Measuring The Strength Of Transforms

Measures of the strength of continental transforms and large strike-slip faults provide a potentially useful means of testing models of continental rheology and evaluating the driving forces of continental deformation (Section 8.5.1). In many intraplate areas, the long-range (1000-5000 km) uniformity of stress orientations and their relative magnitudes inferred from measures of strain or displacement suggest that plate-driving forces provide the largest component of the total stress field (Zoback, 1992). Models of GPS-derived horizontal velocities in some regions, such as southern California, tend to support this view (McCaffrey, 2005). However, in other areas, such as the Basin and Range Province (Section 7.3), stresses caused by lateral variations in crustal buoyancy (Section 7.6.3) also appear to contribute significantly to the horizontal stress field (Sonder & Jones, 1999; Bennett et al., 2003).

There have been numerous attempts to evaluate the strength of the San Andreas Fault using various geologic and geophysical indicators (Zoback et al., 1987; Zoback, 2000). For some fault segments (Fig. 8.25), stress data suggest that the direction of maximum horizontal compression (01, Section 2.10.1) lies at a high angle (P) to the fault zone. In central California these angles are as high as P = 85°. In southern California they are lower at P = 68° (Townend & Zoback, 2004). These observations are problematic because classical theories of faulting (Section 2.10.2) cannot explain compression at high angles to a strike-slip fault with such a small component of convergence. Moreover, in the case of the San Andreas Fault, a paradox exists in that heat flow observations (Lachenbruch & Sass, 1992) show no frictionally generated heat, so that the fault must slip in response to very low shear stresses.

One possible explanation of the high-angle stress directions in California is that the San Andreas is an extremely weak fault that locally reorients the regional stresses (Mount & Suppe, 1987; Zoback et al., 1987; Zoback, 2000). In this interpretation, shear stresses far from the fault are high and contained by the frictional strength of the crust, but shear stresses on planes parallel to the "weak" faults of the San Andreas system must be quite low. Consequently, the principal stresses become reoriented so as to minimize shear stresses on planes parallel to the San Andreas Fault. This requires a rotation such that the direction of maximum horizontal compressive stress (01) becomes nearly orthogonal to the fault if the regional compression direction is at an angle in excess of 45° to the fault, which occurs at present. However, if this angle is less than 45°, the maximum horizontal compression is rotated into approximate parallelism with the fault. This latter type of rotation may have characterized the San Andreas Fault at some time in the past when relative plate motions were different than they are now.

This model of a weak continental strike-slip fault offers one explanation of conflicting geologic and geophysical data in California. However, alternative interpretations involving a strong or an intermediate-strength San Andreas Fault also have been proposed. These latter models are based on frictional theories of faulting, which suggest that 01 rotates to ~45° from the fault trace within a ~20-30-km-wide zone in the Big Bend region (Scholz, 2000). Scholz (2000) interpreted reports of high 01 angles in this area as representing local stresses related to folding instead of regional stresses. He also concluded that the presence and sense of the stress rotation fits predictions of a strong fault rather than a weak one. High fluid pressure (Section 8.6.3) is a possible mechanism for decreasing the strength of the fault and could explain some rotation of the stresses (Rice, 1992). Alternatively, the strength of the fault and the adjacent crust generally could be much lower than predicted by considerations of fault mechanics (Hardebeck & Michael, 2004).

These conflicting observations and interpretations concerning the strength of large strike-slip faults have yet to be resolved. In the case of the San Andreas Fault, part of the controversy may be related to different mechanical behaviors of the creeping versus locked

Surface Trace of San Andreas Fault Middle Mountain_

Surface Trace of San Andreas Fault Middle Mountain_

Figure 8.25 (a) Maximum horizontal compression from southern California (image provided by J. Townend and M. Zoback and modified from Townend & Zoback, 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Stress determinations are as follows: inward-point arrows, borehole breakouts; stars, hydraulic fracturing experiments;plain straight lines, earthquake focal mechanisms. Inset summarizes the angle (b) between the maximum principal compressive stress and the local fault strike within 10 km of the San Andreas Fault (SAF). The angle of 68 ± 7° suggests a relatively low frictional strength for a 400-km-long fault segment. (b) Vertical profile showing location of SAFOD drill hole experiment near Parkfield California (after Hickman et al., 2004, with permission from the American Geophysical Union). Magnetotelluric resistivity readings are from Unsworth & Bedrosian (2004). White circles are earthquake hypocenters. Ovals in drill holes represent down-hole sensors. Contours show resistivity in ohm-meters.

Figure 8.25 (a) Maximum horizontal compression from southern California (image provided by J. Townend and M. Zoback and modified from Townend & Zoback, 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Stress determinations are as follows: inward-point arrows, borehole breakouts; stars, hydraulic fracturing experiments;plain straight lines, earthquake focal mechanisms. Inset summarizes the angle (b) between the maximum principal compressive stress and the local fault strike within 10 km of the San Andreas Fault (SAF). The angle of 68 ± 7° suggests a relatively low frictional strength for a 400-km-long fault segment. (b) Vertical profile showing location of SAFOD drill hole experiment near Parkfield California (after Hickman et al., 2004, with permission from the American Geophysical Union). Magnetotelluric resistivity readings are from Unsworth & Bedrosian (2004). White circles are earthquake hypocenters. Ovals in drill holes represent down-hole sensors. Contours show resistivity in ohm-meters.

segments of the fault or to the different methods of inferring stresses. To resolve these problems independent measurements of principal stress orientations and magnitudes from within large, tectonically active faults are needed. The San Andreas Fault Observatory at Depth (SAFOD) drilling program involves such measurements. This program involves drilling into the hypocentral zone of repeating M ~ 2 earthquakes on a creeping segment of the San Andreas Fault near Park-field, California (Fig. 8.1) at a depth of about 3 km. The goals include establishing an observatory in close prox imity to these repeating earthquakes to obtain down-hole measurements of the physical and chemical conditions under which earthquakes occur and to exhume rock and fluid samples for laboratory analyses (Hickman et al., 2004). Although there is still considerable uncertainty in the preliminary estimates of horizontal stress magnitudes, stress observations near the bottom of a 2.2-km-deep pilot hole (Fig. 8.25b) (Hickman & Zoback, 2004) and heat flow measurements (Williams et al., 2004) suggest a locally weak San Andreas Fault in an otherwise strong crust.

Subduction zones

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