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 2
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 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 strikeslip 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 expell 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-
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 380 5 km, show up to 50% deformed hydrothermal vein bending, and continue to slip due to an aquired material in the central ultracataclasite zones. which are weakness. Some compensating deformation of the only 1-10 m thick [Chester et al, 1993]. The presence hanging wall would be required; in fact, almost all of veins becomes significant if one accepts that these hanging walls seen in the Basin and Range province are faults probably slipped at low shear stresses, with small extensively fractured and faulted shear stresses, pore pressure that is equal to the least An important new idea which has arisen in the last compressive principal stress(to open a crack) cannot be four years is that the lower crust of the Basin and Range ery much less than lithostatic province should behave as a viscous fluid, flowing in to The source of high pore pressures is less clear fill the voids that extreme extension would otherwise form. Simple two-dimensional calculations [Bird, of fault gouges creates these high pressures, and that 1991] show that in this province the high heat-flow some earthquake precursors are due to fluid flow when should cause Moho topography to be destroyed in 1-20 barriers are breached between separate reservoirs. Rice million years(Ma) by lateral extrusion of the lower [1992] presented a different model, in which water from crust. In fact, observed variations of crustal thickness the edge of the Basin/Range province are much less tha faults, creating haloes of high pore pressure, due to they would be in balanced cross-sections, implying pressure-dependent permeability. (The self-sealing of either lateral extrusion or massive intrusion; McCarthy unidirectional hydrothermal systems due to silica pre and Parsons [1994] use seismic data to limit the amount cipitation may also be important of intrusion If high pore pressure is localized only along faults, it The spatial and temporal relationships between the should cause local rotation of the principal stress axes, different core complexes(the integration of regional and of secondary shear surfaces; in fact, this is observed velocity fields) and their ultimate cause remain along the San Andreas fault [Byerlee, 1992]- problems for the future. One place where there is clarity is in a benchmark study of the patterns of fault Extension in the basin and Range province around the moving Yellowstone plume, by Pierce and Morgan [1992]; because much of this deformation was The argument about the kinematic history clearly distinct from earlier distributed Basin/Range detachment faults in metamorphic core complexes has extension, it may serve as a model for the interpretation become quite heated. One school believes that these of structure along other, more ancient plume tracks faults form and slip at high(65%)dip angles, but that the footwall bends to near-horizontal dip as it nears the surface. Buck [1993 extended this bending-footwall Block Rotations Along the Pacific margin model for normal fault rotation and showed why low A problem presented in the last report was the angle detachment faults should only be found in regions explanation of anomalous paleomagnetic declinations of anomalous heat flow. The classic example of the (mostly deflected clockwise, some over 90%)in the active normal fault along the front of the black Transverse Ranges of California. In a major revision of Mountains in Death Valley has been shown to be his previous model for Miocene rotations in California segmented in dip, from a maximum of 60w in Luyendyk [1991] that locally extreme subsurface to a minimum of 17 w where the footwall is extension occurred between rotating blocks, adding the exposed to the east [Miller, 1991] of freedom However, striking evidence has also been presented Nicholson et al.[ 1994] reconstructed the probable cause for the view that at least some detachments slip while in of this transtensional rotation event: the subducting a near-horizontal orientation. Dokka [1993]used a new Monterey plate froze onto the Pacific plate 20 Ma age technique of paleodepth determination to show that the suddenly changing the direction of shear tractions on the Newberry Mountains detachment in California had an nitial dip of only 20 to 27. The Rawhide detachment northeastward to northwestward fault in Arizona was active at a low dip scott and gon today, the remarkable discovery of Lister, 1992), as shown by a marker tuff and truncated seafloor fracture zones cutting upward through the normal faults(in a section that cannot be balanced)in forearc wedge on the North America plate [Goldfinger the upper plate. In southeast Arizona, a Miocene et al., 1992] suggest that the subduction zone is locked detachment with a dip of 20 down to 6 km also and that clockwise rotation must be occurring in a projects updip to within 100 m of Quaternary scarps dextral transpressive setting Johnson and Loy, 1992], which seems to show that the In the future, the interpretation of paleomagnetic data fault remains active at this dip will be much more open to all earth scientists, thanks to If well-developed faults of large slip are intrinsically the creation of a searchable database [Harbert, 1993 weak, however, these two views may not be compatible. Detachments faults could form at high dips(while still strong), rotate to low dips by footwall
