Walker Lane Belt Research Articles

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north‐south within the magmatic regions and northwest‐southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest‐southeast dextral shear of the Walker Lane belt to the east‐west extension characteristic of the Cascade arc in central Oregon. Faulting along north‐south strikes in the volcanically active areas indicates the east‐west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Source parameters and tectonic setting of the 1990 Lee Vining, California, earthquake sequence

Abstract On 24 October 1990, a magnitude 5.0 (Md) earthquake occurred near Lee Vining, California. It was a strike-slip event with a hypocenter 5 km east and 11.5 km below the surface trace of the Sierran frontal fault, which at this locality has 21 m of Holocene vertical offset. A time-domain deconvolution technique was used to estimate the source-time function of this event using a small foreshock as an empirical Green's function. The resulting source-time function indicates a simple rupture process. The moment for this event is 3.0 × 1023 dyne-cm, and the static stress drop is 36 bars. Moderate-sized earthquakes occurring in the western Basin and Range since 1978 have been concentrated in the central part of the Walker Lane Belt. This is an area of diverse topography and numerous strike-slip faults that lies between the Sierra Nevada Mountains on the west and typical Basin and Range topography on the east. Most of these earthquakes have strike-slip focal mechanisms, and with the exception of earthquakes in the Long Valley Caldera area, they are consistent with left-lateral slip on faults that trend northeast and right-lateral slip on faults that trend northwest.

Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone

From the aftershocks of the 1992 Landers earthquake, we infer the orientation of the principal strain rate axes (d1 > d2 > d3; d1 lengthening), their relative magnitude, and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic P and T axes. The seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central Mojave block. Regionally, the aftershock data define two major domains within the central Mojave block: (1) the western Mojave block, including the San Bernardino Mountains and the epicentral area of the Big Bear earthquake, which is characterized by E‐W d1 (lengthening) and N‐S d3 (shortening); and (2) the central Mojave block, including the Landers surface rupture zone, which is characterized by NW‐SE d1 and NE‐SW d3. Inversion for the principal strain axes of geodetically measured coseismic displacements across the Big Bear and Landers seismogenic zones gives results similar to the aftershock inversions for those areas, indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses. The background seismicity for 1981 to 1991 shows the same characteristic d1 and d3 orientations for the two domains, indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation. The micropolar inversion also provides values of the relative vorticity parameter W, which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault‐bounded blocks rotating within tabular seismogenic shear zones. The observed fault geometry along the Kickapoo fault suggests a pinned‐block model for the local block rotation that is consistent with the values of W obtained from our inversions. We interpret the regional NW‐SE orientation of d1 in the central Mojave block to be characteristic of the dextral eastern California shear zone, which transfers approximately 22% of the Pacific‐North American plate motion from the San Andreas system to the Walker Lane Belt in eastern California. Our results and geodetic determinations of the secular shear strain in the central Mojave block indicate that the locus of NW dextral shear generally lies between the San Bernardino Mountains and the Pisgah fault.

The chronology of Cenozoic volcanism and deformation in the Yerington area, western Basin and Range and Walker Lane

High-precision 40 Ar/ 39 Ar isotopic ages obtained from Cenozoic volcanic rocks and subvolcanic intrusions document the age of initiation and the temporal evolution of extensional and strike-slip faulting in the western Basin and Range Province. In the northern Wassuk Range, faulting began between ca. 26 and 24.7 Ma; both normal and strike-slip faults are bracketed between 23.1 and 22.2 Ma, and between 15 and 14 Ma. These ages document inception of the Ancestral Walker Lane, a northwest-trending zone of right-transtensional faulting in western Nevada that separated extending crust on the east from the unextended Sierra Nevada block on the west at lat 39°N. We speculate that the southwesterly migrating, episodic Oligocene–early Miocene, east-west extensional faulting in the Basin and Range thinned and weakened the crust, allowing right-slip faults to develop in the Walker Lane in response to San Andreas right-shear in California. Southwest of the Walker Lane there was no faulting prior to 15 Ma. Here, in the Yerington district, andesitic magmatism began at ca. 15 Ma and was followed by >150% east-west extension along closely spaced (1–2 km) normal faults with up to 4 km of offset each (Proffett, 1977). These faults tilted older Cenozoic rocks 35°–40°W. Our new 40 Ar/ 39 Ar ages substantially revise earlier K-Ar ages of the timing of extension and establish that andesite lava flows cut by normal faults are 13.8–15 Ma, and that these faults are intruded by 12.6–13.0 Ma dacites. Rapid extension is thus bracketed to a 0.7–1.7 m.y. interval at 95% confidence, indicating local, east-west strain rates of 2–4 × 10 −14 /s (5–10 mm/yr). Following this period, lower rates of extension prevailed near Yerington along more widely spaced normal oblique-slip faults that localized clastic sedimentation of the Wassuk Group between 11 and 8 Ma. These faults and sedimentary rocks are more abundant southwest of Yerington in a belt parallel to the Walker Lane in previously little-extended crust. From 7 Ma to present, normal right-oblique slip faults with a lower rate of extension than the previous two periods produced the modern ranges near Yerington and extend 100 km southwest of the Walker Lane, which continues to be the locus of strike-slip faulting. Thus, since 15 Ma the margin of the Basin and Range has moved progressively 100 km west creating the broad Walker Lane belt and lower strain rates near Yerington.

