The San Andreas Fault

The San Andreas Fault formed in Oligocene times (Atwater, 1970, 1989) when the Pacific-Farallon spreading ridge collided with the western margin of North

Arava valley Arava Fault

Arava valley Arava Fault

Distance (km)

Figure 8.11 P-wave velocity model of the crust and mantle below the Arava Fault within the southern segment of the Dead Sea Transform (image provided by M. Weber and modified from the DESERT Group, 2004, with permission from Blackwell Publishing). Profile location is shown in Fig. 8.3a. Vertical exaggeration is 2:1. Triangles indicate shot points along a wide angle seismic reflection and refraction survey used to obtain the velocities (km s~'). Hatched area near crust-mantle boundary represents zone of strong lower crustal reflections. The boundaries and P-wave velocities located northwest of the fault are from Ginzburg et al. (I979a,b) and Makris et al. (1983). Those to the southeast of the fault are based on El-Isa et al. (I987a,b).

Distance (km)

Figure 8.11 P-wave velocity model of the crust and mantle below the Arava Fault within the southern segment of the Dead Sea Transform (image provided by M. Weber and modified from the DESERT Group, 2004, with permission from Blackwell Publishing). Profile location is shown in Fig. 8.3a. Vertical exaggeration is 2:1. Triangles indicate shot points along a wide angle seismic reflection and refraction survey used to obtain the velocities (km s~'). Hatched area near crust-mantle boundary represents zone of strong lower crustal reflections. The boundaries and P-wave velocities located northwest of the fault are from Ginzburg et al. (I979a,b) and Makris et al. (1983). Those to the southeast of the fault are based on El-Isa et al. (I987a,b).

O cd

Arava Fault

Ductile : 4 km extension

Ductile : 4 km extension

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Brittle

Ductile

Figure 8.12 Sketches showing the processes involved in producing the crustal section shown in Fig. 8.11 (image provided by J. Mechie and modified from the DESERT Group, 2004, with permission from Blackwell Publishing). (a) Crustal structure at-17 Myr, before initiation of strike-slip motion. (b) Strike-slip displacement of-105 km results in significantly different structure east and west of the Arava Fault. (c) Minor (-4 km) of extension results in subsidence and flexure of the western block and uplift of the eastern block. Moho shows a similar deflection. (d) Erosion and sedimentation produces present structure. Note that processes (b-d) act simultaneously.

Figure 8.12 Sketches showing the processes involved in producing the crustal section shown in Fig. 8.11 (image provided by J. Mechie and modified from the DESERT Group, 2004, with permission from Blackwell Publishing). (a) Crustal structure at-17 Myr, before initiation of strike-slip motion. (b) Strike-slip displacement of-105 km results in significantly different structure east and west of the Arava Fault. (c) Minor (-4 km) of extension results in subsidence and flexure of the western block and uplift of the eastern block. Moho shows a similar deflection. (d) Erosion and sedimentation produces present structure. Note that processes (b-d) act simultaneously.

Arava Fault

America (Fig. 5.28a, Section 5.11). The fault joins the Mendocino Triple Junction with the Gulf of California and is the only continuous structure within the plate boundary zone (Fig. 8.1). Displacement on the fault is dominantly strike-slip (Fig. 7.10), although in places it also is associated with localized transpression and transtension (Section 8.2). Heat flow measurements (Sass et al., 1994), seismicity (Fig. 7.8), and seismic reflection and refraction surveys (Henstock & Levander, 2000; Godfrey et al., 2002) indicate that the fault has formed in very heterogeneous lithosphere characterized by large lateral variations in thickness, strength, and thermal properties.

Most of the evidence from northern and central California suggests that the San Andreas Fault penetrates into the lower crust as a near vertical structure and may offset the Moho (Holbrook et al., 1996; Hole et al., 2000). From west to east, the top and bottom of a 5- to 6-km-thick lower crust drops by up to 4 km across the San Andreas Fault. The Moho is similarly offset, albeit by only ~2 km. Seismic velocities in the upper mantle show a small change across the profile, from 8.1 km s-1 beneath the Pacific to about 7.9 km s-1 beneath the Coast Ranges, suggesting that the latter are characterized by slightly lower densities and temperatures that are higher by ~550 K between 30 and 50 km depth (Henstock & Levander, 2000).

Beneath the Transverse Ranges, a velocity model (Fig. 8.13), constructed using active-source seismic data, reveals the presence of an 8-km-thick crustal root centered beneath the surface trace of the San Andreas Fault (Godfrey et al., 2002). The presence of this crustal root indicates that the transpression associated with the Big Bend in the San Andreas Fault (Section 8.2) affects the entire crust. The data also show an offset Moho. Estimates of the magnitude of the offset are variable, mostly because they depend on the specific velocities used. Published estimates show the Moho to be at least one and possibly several kilometers deeper on the northern side of the fault. A similar, small Moho disruption also occurs beneath the Eastern California Shear Zone (Zhu, 2000) (Fig. 8.1). These offsets suggest that a narrow zone of brittle and ductile deformation surrounding the southern segment of the San Andreas Fault also extends vertically through the entire crust.