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 380 5 km, show up to 50% deformed hydrothermal vein material in the central ultracataclasite zones, which are only 1-10 m thick [Chester et al., 1993]. The presence of veins becomes significant if one accepts that these faults probably slipped at low shear stresses; with small shear stresses, pore pressure that is equal to the leastcompressive principal stress (to open a crack) cannot be very much less than lithostatic. The source of high pore pressures is less clear. Byerlee [1993] proposed that interseismic compaction of fault gouges creates these high pressures, and that some earthquake precursors are due to fluid flow when barriers are breached between separate reservoirs. Rice [1992] presented a different model, in which water from the mantle or lower crust rises preferentially along faults, creating haloes of high pore pressure, due to pressure-dependent permeability. (The self-sealing of unidirectional hydrothermal systems due to silica precipitation may also be important.) If high pore pressure is localized only along faults, it should cause local rotation of the principal stress axes, and of secondary shear surfaces; in fact, this is observed along the San Andreas fault [Byerlee, 1992]. Extension in the Basin and Range Province The argument about the kinematic history of detachment faults in metamorphic core complexes has become quite heated. One school believes that these faults form and slip at high (65°) dip angles, but that the footwall bends to near-horizontal dip as it nears the surface. Buck [1993] extended this bending-footwall model for normal fault rotation and showed why lowangle detachment faults should only be found in regions of anomalous heat flow. The classic example of the active normal fault along the front of the Black Mountains in Death Valley has been shown to be segmented in dip, from a maximum of 60°W in the subsurface to a minimum of 17°W where the footwall is exposed to the east [Miller, 1991]. However, striking evidence has also been presented for the view that at least some detachments slip while in a near-horizontal orientation. Dokka [1993] used a new technique of paleodepth determination to show that the Newberry Mountains detachment in California had an initial dip of only 20 to 27°. The Rawhide detachment fault in Arizona was active at a low dip [Scott and Lister, 1992], as shown by a marker tuff and truncated normal faults (in a section that cannot be balanced) in the upper plate. In southeast Arizona, a Miocene detachment with a dip of 20° down to 6 km also projects updip to within 100 m of Quaternary scarps [Johnson and Loy, 1992], which seems to show that the fault remains active at this dip. If well-developed faults of large slip are intrinsically weak, however, these two views may not be incompatible. Detachments faults could form at high dips (while still strong), rotate to low dips by footwall bending, and continue to slip due to an aquired weakness. Some compensating deformation of the hanging wall would be required; in fact, almost all hanging walls seen in the Basin and Range province are extensively fractured and faulted. An important new idea which has arisen in the last four years is that the lower crust of the Basin and Range province should behave as a viscous fluid, flowing in to fill the voids that extreme extension would otherwise form. Simple two-dimensional calculations [Bird, 1991] show that in this province the high heat-flow should cause Moho topography to be destroyed in 1-20 million years (Ma) by lateral extrusion of the lower crust. In fact, observed variations of crustal thickness at the edge of the Basin/Range province are much less than they would be in balanced cross-sections, implying either lateral extrusion or massive intrusion; McCarthy and Parsons [1994] use seismic data to limit the amount of intrusion. The spatial and temporal relationships between the different core complexes (the integration of regional velocity fields) and their ultimate cause remain problems for the future. One place where there is clarity is in a benchmark study of the patterns of faulting around the moving Yellowstone plume, by Pierce and Morgan [1992]; because much of this deformation was clearly distinct from earlier distributed Basin/Range extension, it may serve as a model for the interpretation of structure along other, more ancient plume tracks. Block Rotations Along the Pacific Margin A problem presented in the last report was the explanation of anomalous paleomagnetic declinations (mostly deflected clockwise, some over 90°) in the Transverse Ranges of California. In a major revision of his previous model for Miocene rotations in California, Luyendyk [1991] now proposes that locally extreme extension occurred between rotating blocks, adding the degrees of freedom to make rotation possible. Nicholson et al. [1994] reconstructed the probable cause of this transtensional rotation event: the subducting Monterey plate froze onto the Pacific plate 20 Ma ago, suddenly changing the direction of shear tractions on the base of the margin of North America from northeastward to northwestward. In Oregon today, the remarkable discovery of seafloor fracture zones cutting upward through the forearc wedge on the North America plate [Goldfinger et al., 1992] suggest that the subduction zone is locked, and that clockwise rotation must be occurring in a dextral transpressive setting. In the future, the interpretation of paleomagnetic data will be much more open to all earth scientists, thanks to the creation of a searchable database [Harbert, 1993]