Pn tomography beneath the Southern Great Basin

P‐wave first arrival times recorded on the Southern Great Basin Seismic Network were used to tomographically image lateral variations in both crustal thickness and Pn velocity beneath the southern Great Basin. The tomographic image indicates that the southern Great Basin has a flat Moho at about 30 km depth. We attribute the flat Moho to both underplating by partial melt from the uppermost mantle and to ductile flow in the crust. Magmatic underplating probably dominates where Pn velocities are low (<7.8 km/s) and ductile flow should dominate where Pn velocities are high (>7.9 km/s). Low velocity regions occur predominantly beneath the east margin of the Walker Lane Belt and the Las Vegas Shear Zone suggesting that these shear zones overlie a zone of relatively weak mantle. A region of high Pn velocity northeast of Nevada Test Site (NTS) may correspond to old lithosphere or to a region that has experienced extensive basalt extraction.

Covariance of structural and stratigraphic trends: Evidence for anticlockwise rotation within the Walker Lane belt Death Valley region, California and Nevada

Large‐magnitude relative rotations of range‐scale structural blocks are inferred to be products of crustal extension in the Death Valley region, in California and Nevada. Reconstructions of the extended terrain restore pre‐Tertiary contractile structures with currently diverse trends into a relatively simple, north‐northeast trending thrust belt. We interpret trends of major contractile structures throughout the Death Valley region as characteristically discordant to, and thus geometrically independent from, stratigraphic trends. We compare trends of pre‐Tertiary contractile structures with paleoflow orientations in Lower Cambrian strata throughout the extended terrain. Significant differences between mean paleoflow orientations are consistent in magnitude and sense with differences observed in the structural trends. Available evidence does not support interpretation of paleoflow data from apparently rotated areas as representing an original paleogeography distinct from that of adjacent areas. Facies variations are present within the contractile structures. We interpret the strong covariance throughout the study area between structural and paleoflow trends of diverse orientations as evidence for vertical axis rotations. Combined rotation estimates from these data indicate clockwise rotations of 85° ± 12° for Bare Mountain and 73° ± 24° for the Striped Hills but anticlockwise rotations of 30° ± 35° and 87° ± 29°, respectively, for the northern and southern Grapevine Mountains. Such large rotations of opposite sense may be related to north‐south shortening accommodated by conjugate strike‐slip faults during large‐magnitude extension in the Death Valley region but are difficult to explain as products of regional dextral shear within the southern Walker Lane belt.

Paleomagnetism and rotation constraints for the middle Miocene southwestern Nevada volcanic field

Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt within the Basin and Range province and thus provide an interesting opportunity to test for late Cenozoic vertical‐axis rotation. Paleomagnetic data from individual ash flow sheets document no significant relative vertical‐axis rotation among localities within central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time‐averaged mean paleomagnetic direction (D = 351.4°, I = 52.7°, α95 = 4.5°) calculated from data from numerous separate rock units suggests that the central SWNVF underwent minimal counterclockwise vertical‐axis rotation (R = −7.1° ± 6.6°) with respect to the North American craton. No clockwise vertical‐axis rotation is found to support projection of dextral faults of the Walker Lane beneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. These clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central SWNVF block and more strongly extended areas to the south and southwest of the field. Negligible rotation of 11.45‐Ma to 13.25‐Ma tuffs relative to the central SWNVF was found at the southeast margin of the field where 90° clockwise rotation at the northwest termination of the Las Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate 13.25 Ma, and thus dextral shear within this part of the Walker Lane belt was not synchronous or connected across the southern margin of the field. Small counterclockwise vertical‐axis rotation relative to the craton, as found for the central SWNVF block, might be a regional feature in the western Great Basin.

