Page 247 - Global Tectonics

CONTINENTAL TRANSFORMS AND STRIKE-SLIP FAULTS 233

of the crust. In this view, the velocity fi eld commonly show that elements of both continuous and discontinu-

is assumed to represent the average deformation of the ous representations fit the observations in many areas.
whole lithosphere, which consists of a thin layer (10– In this section, the results of geodetic measurements

20 km) that deforms by faulting above a thick layer and velocity field modeling are discussed in the context
(80–100 km) that deforms by ductile creep (Jackson, of the San Andreas Fault system, whose structural
2004). In rigid block models, faults are predicted to be diversity illustrates the variety of ways in which strain
widely spaced, slip rapidly, and extend vertically through may be accommodated along and adjacent to continen-
the entire lithosphere, terminating as large ductile shear tal transforms.
zones in the upper mantle (McCaffrey, 2005). These
latter properties suggest that deforming continental
lithosphere exhibits a type of behavior that resembles 8.5.2 Relative plate motions
plate tectonics. In both types of model, the deformation

may be driven by a combination of forces, including and surface velocity fields
those acting along the edges of crustal blocks, basal
tractions due to the flow of the lower crust and upper In the southwestern USA, relative motion between the

mantle, and gravity. Pacific and North American plates occurs at a rate of
−1
Determining the degree to which continental defor- about 48–50 mm a (DeMets & Dixon, 1999; Sella et al.,
mation is continuous or discontinuous has proven dif- 2002). Geodetic and seismologic data suggest that up to
ficult to achieve with certainty in many areas. One 70% of this motion presently may be accommodated

reason for the difficulty is that the short-term (decade- by dextral slip on the San Andreas Fault (Argus &

scale) surface velocity field measured with geodetic data Gordon, 2001). Out of a total of approximately 1100–
usually appears continuous at large scales (kilometers 1500 km of strike-slip motion, since the Oligocene
to hundreds of kilometers) (Section 2.10.5). This char- (Stock & Molnar, 1988), only 300 and 450 km of right
acteristic results because geodetic positioning tech- lateral slip have accumulated along the southern and

niques provide velocity estimates at specific points in northern reaches of the San Andres Fault, respectively
space, with the density of available points depending on (Dillon & Ehlig, 1993; James et al., 1993). The remaining
the region and the scale of the investigation (Bos & movements, therefore, must be accommodated else-
Spakman, 2005). Most interpretation methods start where within the diffuse zone of deformation that
with some interpolation of the geodetic data, with the stretches from the Coast of California to the Basin and

final resolution depending on the number and distribu- Range (Fig. 8.1).
tion of the available stations (Jackson, 2004). In addi- Along its ∼1200 km length, the San Andreas Fault is
tion, available information on fault slip rates is divided into segments that exhibit different short-term
incomplete and those that are calculated over short tim- mechanical behaviors. Some segments, such as those
escales may not be representative of long-term slip rates located north of Los Angeles and north of San Fran-
(Meade & Hager, 2005; McCaffrey, 2005). If accurate cisco, have ruptured recently, generating large historical
long-term slip rates on all crustal faults were available, earthquakes, and now exhibit little evidence of slip.
then the problem could be solved directly. Furthermore, These segments appear locked at depth and are now
even though continental deformation is localized along accumulating significant nonpermanent (elastic) strains

faults over the long term, the steady-state motion of an near the surface, making them a major potential earth-
elastic upper crust over the short-term contains little quake hazard (Section 2.1.5). Between the two locked
information about the rheology of the deforming mate- segments is a 175-km-long fault segment in central
rial, so the importance of the faults in the overall California that is characterized by aseismic slip, shallow
mechanical behavior of the lithosphere is unclear. This (<15 km depth) microearthquakes, and few large his-
latter uncertainty clouds the issue of whether deforma- torical earthquakes. Along this segment, the aseismic

tion along continental transforms is driven mostly by slip reflects a relatively steady type of creep that results
edge forces, basal tractions, or gravitational forces from frictional properties promoting stable sliding on
(Savage, 2000; Zatman, 2000; Hetland & Hager, 2004) the fault plane (Scholz, 1998). These different behaviors,
(Section 8.5.3). and especially the occurrence of aseismic creep on or

Despite the difficulties involved in quantifying con- near faults at the surface, complicate the estimation of
tinental deformation, geoscientists have been able to horizontal velocity fields (Section 8.5.3).