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 381 Stress Field of North America the explanation for the Quaternary Wilshire arch under The publication of the World Stress Map has Los Angeles and Hollywood [Hummon et al, 1994] focussed thinking on the big picture, Zoback [1992] showed that to first order the world stress field is the Alaskan neotectonics result of ridge-push and continental collision resistance, with little evidence for strong basal tractions A compilation of 621 stress indicators in Alaska Richardson and Reding [1992] used thin-plate elastic shows a fan pattern of o, directions radiating from the syntaxis in the St. Elias range [Estabrook and Jacob, models to show that both shear and super-lithostatic normal tractions on the san andreas and caribbean 1991- In a uniform elastic material, such a stress pattern would suggest a stress singularity, or at least transforms are only 5 to 10x10 Pa, and that this value high values. However, a nonlinear thin-plate finite can be explained primarily by ridge-push effects. A collection of surface-wave moment tensors from 51 element model with faults [Bird, 1992b] shows that the western U.S. events confirms the o. directions of the stress magnitudes are low, and that the pattern is due to World Stress Map, and shows a convergence of fault with terrane collision of the yakutat block. thus tensional axes(o3) on the Mendocino triple junction Alaska is no exception to the general weakness of major As efforts are gradually applied to study paleostress faults. The same model also predicted rapid westward tra attention should be paid to the exhaustive study of consistent with the evidence of seismic slip vectors Bergerat et al. [1992 who used joints and faults to infer a 9-stage stress history for the Colorado Plateau [McCaffrey, 1992] the Jul be calcite twinning in limestones, which Craddock et al. [1993] Neotectonics of the new madrid seismic used to map the Paleozoic strain(and paleostress)field in the eastern U.S Li and Schweig [1993] questioned the view that the Mississippi embayment and New Madrid seismic zone California Neotectonics represents simple reactivations of the Precambrian Reelfoot rift. After observed S-waves that were Furlong [1993] presented an elegant synthesis of converted from P-waves to improve their velocity how the northward migration of the Mendocino triple model, chiu et al.[1993] relocated events and found a junction removed the Gorda plate from under North America, creating a "mantle San Andreas transform simple Califormia-type pattern with thrusting in a left which was to the northeast of the surficial margin; with step of a dextral strike-slip system. The important time, surficial fault activity jumped inboard, as from the questions remaining concern the overall rate of San Andreas to the Hayward-Calaveras fault system in deformation, and how(if) these faults connect to plate the San Francisco Bay area. The complicating effects of boundaries at each end. Liu et al. [1992] reoccupied a slightly convergent Pacific plate velocity since 7 Ma triangulation stations with GPS and found rapid strain are being recognized; in particular, seismic studies show accumulation (0.08 urad/a) with a N67 E shortening probable oceanic crust of the Pacific(?) plate direction(in agreement with stress data) and a relative underthrust along the California margin from Morro elocity of 5-7 mm/year across the network. Such a rate Bay north to San Francisco Page and Brocher, 1993] would obviously imply significant seismic hazard along strike from the 1811-1812 The maturity of California neotectonic studies is shown by the fact that Bird and Kong [1994] were able confirmation of the result by homogeneous geodetic to predict fault slip rates and geodesy to within 3 methods should be a high priority. If it is confirmed, we mm/year in a finite-element model; such models may may have a unique opportunity to study new faults in a serve as supplements to hard data in seismic hazard relatively simple midcontinental setting In the area of hazard studies, it is being realized that Huge Displacements of the Mantle few new(Pliocene-Quaternary) thrust faults in the Lithosphere Transverse Ranges actually break the surface, instead anticlines involving Holocene sediments should b Bird[1992a;,1994 assumed to overlie seismically dangerous blind thrusts formation of the Rocky Mountains and the Basin/ Shaw and Suppe [1994 used reflection sections and province by a single flat subduction event, whi balanced-section methods to infer three active thrust suggested had sheared away and displaced the entire under the Santa Barbara Channel, with slip rates that tectonic mantle lithosphere of the western U.S dd up to 3 mm/year (or half the geodetically According to this model, the only tectonic mantle determined rate of shortening ). A Wilshire fault with a lithosphere( defined as cold and strong) remaining in the slip rate of 1. 5-3.2 mm/year has also been proposed western U.S. should be a =40-km layer which has formed by cooling since mid-Tertiary times
