REVIEWS OF GEOPHYSICS. SUPPLEMENT. PAGES 379-383. JULY 1995 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1991-1994 Lithosphere dynamics and continental deformation Peter Bird Department of Earth and Space Sciences, University of California, Los Angeles The unifying theme in this section is the remarkable Andreas fault [Olson and Zoback, 1992. Mount and weakness of major faults. I will consider the diverse Suppe [1992 collected borehole-elongation data new evidence for weakness, and the evidence for high showed that in both California and Sumatra, the most re localized in faults as a fundamental cause compressive(o,)direction is 70-90 from major strike With this background one can better understand why slip faults, requiring them to be very low-friction faults remain active even after large rotations with surfaces. In the Sumatran arc the minimum angle respect to stress: I will look at large Neogene($23.7 between slip vectors and the trench is 65-75, implying million year old) rotations about horizontal axes in the that the dextral fault along the arc is no stronger than the Basin and Range province, and about vertical axes along water-lubricated subduction shear zone [McCaffrey the Pacific margin. Recent developments will be 1992] summarized from studies of Neogene tectonics In one of the most exciting developments of the last (neotectonics)in California, Alaska, and the Mississippi four years, Hauksson [1994] showed that the regional embayment, in the context of a weak North American stress field in the area of the Landers earthquake stress field that results mainly from topographic forces permanently rotated 7-20 in that event, with the local To close, I will present new geophysical studies relevant rotation approximately proportional to the local fault to the continuing controversy over whether the basic slip(and stress drop). This proves that the seismic structure of the North American mantle lithosphere was stress drop was a large fraction of the initial shear stress, altered by an early Tertiary episode of flat subduction and therefore that the fault was weak even at the start of slip. This method of quantitative shear stress estimation Weakness of Major Faults is unique in the large volume of crust that it samples, unfortunately, it can only be applied where a large Both laboratory and in-situ studies of crustal rocks earthquake falls within a well-established seismic far from faults have typically shown them to have high network coefficients of friction (0.65-0.85, dimensionless) a nonlinear thin-plate finite-element model of Zoback et al. [1993] find such behavior to 6 km depth in California and its faults [Bird and Kong, 1994] was he ktb borehole in Germany. How optimized with respect to geologic, geodetic, and stress something different about major faults data, with the result that friction on major faults is only In the last Report, Hickman [1991] summarized the 0.. 17. Bird [ 1992b] found typical fault friction vidence for a weak San Andreas fault: the lack of a Alaska to be at least this low with the same method heat-flow anomaly, and of proven athermal a hint that fault weakness may persist for very long circulation to remove the heat by advection. The lack of times is given by the reactivation of Cretaceous thrust any heat-flow anomaly in the new Cajon Pass scientific faults in Wyoming-Utah as Quaternary normal faults borehole next to the san andreas fault limits the vertical [West.1993] integral of shear traction to 10 N/m[Lachenbruch and a nearly perpendicular/parallel Pore water at High pressure relationship between principal stress axes and the fault Schol et al. [1993 reviewed the literature on new plane limits the possible shear tractions to low values faults, and concluded that they are self-similar, implying In-situ stress measurements in the Cajon Pass hole show that the strain-weakening that created the fault is that crustal blocks have high friction internally, but that followed by further slip-weakening. However, since the most-compressive horizontal principal stress major slip-weakening is not seen in the laboratory, local direction (o, is perpendicular to the San Andreas, anomalous pore pressure is a more popular explanation precluding any dextral shear on that fault today [zoback for fault weakness. Such pressures would have to and Healy, 1992]. Next to the southern San Andreas approach lithostatic pressure, which is the weight/area fault, Pliocene-Quaternary folds are forming with axes of the rock overburden parallel to the fault, and extension along these axes Saline hot springs in the California Coast Rang [Burgmann, 1991]. On the part of the San Andreas fault pell ancient fluids from Cretaceous shales(or that slipped in the Loma Prieta earthquake, stress sources)and imply some degree of anomalo directions from aftershocks imply no measurable shea pressure [Unruh et al, 1992. Fluid inclusions in the traction remaining after slip on the main fault [Zoback exhumed footwall of the Dixie Valley fault in Nevada and Beroza, 1993]. On the San Francisco peninsula, for record essentially lithostatic pore pressure at(305.C, 35 km north of the Loma Prieta rupture, fault plane 1.57x10 Pa)[Parry et aL., 1991]. Internal structures of solutions also show o, almost perpendicular to the San the San Gabriel and Punchbowl faults exhumed from 2REVIEWS OF GEOPHYSICS, SUPPLEMENT, PAGES 