Since this pattern of crustal and mantle coupling and kinematics is a result of the processes started with MTJ passage, we expect it to have occurred in the past, and likely to occur in the future. In this way a series of terranes can be excised from the western edge of North America and emplaced onto and incorporated into the Pacific plate. In fact there are a series of terranes and basins along the San Andreas (McCulloch, 1989) which may have been emplaced this way. Tectonic consequences Understanding the plate tectonic kinematics on both crustal and lithospheric scales for the formation and evolution of the San Andreas provides the framework to investigate the tectonism of the boundary. Although aspects of the tectonism all along the boundary are linked to the lithospheric evolution, there are two regions which experience tectonic activity tightly coupled to the plate boundary evolution. At the Mendocino Triple Junction and in the San Francisco Bay region fundamental changes in plate behavior produce a diagnostic tectonic response. In the immediate vicinity of the MTJ, there is strong evidence of extremely rapid uplift (Merrits and Vincent, 1989; Dumitru, 1991). This uplift is probably linked to two separate processes associated with MTJ passage. First, the emplacement of hot asthenosphere at shallow levels in the slab window is expected to produce a regional uplift from the combined buoyancy and heating (Furlong, 1984). In addition, the eastern continuation of the Pacific plate beneath the western margin of North America, the terranes carried by the

Pacific plate adjacent to the boundary, and the geographic vagaries of the western margin ot North America all lead to the potential for locaiized convergence along the margin. One such example of this is the extremely rapid uplift of the King Range terrane (Underwood ct al.. 1988; Dumitru, 1991;) which may represent the rcaccretion of a terrane carried by the Pacific plate to North America. There is also evidence in crustal seismic tomography in the MTJ region (Verdonck et al., 1992) for a pattern of crustal thickening above the slab window. Whether this occurs by shortening along faults (e.g., Kelsey and Cashman, 19831 or by thermally activated crustal flow, is at prcsent unclear, but the localization of this zone of thickened crust to the region above the slab window indicates an ongoing dynamic process. This crustal structure then may be linked to both the convergent processes and thermal-buoyancy processes which produce uplift in the wake of the MTJ. A myriad of tectonic activities in the San Francisco Bay region can be directly linked to the plate boundary geometry and its evolution. Perhaps of utmost importance is the role the lithospheric system plays in earthquake genesis and producing the patterns of seismic activity. The Loma Prieta earthquake of 1989 occurred along the segment of the San Andreas bounding the southern and western margins of the Bay Area Block (Fig. 11). Because of its position and nature of rupture, this earthquake has been interpreted to represent the decoupling of the Santa Cruz block from its underlying mantle which continues along the path shown in Figure 11 (Furlong and Langston, 1990; Langston et al., 19901. In this interpretation, Loma Prieta type events produce the detachment surface which underlies San Francisco Bay, decouple the Santa Cruz Block, and allow it to abduct or overthrust onto the adjacent Pacific plate. The development of the detachment surface indicated in Figure 3 below San Francisco Bay connecting the San Andreas and East Bay faults raises additional concerns for earthquake hazards in the region. Historically, there is an intriguing pattern of paired earthquakes within this region

DEVELOPMENT

OF THE SAN ANDREAS

161

FAULT SYSTEM

Paired Bay Area Earthquakes ? 123”

122”

38”

37”

Fig. 12. Patterns of San Francisco Bay region earthquakes indicating possible linkage between events on each of the fault systems. Role of lower crustal detachment surface in aiding this coupling is at present unresolved.

of the San Andreas, where major events on one fault strand are followed within a few years by events along other fault segments (Ellsworth, 1990). The pattern of potential earthquake pairing is shown in Figure 12, with all earthquakes since 1836 satisfying the pattern. Unfortunately the time interval of the observations is too short to adequately test the significance of this observation, but nonetheless it raises some intriguing questions in regard to stress/strain coupling across detachment surface. yodeling of the stress and strain r~ifications various plate boundary geometries (Furlong and Verdonck, 1993) indicate that the existence of a low-strength detachment structure in the lower crust will allow more rapid interaction between faults in the region, but in all cases the level of stress loading caused by an event such as Loma Prieta on adjacent faults is at most a few MPa. As mentioned earlier, BASIX (Bay Area Seismic Imaging eXperiment1 is attempting to image and define such a lower crustal structure (McCarthy et al., 1992). Although final results are not yet available, preliminary seismic data indicate a structure beneath the Bay Area Block in the

depth range of 15-20 km, appropriately located to be a candidate structure for the detachment surface. The decoupling of the Santa Cruz block and its westward translation onto Pacific plate produces a series of tectonic events which lead to the development of an exciting new type of sedimentary basin, manifest in San Francisco Bay (Prims and Furlong, 1992). The coincidence of the San Andreas with the deeper plate boundary through central California and the development of a transfer fault to connect that segment with the Peninsular San Andreas is a process which should occur repeatedly during the evolution of the plate boundary. The present configuration is simply the latest in a series of such crustal fault geometries. Such a history is shown schematically in Figure 13, where the faults overlying the mantle boundary eventually mature to the point that the outboard San Andreas is essentially abandoned. This produces the fault geometry that leads to decoupling and abduction of what in the San Francisco region is the granitic Salinian terrane which (as part of the Santa Cruz block of Fig. 11) overthrusts the Pacific plate. Of importance to basin development is the excess mas represented by the Salinian terrane. It serves as a load on the Pacific plate producing flexure and subsidence. The Pacific plate does not end at the Peninsular San Andreas, but extends beneath the San Francisco Bay to the East Bay Faults. The load of the Salinian terrane thus causes subsidence of not only the Pacific beneath the Salinian, but also the Pacific plate below the Bay Area block. The seismic activity and significant plate motion accommodated on the East Bay Faults indicates at least partial decoupling of the Bay Area block from North America, allowing it to subside with the Pacific plate flexure (Fig. 14). In this interpretation the San Francisco Bay represents a flexural depression generated by the loading of a finite extent Pacific plate (e.g., “broken plate” flexure) by the Salinian terrane. Timing and magnitude of San Francisco Bay subsidence are in complete agreement with the timing of Salinian abduction and East Bay fault decoupling. The development of San Francisco Bay represents the present occurrence of the process out-