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 381 Stress Field of North America The publication of the World Stress Map has focussed thinking on the big picture; Zoback [1992] showed that to first order the world stress field is the result of ridge-push and continental collision resistance, with little evidence for strong basal tractions. Richardson and Reding [1992] used thin-plate elastic models to show that both shear and super-lithostatic normal tractions on the San Andreas and Caribbean transforms are only 5 to 10×106 Pa, and that this value can be explained primarily by ridge-push effects. A collection of surface-wave moment tensors from 51 western U.S. events confirms the σ1 directions of the World Stress Map, and shows a convergence of tensional axes (σ3 ) on the Mendocino triple junction [Patton and Zandt, 1991]. As efforts are gradually applied to study paleostress, attention should be paid to the exhaustive study of Bergerat et al. [1992], who used joints and faults to infer a 9-stage stress history for the Colorado Plateau since the Jurassic. Another source of data can be calcite twinning in limestones, which Craddock et al. [1993] used to map the Paleozoic strain (and paleostress) field in the eastern U.S. California Neotectonics Furlong [1993] presented an elegant synthesis of how the northward migration of the Mendocino triple junction removed the Gorda plate from under North America, creating a "mantle San Andreas transform" which was to the northeast of the surficial margin; with time, surficial fault activity jumped inboard, as from the San Andreas to the Hayward-Calaveras fault system in the San Francisco Bay area. The complicating effects of a slightly convergent Pacific plate velocity since 7 Ma are being recognized; in particular, seismic studies show probable oceanic crust of the Pacific(?) plate underthrust along the California margin from Morro Bay north to San Francisco [Page and Brocher, 1993]. The maturity of California neotectonic studies is shown by the fact that Bird and Kong [1994] were able to predict fault slip rates and geodesy to within 3 mm/year in a finite-element model; such models may serve as supplements to hard data in seismic hazard estimation. In the area of hazard studies, it is being realized that few new (Pliocene-Quaternary) thrust faults in the Transverse Ranges actually break the surface; instead, anticlines involving Holocene sediments should be assumed to overlie seismically dangerous blind thrusts. Shaw and Suppe [1994] used reflection sections and balanced-section methods to infer three active thrusts under the Santa Barbara Channel, with slip rates that add up to 3 mm/year (or half the geodeticallydetermined rate of shortening). A Wilshire fault with a slip rate of 1.5-3.2 mm/year has also been proposed as the explanation for the Quaternary Wilshire arch under Los Angeles and Hollywood [Hummon et al., 1994]. Alaskan Neotectonics A compilation of 621 stress indicators in Alaska shows a fan pattern of σ1 directions radiating from the syntaxis in the St. Elias range [Estabrook and Jacob, 1991]. In a uniform elastic material, such a stress pattern would suggest a stress singularity, or at least high values. However, a nonlinear thin-plate finiteelement model with faults [Bird, 1992b] shows that the stress magnitudes are low, and that the pattern is due to the juxtaposition of transpression on the Fairweather fault with terrane collision of the Yakutat block. Thus, Alaska is no exception to the general weakness of major faults. The same model also predicted rapid westward transport of the west Aleutian forearc, which is consistent with the evidence of seismic slip vectors [McCaffrey, 1992]. Neotectonics of the New Madrid Seismic Zone Li and Schweig [1993] questioned the view that the Mississippi embayment and New Madrid seismic zone represents simple reactivations of the Precambrian Reelfoot rift. After observed S-waves that were converted from P-waves to improve their velocity model, Chiu et al. [1993] relocated events and found a simple California-type pattern with thrusting in a left step of a dextral strike-slip system. The important questions remaining concern the overall rate of deformation, and how (if) these faults connect to plate boundaries at each end. Liu et al. [1992] reoccupied triangulation stations with GPS and found rapid strain accumulation (0.08 µrad/a) with a N67°E shortening direction (in agreement with stress data) and a relative velocity of 5-7 mm/year across the network. Such a rate would obviously imply significant seismic hazard along strike from the 1811-1812 aftershock zone, so confirmation of the result by homogeneous geodetic methods should be a high priority. If it is confirmed, we may have a unique opportunity to study new faults in a relatively simple midcontinental setting. Huge Displacements of the Mantle Lithosphere? Bird [1992a; 1994] modeled the simultaneous formation of the Rocky Mountains and the Basin/Range province by a single flat subduction event, which he suggested had sheared away and displaced the entire tectonic mantle lithosphere of the western U.S.. According to this model, the only tectonic mantle lithosphere (defined as cold and strong) remaining in the western U.S. should be a ≈40-km layer which has formed by cooling since mid-Tertiary times