In addition to an offset Moho, the velocity model shown in Fig. 8.13 indicates that relatively slow seismic velocities (6.3 km s-1), which are consistent with weak, quartz-rich lithologies, characterize the middle and

San Santa

San Santa

Figure 8.13 Velocity model of the crust and mantle below the Los Angeles Basin (LAB) (modified from Godfrey et al., 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). Section parallels thick black profile line shown in Fig. 8.8a. Dashed lines are velocity contours. Numbers are velocities in km s~'. Vertical exaggeration is 2:1. Upper 10km is constrained by crustal refractions. The shaded region shows the part of the mantle constrained by Pn wave data, thick black lines indicate constrained part of the Moho. LVZ, low velocity zone; NIF, Newport-Inglewood Fault; PVF, Palos Verde Fault; SAF, San Andreas Fault; SGF, San Gabriel Fault; SGM, San Gabriel Mountains; SGV, San Gabriel Valley; SMF, Sierra Madre Fault; VT, Vincent Thrust; WF, Whittier fault.

Figure 8.13 Velocity model of the crust and mantle below the Los Angeles Basin (LAB) (modified from Godfrey et al., 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union). Section parallels thick black profile line shown in Fig. 8.8a. Dashed lines are velocity contours. Numbers are velocities in km s~'. Vertical exaggeration is 2:1. Upper 10km is constrained by crustal refractions. The shaded region shows the part of the mantle constrained by Pn wave data, thick black lines indicate constrained part of the Moho. LVZ, low velocity zone; NIF, Newport-Inglewood Fault; PVF, Palos Verde Fault; SAF, San Andreas Fault; SGF, San Gabriel Fault; SGM, San Gabriel Mountains; SGV, San Gabriel Valley; SMF, Sierra Madre Fault; VT, Vincent Thrust; WF, Whittier fault.

lower crust beneath the Mojave Desert. By contrast, the lower crust located south of the San Andreas Fault is characterized by relatively fast velocities (6.6-6.8 km s-1), suggesting that the region south of the fault is composed of strong feldspar- and/or olivine-rich rocks. This velocity structure is compatible with the idea that the weak crust north of the fault has flowed southward, creating the thick root beneath the Transverse Ranges (Fig. 8.14). In support of this hypothesis, several prominent bright spots beneath the San Gabriel Mountains, where reflector amplitudes are especially high (zones A and B in Fig. 8.14), suggest the presence of fractures and fluids that have penetrated along a thrust décollement surface (Section 8.2) at the base of the brittle seismo-genic zone (Fuis et al., 2001). The pattern implies that the décollement is associated with a weak, ductilely flowing crust beneath the brittle upper crust.

The structure of the subcontinental mantle beneath the Transverse Ranges has been studied using the principles of seismic anisotropy (Section 2.1.8). In this region a near vertical, 60- to 80-km-wide, high velocity, high density body extends some 200 km downward into the upper mantle below the surface trace of the San Andreas Fault (Kohler, 1999). The significance of the anomaly is uncertain, but it may represent a zone of sinking material that helps to drive lower crustal flow

1857 7.8 FORT TEJON

1857 7.8 FORT TEJON

Figure 8.14 Schematic block diagram showing the three-dimensional geometry of active faults of the Los Angeles region (image provided by G. Fuis and modified from Fuis et al., 2001, with permission from the Geological Society of America). Moderate and large earthquakes are shown with black stars, dates, and magnitudes. Small white arrows show block motions in vicinities of bright reflective regions A and B. Large white arrows show relative convergence direction of Pacific and North American plates. Regions A and B are zones of cracks that transport fluids migrating up from depth. A décollement surface ascends from cracked region A at San Andreas Fault, above which brittle upper crust is imbricated along thrust and reverse faults and below which lower crust is flowing toward San Andreas Fault (black arrows), depressing the Moho. Mantle of Pacific plate sinks beneath the San Gabriel Mountains.

Figure 8.14 Schematic block diagram showing the three-dimensional geometry of active faults of the Los Angeles region (image provided by G. Fuis and modified from Fuis et al., 2001, with permission from the Geological Society of America). Moderate and large earthquakes are shown with black stars, dates, and magnitudes. Small white arrows show block motions in vicinities of bright reflective regions A and B. Large white arrows show relative convergence direction of Pacific and North American plates. Regions A and B are zones of cracks that transport fluids migrating up from depth. A décollement surface ascends from cracked region A at San Andreas Fault, above which brittle upper crust is imbricated along thrust and reverse faults and below which lower crust is flowing toward San Andreas Fault (black arrows), depressing the Moho. Mantle of Pacific plate sinks beneath the San Gabriel Mountains.

and enhance crustal contraction beneath the Transverse Ranges (Godfrey et al., 2002).

Measurements of shear wave (SKS) splitting have revealed an anisotropic upper mantle whose properties change with depth beneath the northern and central segments of the San Andreas Fault (Ozalaybey & Savage, 1995; Hartog & Schwartz, 2001). Ozalaybey & Savage (1995) interpreted these data in terms of two superimposed layers. The lower layer contains an east-west direction of fast polarization that may originate from asthenospheric flow caused by the migration of the Mendocino triple junction ~15 million years ago. Alternatively, the pattern may reflect a fossil anisot-ropy. The upper layer contains a fast polarization direction that parallels the trace of the San Andreas Fault and is well expressed on the northeast side of the San Andreas Fault where the lithosphere is relatively thin and hot. It is poorly developed on the southwest side where the lithosphere is relatively thick. The localization of this upper layer near the San Andreas Fault suggests that the anisotropy originates from deformation in a steep 50- to 100-km-wide mantle shear zone (Teyssier & Tikoff, 1998). Its thickness is not well constrained but it may reach 115-125 km thick and involve the asthenospheric mantle. The change in polarization direction with depth directly below the fault could result from either a change in the amount of strain due to right lateral shearing (Savage, 1999) or a change in strain direction (Hartog & Schwartz, 2001). Additional work is needed to establish the relationship between the postulated mantle shear zone and faulting in the upper crust.

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