lined in Figure 13. With the further northward movement of the MTJ, the situation shown will repeat, transferring the Bay Area block onto the Pacific plate where it will be an offshore basin along the margin. We expect that this process has occurred in the past along the San Andreas, producing the sequence of sedimentary basins which lie on and travel with the Pacific plate, but likely formed in the transitional regions during plate boundary evolution. The development of San Francisco Bay has always been enigmatic as plate motions and plate boundary geometries are incompatible with the “pull apart” or transtensional models of basin evolution. Recognizing that the San Francisco Bay is in reality a flexural basin formed in this manner explains not only its formation, but provides a testable model for the development of other basins in offshore California.

Discussion

Fig. 13. Kinematic evolution of crustal fault system in San Francisco region. This pattern of fault evolution emplaces the Salinian terrane over the Pacific plate providing a mass excess which causes subsidence in the San Francisco Bay Area block.

The evolution of the San Andreas plate boundary system represents the coupling of mantle processes initiated by plate tectonic events with crustal deformation. Is this type of coupling between crust and mantle to be expected along other plate boundaries? In particular is this interaction unique to the San Andreas or will we see similar effects along other transform boundaries? Without much of the corroborating information

Salinian Block

Fig. 14. Schematic cross section through Salinian and Bay Area blocks showing relative positions of Salinian block load with extent of the Pacific plate and potential for Bay area subsidence during emplacement of Salinian block.

DEVELOPMENT

OF THE

SAN ANDREAS

FAULT

for the San Andreas system (seismic tomography, petrology, geodetic measurements,

heat flow, etc.) the linkages and coupling. Similar interactions may be uncovered for other regions, but certainly the importance of the innumerable investigations of the San Andreas which preceded our studies cannot be minimized. Without comparable data bases it is hard to be specific, but several plate boundary environments exhibit some similar tectonic characteristics. The Queen Charlotte fault system located off the western coast of Canada has many of the same characteristics as the San Andreas. It also serves as the boundary between the Pacific and North American plates, its history is linked to the change from convergent to transform plate interactions, and it lengthens through time (although the lengthening is associated with a ridge migration). The Queen Charlotte system is much less accessible than the San Andreas and consequently much less is known of its structure and deformational style. Overall though I would expect it to share similar characteristics to the San Andreas, in particular the coupling between mantle processes and crustal deformation. Oceanic transforms provide a tectonic regime where I think we can begin to delineate the link between mantle and crustal processes. There appears to be a maximum plate velocity beyond which oceanic transforms do not exist (Naar and Hey, 19891, and the age offset (or equivalently the rheologic character of the plate boundary) along oceanic transforms appears to be directly coupled to plate velocities (Furlong, 1992). In fact the overall evolution of oceanic transforms, and the development of propagating ridge segments may all be part of such coupled processes. The rheologic evolution described for the San Andreas appears to have similar analogs in the oceanic transform regime. The thermal history along an oceanic transform is not simply retrograde (cooling) which leads to an interesting sequence of grain size reduction followed by grain growth as the plate boundary approaches the end of the ridge prior to becoming a fracture zone (Furlong, 1992). This allows the strengthening of oceanic transforms and the development of strong fracit would

have

been

difficult

163

SYSTEM

to identify

ture zones (Bergman and Solomon, 1992; Bonneville and McNutt, 1992). Finally, these studies of the San Andreas have opened my eyes to the links between large scale mantle processes and crustal and supracrustal responses. Not only are the crustal faults a natural consequence of the plate tectonic motions, and earthquake hazards entwined with the rheology of the mantle, but new types of basin generation mechanisms are possible when the crust and mantle interact as they do along the San Andreas. Acknowledgments

The research described is derived from many collaborations. No one person (or at least not I> could have the breadth of expertise to tie together all of the necessary parts of the puzzle. Among the people most influential in putting together this story are George Zandt, Harley Benz, Dave Verdonck, Mian Liu, Jordi Prims, and Jill McCarthy. Funding for this research has come from the U.S. National Science Foundation (EAR-8816966 and EAR-91041851, the U.S. Geological Survey (NEHRP 1434-92-G-22131, NASA (DOSE NAG5-1909), and the Petroleum Research Fund of the ACS (23496-AC). All are greatly fully acknowledged. Additionally, the better part of a year spent at the Institute of Earth Sciences at the University of Utrecht provided superb intellectual stimulus and exposed me to many of the concepts of mantle rheology which have been used here. References Atwater, T., 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geol. Sot. Am. Bull., 81: 3513-3536. Benz, H.M., Zandt, G. and Oppenheimer, D.H., 1992. Lithospheric structure of northern California from teleseismic images of the upper mantle. J. Geophys. Res., 97: 47914807. Bergman, E.A. and Solomon, SC., 1992. On the strength of oceanic fracture zones and their influence on the intraplate stress field. J. Geophys. Res., 97: 15,365-15,377. Bonneville, A. and McNutt, M., 1992. Shear strength of the great Pacific fracture zones. Geophys. Res. Lett., 19: 2023-2026.

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