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION Geochemical objections have been raised to this model Bird, P, and X. Kong, Computer leg, Livaccari and Perry, 1993: 1994, especially that nth of major faults. Geol. geochemical lithosphere(defined by certain element and sotope concentrations) is still present. But several Bird, P, Isotopic evidence for preservation of Cordilleran ithospheric mantle during the Sevier-Laramide orogeny recent studies have recently shown that the seismic lithosphere ( defined by high velocity and low 1994 United States: Comment, Geology, attenuation) has roughly the predicted structure Buck, W.R., Effect of lithospheric thickness on the formation of high- and low-angle normal faults, Geology, 21,933- Humphreys and Dueker[1994] performed a regional 936.1993 tomographic inversion which confirmed that upper Burgmann, R, Transpression along the southen San Andreas mantle seismic velocities are systematically slower in ult, Durmid Hill, California, Tectonics, 10. 1152-1163 the western U. S. than in the east, with the differences confined to the uppermost 300 km. In a profile from Byerlee, J, The change in orientation of subsidiary shears near Utah to Kansas. p-s conversions at the moho show that the crust thickens eastward from the colorado Byerlee, J, Model for episodic flow of high-pressure water in Plateau to the Great Plains, so that the high topography ult zones before earthquakes, Geology, 21, 303-306. of the former must be compensated in the mantle Chester. F.M. J. P. Evans. and R. L. Biegel Internal structure [Sheehan et al, 1992]. An inversion of teleseismic data and weakening mechanisms of the San Andreas fault, J. alderman and Davis, 1991] shows mantle lithosphere Geophys.Res,98,771-786,1993 is 80 km thicker on the east side of the rio grande rift Chiu, J-M, A. C. Johnston, and R. B. Herrmann, A than on the west. This compares well with analysis of collaborative research: Analysis of PANDA data and gravity data by Cordell et al. [1991] which shows continuation of PANDA experiment Madrid seismic mantle lithosphere thicknesses of 200 km to the East, but only 50-125 km to the west. Also, Beghoul et al. U.S. Geol. Sur. Open -File Rep, 93-195, 247-250, 1993 [1993]used teleseismic travel times to show that mantle Cordell, L, Y. A. Zorin, and G. R. Keller, The decompensative ithosphere is typically 20-50 km under the Basin/Range Grande rift, J Geophys. Res, 96. 6557-6568, 1991 and Colorado Plateau, but 150-190 km under the Great Craddock, J P, M. Jackson, B. A. van der Pluijm, and R. T. Plains. A tomographic image of uppermost-mantle(Pn) Versical, Regional shortening fabrics in eastern North velocity in the western U.S. [Hearn et al, 1991] shows he Appalachian- hat within the low-velocity region, local seismic Ouachita orogenic belt, Tectonics, 12 257-264, 1993 Dokka, R. K, Original dip and subsequent modification of a velocity is lowest in areas of Neogene extension, and along the Yellowstone plume track California, Geology, 21, 711-714, 1993 Teleseismic shear wave splitting and polarization Estabrook C.H. and K. h. jacob Stress ind provide an exciting new tool to determine the stretchi Neotectonics of North America direction of the upper mantle fabric. At 3 sites in the Slemmons. E. R. EngdahL. M. D. Zo Blackwell, pp. 387 west-central U.S.. these directions are east/northeast 1991 west/southwest [Silver and Chan, 1991]. If these Furlong, K. P, Thermal- rheologic evolution of the fabrics are in the lithosphere, they are inconsistent with mantle and the development of the San andreas Birds model; but if they are in the asthenosphere below, system, Tectonophysics, 223, 149-164 oldfinger, C, L D. Kulm, R S. Yeats, B. Applegate, M.E. they are entirely consistent ast shall MacKay, and G. F. Moore, Transverse structural trends subduction. Improved depth resolution should be a along the Oregon convergent margin: Implications for priority ntial and crustal rotations Halderman, T P, and P. M. Davis, Q beneath the Rio grande References nd B. L. Isacks, Lithosphe Harbert, W, Paleomagnetic database search possible, Eos structure of tibet and western North America: mechanisms Trans.AGU,74,100-101,1993. plift and a comparative study, J. Geophys. Res, 98 Hauksson, E, State of stress from focal mechanisms before Bergerat, F, C. Bouroz-Weil, and and after the 1992 Landers earthquake sequence, Bull er. Paleostress Seismol.Soc.Am,84,917-934,1994. Hearn, T, N. Beghoul, and M. Barazangi, Tomography of the U.S.A., Tectonophysics, 206, 219-2 Bird, P, Lateral extrusion of lower crust from under hgh hiceophys United States from re arrival times. Res,96,16,36%-16,381,1991 S.H., Stress in the lithosphere and 10,275-10,286,1991 Bird, P. Deformation and uplift of North America in the Union of Geodesy and Geophysics dited by M. A. Shea the IBM 1990 Contest Prize Papers, I, edited by K. R nd E. De ts, J F. Dolan, K.E. 105, Baldwin Press, Athens, Georgia, 1992a. ch, and G. J. Huftile, Wilshire fault: Earthquakes in Bird, P, Computer simulations of tectonics around Hollywood?, Geology, 22. 291-294, 1994 Alaskan syntaxis(abstract), Eos Trans. AGU, 73, Fall Humphreys, E. D, and K. G. Ducker, Western U.S.upper Meeting Suppl., 504, 1992b mantle structure, J. Geop/ys. Res, 99, 9615-9634, 1994