379-383, JULY 1995 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1991-1994 Lithosphere dynamics and continental deformation Peter Bird Department of Earth and Space Sciences, University of California, Los Angeles The unifying theme in this section is the remarkable weakness of major faults. I will consider the diverse new evidence for weakness, and the evidence for high pore pressure localized in faults as a fundamental cause. With this background one can better understand why faults remain active even after large rotations with respect to stress: I will look at large Neogene (≤23.7 million year old) rotations about horizontal axes in the Basin and Range province, and about vertical axes along the Pacific margin. Recent developments will be summarized from studies of Neogene tectonics (neotectonics) in California, Alaska, and the Mississippi embayment, in the context of a weak North American stress field that results mainly from topographic forces. To close, I will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the North American mantle lithosphere was altered by an early Tertiary episode of flat subduction. Weakness of Major Faults Both laboratory and in-situ studies of crustal rocks far from faults have typically shown them to have high coefficients of friction (0.65-0.85, dimensionless). Zoback et al. [1993] find such behavior to 6 km depth in the KTB borehole in Germany. However, there is something different about major faults. In the last Report, Hickman [1991] summarized the evidence for a weak San Andreas fault: the lack of a heat-flow anomaly, and of proven hydrothermal circulation to remove the heat by advection. The lack of any heat-flow anomaly in the new Cajon Pass scientific borehole next to the San Andreas fault limits the vertical integral of shear traction to 1011 N/m [Lachenbruch and Sass, 1992]. In many places, a nearly perpendicular/parallel relationship between principal stress axes and the fault plane limits the possible shear tractions to low values. In-situ stress measurements in the Cajon Pass hole show that crustal blocks have high friction internally, but that the most-compressive horizontal principal stress direction (σ1 ) is perpendicular to the San Andreas, precluding any dextral shear on that fault today [Zoback and Healy, 1992]. Next to the southern San Andreas fault, Pliocene-Quaternary folds are forming with axes parallel to the fault, and extension along these axes [Burgmann, 1991]. On the part of the San Andreas fault that slipped in the Loma Prieta earthquake, stress directions from aftershocks imply no measurable shear traction remaining after slip on the main fault [Zoback and Beroza, 1993]. On the San Francisco peninsula, for 35 km north of the Loma Prieta rupture, fault plane solutions also show σ1 almost perpendicular to the San Andreas fault [Olson and Zoback, 1992]. Mount and Suppe [1992] collected borehole-elongation data and showed that in both California and Sumatra, the most compressive (σ1 ) direction is 70-90° from major strike￾slip faults, requiring them to be very low-friction surfaces. In the Sumatran arc the minimum angle between slip vectors and the trench is 65-75°, implying that the dextral fault along the arc is no stronger than the water-lubricated subduction shear zone [McCaffrey, 1992]. In one of the most exciting developments of the last four years, Hauksson [1994] showed that the regional stress field in the area of the Landers earthquake permanently rotated 7-20° in that event, with the local rotation approximately proportional to the local fault slip (and stress drop). This proves that the seismic stress drop was a large fraction of the initial shear stress, and therefore that the fault was weak even at the start of slip. This method of quantitative shear stress estimation is unique in the large volume of crust that it samples; unfortunately, it can only be applied where a large earthquake falls within a well-established seismic network. A nonlinear thin-plate finite-element model of California and its faults [Bird and Kong, 1994] was optimized with respect to geologic, geodetic, and stress data, with the result that friction on major faults is only 0.12-0.17. Bird [1992b] found typical fault friction in Alaska to be at least this low, with the same method. A hint that fault weakness may persist for very long times is given by the reactivation of Cretaceous thrust faults in Wyoming-Utah as Quaternary normal faults [West, 1993]. Pore Water at High Pressure Scholz et al. [1993] reviewed the literature on new faults, and concluded that they are self-similar, implying that the strain-weakening that created the fault is followed by further slip-weakening. However, since major slip-weakening is not seen in the laboratory, local anomalous pore pressure is a more popular explanation for fault weakness. Such pressures would have to approach lithostatic pressure, which is the weight/area of the rock overburden. Saline hot springs in the California Coast Ranges ex￾pell ancient fluids from Cretaceous shales (or deeper sources) and imply some degree of anomalous pore pressure [Unruh et al., 1992]. Fluid inclusions in the exhumed footwall of the Dixie Valley fault in Nevada record essentially lithostatic pore pressure at (305°C, 1.57×106 Pa) [Parry et al., 1991]. Internal structures of the San Gabriel and Punchbowl faults, exhumed from 2-