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 382 Geochemical objections have been raised to this model [e.g., Livaccari and Perry, 1993; 1994], especially that geochemical lithosphere (defined by certain element and isotope concentrations) is still present. But several recent studies have recently shown that the seismic lithosphere (defined by high velocity and low attenuation) has roughly the predicted structure. Humphreys and Dueker [1994] performed a regional tomographic inversion which confirmed that uppermantle seismic velocities are systematically slower in the western U.S. than in the east, with the differences confined to the uppermost 300 km. In a profile from Utah to Kansas, P→S conversions at the Moho show that the crust thickens eastward from the Colorado Plateau to the Great Plains, so that the high topography of the former must be compensated in the mantle [Sheehan et al., 1992]. An inversion of teleseismic data [Halderman and Davis, 1991] shows mantle lithosphere is 80 km thicker on the east side of the Rio Grande rift than on the west. This compares well with analysis of gravity data by Cordell et al. [1991] which shows mantle lithosphere thicknesses of 200 km to the East, but only 50-125 km to the west. Also, Beghoul et al. [1993] used teleseismic travel times to show that mantle lithosphere is typically 20-50 km under the Basin/Range and Colorado Plateau, but 150-190 km under the Great Plains. A tomographic image of uppermost-mantle (Pn) velocity in the western U.S. [Hearn et al., 1991] shows that within the low-velocity region, local seismic velocity is lowest in areas of Neogene extension, and along the Yellowstone plume track. Teleseismic shear wave splitting and polarization provide an exciting new tool to determine the stretching direction of the upper mantle fabric. At 3 sites in the west-central U.S., these directions are east/northeastwest/southwest [Silver and Chan, 1991]. If these fabrics are in the lithosphere, they are inconsistent with Bird's model; but if they are in the asthenosphere below, they are entirely consistent with past shallow-angle subduction. Improved depth resolution should be a priority. References Beghoul, N., M. Barazangi, and B. L. Isacks, Lithospheric structure of Tibet and western North America: Mechanisms of uplift and a comparative study, J. Geophys. Res., 98, 1997-2016, 1993. Bergerat, F., C. Bouroz-Weil, and J. Angelier, Paleostresses inferred from macrofractures, Colorado Plateau, western U.S.A., Tectonophysics, 206, 219-243, 1992. Bird, P., Lateral extrusion of lower crust from under high topography, in the isostatic limit, J. Geophys. Res., 96, 10,275-10,286, 1991. Bird, P., Deformation and uplift of North America in the Cenozoic era, in Scientific Excellence in Supercomputing: the IBM 1990 Contest Prize Papers, 1, edited by K. R. Billingsley, H. U. Brown, III, and E. Derohanes, pp. 67- 105, Baldwin Press, Athens, Georgia, 1992a. Bird, P., Computer simulations of tectonics around the Alaskan syntaxis (abstract), Eos Trans. AGU, 73, Fall Meeting Suppl., 504, 1992b. Bird, P., and X. Kong, Computer simulations of California tectonics confirm very low strength of major faults, Geol. Soc. Am. Bull., 106, 159-174, 1994. Bird, P., Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States: Comment, Geology, 22, 670-671, 1994. Buck, W. R., Effect of lithospheric thickness on the formation of high- and low-angle normal faults, Geology, 21, 933- 936, 1993. Burgmann, R., Transpression along the southern San Andreas fault, Durmid Hill, California, Tectonics, 10, 1152-1163, 1991. Byerlee, J., The change in orientation of subsidiary shears near faults containing pore fluid under high pressure, Tectonophysics, 211, 295-303, 1992. Byerlee, J., Model for episodic flow of high-pressure water in fault zones before earthquakes, Geology, 21, 303-306, 1993. Chester, F. M., J. P. Evans, and R. L. Biegel, Internal structure and weakening mechanisms of the San Andreas fault, J. Geophys. Res., 98, 771-786, 1993. Chiu, J.-M., A. C. Johnston, and R. B. Herrmann, A collaborative research: Analysis of PANDA data and continuation of PANDA experiment in the central New Madrid seismic zone, National Earthquake Hazards Reduction Program, Summaries of Technical Reports, 34, U.S. Geol. Surv. Open-File Rep., 93-195, 247-250, 1993. Cordell, L., Y. A. Zorin, and G. R. Keller, The decompensative gravity anomaly and deep structure of the region of the Rio Grande rift, J. Geophys. Res., 96, 6557-6568, 1991. Craddock, J. P., M. Jackson, B. A. van der Pluijm, and R. T. Versical, Regional shortening fabrics in eastern North America: Stress transmission from the AppalachianOuachita orogenic belt, Tectonics, 12, 257-264, 1993. Dokka, R. K., Original dip and subsequent modification of a Cordilleran detachment fault, Mojave extensional belt, California, Geology, 21, 711-714, 1993. Estabrook, C. H., and K. H. Jacob, Stress indicators in Alaska, in Neotectonics of North America, edited by D. B. Slemmons, E. R. Engdahl, M. D. Zoback, and D. D. Blackwell, pp. 387-399, Geol. Soc. Am., Boulder, Col., 1991. Furlong, K. P., Thermal-rheologic evolution of the upper mantle and the development of the San Andreas fault system, Tectonophysics, 223, 149-164, 1993. Goldfinger, C., L. D. Kulm, R. S. Yeats, B. Applegate, M. E. MacKay, and G. F. Moore, Transverse structural trends along the Oregon convergent margin: Implications for Cascadia earthquake potential and crustal rotations, Geology, 20, 141-144, 1992. Halderman, T. P., and P. M. Davis, Qp beneath the Rio Grande and East African Rift zones, J. Geophys. Res., 96, 10,113- 10,128, 1991. Harbert, W., Paleomagnetic database search possible, Eos Trans. AGU, 74, 100-101, 1993. Hauksson, E., State of stress from focal mechanisms before and after the 1992 Landers earthquake sequence, Bull. Seismol. Soc. Am., 84, 917-934, 1994. Hearn, T., N. Beghoul, and M. Barazangi, Tomography of the western United States from regional arrival times, J. Geophys. Res., 96, 16,369-16,381, 1991. Hickman, S. H., Stress in the lithosphere and the strength of active faults, in U.S. National Report to International Union of Geodesy and Geophysics, 1987-1990: Contributions in Tectonophysics, edited by M. A. Shea, pp. 759-775, Am. Geophys. U., Washington, 1991. Hummon, C., C. L. Schneider, R. S. Yeats, J. F. Dolan, K. E. Sieh, and G. J. Huftile, Wilshire fault: Earthquakes in Hollywood?, Geology, 22, 291-294, 1994. Humphreys, E. D., and K. G. Dueker, Western U.S. upper mantle structure, J. Geophys. Res., 99, 9615-9634, 1994
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION Johnson, R. A, and K. L. Loy, Seismic reflection evidence for A. F, G. A. Abers, J. A. Lawrence, Q. Chen, and A ing in southeastern Arizona erner-Lam. Crustal thickness variations across the Geology,20.597-600,199 y Mountain front from teleseismic observations Lachenbruch, A. H, and J. H. Sass, Heat flow from Cajon ract), Eos Trans. AGU, 73, Fall Meeting Suppl., 372, Pass, fault strength, and tectonic implications, J. Geophys Silver, P. G, and w. W. Chan, Shear wave splitting an Li,Y, and E.S. Schweig, Has the Reelfoot rift zone controlled ntal mantle deformation, J. Geophys. Res tectonic activity since the Late Cretaceous?(abstract), Eos 6.429-16.454.1991 Trans. AGU, Spring Meeting Suppl., 281, 199 Unruh, J. R, M. L. Davisson, R. E. Criss, and E M. Moores Liu, L. B, M. D. Zoback, and P. Segall, Rapid intraplate strain plications of perennial saline springs for abnormally hig umulation in the New Madrid seismic zone. Science fluid pressures and active thrusting in western California 257,1666-1669,1992 Geology.,20.431-434,1992 Livaccari, R. F, and F. V. Perry, c evidence for West. M. W. Extensional reactivation of thrust preservation of Cordilleran lithospheric mantle during the ccompanied by coseismic surface rupture, southw Sevier-Laramide orogeny, western U Wyoming and north-central Utah, Geol. Soc.Am. 105,1137-1150,1993 Livaccari, R. F, and F. V. Perry, Isotopic Zobackio ss auld in the beg a n Evidence for M 6.9) Loma pertly Sevier-Laramide or western United States: Reply California, earthquake and its aftershocks, Geology, 21 l8l-185.1993 Luvendvk. B. P. A model for Neogene crustal rotations Zoback, M. D, and J. H. Healy, In situ stress measu 3.5 km depth in the Cajon Pass scientific resear Geol.Soc.Am.Bmll,03,.1528-1536,1991 Implications for echanics of crustal Mc Caffrey, R, Oblique plate convergence, slip vectors, and Geophys.Res,97,5039-5057,199 Geophys.Res.,97,8905-8915, Zoback, M. D, R. Apel, J. Baumgartner, M. Brudy, R. Emmermann, B. Engeser, K. Fuchs, W. Kessels, H. Mc Carthy, J, and T. Parsons, Insights Rischmuller. F. Rummel. and L. Ver Cenozoic evolution of the Basin and Range- trength inferred from stress measurements to 6 km depth in Plateau transition KTB borehole, Nature (London), 365, 633-635, 199 reflection data, Geol. Soc. Am. BulL. 106, 747-759, 1994 Zoback, M. L, First- and second-order patterns of stress in the Miller, M. G, High-angle origin of the currently low-angle lithosphere: The world stress map project, J. Geophys. Res 97,11,703-11,728,1992 Geolog,19,372-375,1991 Mount, V.S., and J. Suppe, Present-day stress orientations P. Bird, Dep Earth and Space Sciences adjacent to strike-slip faults: California and Sumatra, J. University of California, Los Angeles, CA 90095-1567 (e- Geophys.Res,97,1l,995-12013,1992 Nicholson, C C. C. Sorlien, T. Atwater, J C. Crowell, and B P. Luyendyk, Microplate capture, rotation of the western (Received 24 June 1994; accepted: 21 November 1994.) Transverse Ranges. and initiation of the San Andreas as a L s Olson, J. A, and M. L. Zoback, Seismic deformation patterns on the San Francisco peninsula(abstract), Eos Trans. AGU, 73. Fall Meeting Suppl., 401, 1992 B. M. and T. M. brocher. thrusting of the central fornia margin over the edge of the Pacific plate during the transform regime, Geology, 21, 635-638, 1993 Parry, W. T, D. Hedderly-Smith, and R. L. Bruhn, clusions and hydrothermal alteration on the dixie vE Nevada, J Geophys. Res, 96, 19,733-19, 748, 1991 tton, H J, and G. Zandt, Seismic moment tensors of wester U. S earthquakes and implications for the tectonic stress field, J. Geophys 18,245-18,259,199 Pierce. K. L. and L. The track of the Yellowstone hot g, and uplift, Geol. Soc. A Mem,179.1-54,1992 Rice, J.R., Fault stress states, pore pressure distributions, and dreas fault. in Fault Stre States, Pore Pressure Distributions, and Transpor Properties of Rocks: A Festschrift in Honor of w. F. brace, edited by B. Evans and T -F. Wong, pp. 475-503, Acad Press. San Diego. 1992 Richardson, R. M, and L. M. Reding, North American plate dynamics, J Geophys. Res, 96, 12, 201 Scholz. C. H. N. H. Dawers. J-Z. Yu and M. H. Masters Fault growth and fault scaling laws: Preliminary results, J. ScofL. R. y. d g. L ister. Detachment faults: Evidence for a Geology,20.833-836 Shaw, J. H, and J Suppe, Active faulting and growth folding in the eastern Santa Barbara Channel, California, Geol. Soc.Am.Bmll,106.607-626,1994
BIRD: LITHOSPHERE DYNAMICS AND CONTINENTAL DEFORMATION 383 Johnson, R. A., and K. L. Loy, Seismic reflection evidence for seismogenic low-angle faulting in southeastern Arizona, Geology, 20, 597-600, 1992. Lachenbruch, A. H., and J. H. Sass, Heat flow from Cajon Pass, fault strength, and tectonic implications, J. Geophys. Res., 97, 4995-5015, 1992. Li, Y., and E. S. Schweig, Has the Reelfoot rift zone controlled tectonic activity since the Late Cretaceous? (abstract), Eos Trans. AGU, Spring Meeting Suppl., 281, 1993. Liu, L. B., M. D. Zoback, and P. Segall, Rapid intraplate strain accumulation in the New Madrid seismic zone, Science, 257, 1666-1669, 1992. Livaccari, R. F., and F. V. Perry, Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States, Geology, 21, 719-722, 1993. Livaccari, R. F., and F. V. Perry, Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States: Reply, Geology, 22, 671-672, 1994. Luyendyk, B. P., A model for Neogene crustal rotations, transtensions, and transpression in southern California, Geol. Soc. Am. Bull., 103, 1528-1536, 1991. McCaffrey, R., Oblique plate convergence, slip vectors, and forearc deformation, J. Geophys. Res., 97, 8905-8915, 1992. McCarthy, J., and T. Parsons, Insights into the kinematic Cenozoic evolution of the Basin and Range-Colorado Plateau transition from coincident seismic refraction and reflection data, Geol. Soc. Am. Bull., 106, 747-759, 1994. Miller, M. G., High-angle origin of the currently low-angle Badwater Turtleback fault, Death Valley, California, Geology, 19, 372-375, 1991. Mount, V. S., and J. Suppe, Present-day stress orientations adjacent to strike-slip faults: California and Sumatra, J. Geophys. Res., 97, 11,995-12,013, 1992. Nicholson, C., C. C. Sorlien, T. Atwater, J. C. Crowell, and B. P. Luyendyk, Microplate capture, rotation of the western Transverse Ranges, and initiation of the San Andreas as a low-angle fault sytem, Geology, 22, 491-495, 1994. Olson, J. A., and M. L. Zoback, Seismic deformation patterns on the San Francisco peninsula (abstract), Eos Trans. AGU, 73, Fall Meeting Suppl., 401, 1992. Page, B. M., and T. M. Brocher, Thrusting of the central California margin over the edge of the Pacific plate during the transform regime, Geology, 21, 635-638, 1993. Parry, W. T., D. Hedderly-Smith, and R. L. Bruhn, Fluid inclusions and hydrothermal alteration on the Dixie Valley fault, Nevada, J. Geophys. Res., 96, 19,733-19,748, 1991. Patton, H. J., and G. Zandt, Seismic moment tensors of western U. S. earthquakes and implications for the tectonic stress field, J. Geophys. Res., 96, 18,245-18,259, 1991. Pierce, K. L., and L. A. Morgan, The track of the Yellowstone hot spot: Volcanism, faulting, and uplift, Geol. Soc. Am. Mem., 179, 1-54, 1992. Rice, J. R., Fault stress states, pore pressure distributions, and the weakness of the San Andreas fault, in Fault Stress States, Pore Pressure Distributions, and Transport Properties of Rocks: A Festschrift in Honor of W. F. Brace, edited by B. Evans and T.-F. Wong, pp. 475-503, Acad. Press, San Diego, 1992. Richardson, R. M., and L. M. Reding, North American plate dynamics, J. Geophys. Res., 96, 12,201-12,223, 1992. Scholz, C. H., N. H. Dawers, J.-Z. Yu, and M. H. Masters, Fault growth and fault scaling laws: Preliminary results, J. Geophys. Res., 98, 21,951-21,961, 1993. Scott, R. J., and G. S. Lister, Detachment faults: Evidence for a low-angle origin, Geology, 20, 833-836, 1992. Shaw, J. H., and J. Suppe, Active faulting and growth folding in the eastern Santa Barbara Channel, California, Geol. Soc. Am. Bull., 106, 607-626, 1994. Sheehan, A. F., G. A. Abers, J. A. Lawrence, Q. Chen, and A. L. Lerner-Lam, Crustal thickness variations across the Rocky Mountain front from teleseismic observations (abstract), Eos Trans. AGU, 73, Fall Meeting Suppl., 372, 1992. Silver, P. G., and W. W. Chan, Shear wave splitting and subcontinental mantle deformation, J. Geophys. Res., 96, 16,429-16,454, 1991. Unruh, J. R., M. L. Davisson, R. E. Criss, and E. M. Moores, Implications of perennial saline springs for abnormally high fluid pressures and active thrusting in western California, Geology, 20, 431-434, 1992. West, M. W., Extensional reactivation of thrust faults accompanied by coseismic surface rupture, southwestern Wyoming and north-central Utah, Geol. Soc. Am. Bull., 105, 1137-1150, 1993. Zoback, M. D., and G. C. Beroza, Evidence for nearly frictionless faulting in the 1989 (M 6.9) Loma Prieta, California, earthquake and its aftershocks, Geology, 21, 181-185, 1993. Zoback, M. D., and J. H. Healy, In situ stress measurements to 3.5 km depth in the Cajon Pass scientific research borehole: Implications for the mechanics of crustal faulting, J. Geophys. Res., 97, 5039-5057, 1992. Zoback, M. D., R. Apel, J. Baumgartner, M. Brudy, R. Emmermann, B. Engeser, K. Fuchs, W. Kessels, H. Rischmuller, F. Rummel, and L. Vernik, Upper-crustal strength inferred from stress measurements to 6 km depth in the KTB borehole, Nature (London), 365, 633-635, 1993. Zoback, M. L., First- and second-order patterns of stress in the lithosphere: The world stress map project, J. Geophys. Res., 97, 11,703-11,728, 1992. P. Bird, Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 (email: pbird@ess.ucla.edu) (Received 24 June 1994; accepted: 21 November 1994.)