Tectonic Controls on Late Cenozoic Strike-Slip Faulting, Volcanism, and Landscape Development in the Mojave Desert, California: ABSTRACT

Recent studies of the late Cenozoic tectonics of the Mojave Desert Block suggest that strain is regionally heterogeneous and has been partitioned into six domains that are separated by major strike-slip faults and extensional zones. Tectonic rotation of these domains as well as their internal deformation by strike-slip faulting have occurred as the result of broadly distributed regional right shear. Sixty-five kilometers of total right slip is reckoned to have occurred along faults of the southern half of the province (between the Helendale and Granite Mountains faults). The broad network of faults of the Mojave, along with kinematically and temporally similar strike-slip faults of the Death Valley region (Furnace Creek and Southern Death Valley fault zones), constitute a regional, through going zone of right shear named the Eastern California shear zone (ECSZ). This zone of intracontinental shear also likely includes the Walker Lane belt of western Nevada. Because of its physical connection to faults the southern portion of the San Andreas fault system, the ECSZ must have also accommodated a portion of Pacific-North American transform motion. In addition to imparting the strong NW structural grain to the region, the tectonic regime has had a profound effect on Neogene paleogeography andmore » has apparently facilitated local magmatism. Faulting and block rotations have created a series of structurally controlled basins and uplifts of many geometries. Extension in several basins has also been accompanied by young basaltic magmatism. These include surface flows such as at Mt. Pisgah (shield volcano) and subsurface dike emplacement beneath Troy Lake.« less

State of stress and modern deformation of the Northern Basin and Range Province

Constraints on the current stress regime of the actively extending northern Basin and Range province are provided by deformation data (focal mechanisms and fault slip studies), hydraulic fracturing in situ stress measurements, borehole elongation (“breakouts”) analyses, and alignment of young volcanic vents. The integrated data indicate significant variations both in principal stress orientations and magnitudes. An approximately E‐W least principal stress direction appears to characterize both the eastern and western margins of the Basin and Range province, whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented NW to N60°W. The contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins. Active deformation along the southeastern and western province margins is characterized by a combination of strike‐slip and normal faulting. Focal mechanisms along northeastern province margin (Wasatch front) and in central Nevada indicate a combination of normal and oblique‐normal faulting. Temporal, regional, and depth‐dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles. However, some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated. Evidence for significant temporal variation (or multiple cycles of variation) in relative stress magnitude comes from the Sierran front‐Basin and Range boundary region where recent earthquakes are predominantly strike slip, whereas the profound relative vertical relief across the Sierra frontal fault zone in the last 9–10 m.y. implies a normal faulting stress regime. Using the best data on stress orientation, relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements. Analysis of well‐constrained slip vectors in the Owens Valley, California, area indicate that large temporal variations in the magnitude of the approximately N‐S oriented maximum horizontal stress are required to explain dominantly dip‐slip and strike‐slip offsets on subparallel faults. Similar faulting relations are observed throughout much of the boundary zone between the Basin and Range‐Sierra Nevada (including the Walker Lane belt). Along the eastern province margin in the Wasatch front area in Utah, available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of <4–5 km. Earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth. Furthermore, superimposed sets of young fault striae along a segment of the Wasatch fault also indicate temporal variations of relative stress magnitudes. Sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion, thermally induced flow, and the influence of the San Andreas plate boundary. Although difficult to date accurately, the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle (1000–5000 years) and multiple earthquake cycles (10,000+ years).

East‐trending dextral faults in the western Great Basin: An explanation for anomalous trends of pre‐Cenozoic strata and Cenozoic faults

Two postulated major east trending strike‐slip fault zones, here named the Coaldale and Excelsior fault zones, are delineated mainly on the basis of faults in Cenozoic rocks in eastern California and western Nevada and coincide with apparent major right‐lateral offsets in the distribution and facies of pre‐Cenozoic rocks. Apparent right‐lateral offset of pre‐Cenozoic rocks on the Coaldale fault zone is 60 to 80 km and on the Excelsior fault zone is 45 to 55 km. This offset accounts for major disruption in trends of pre‐Cenozoic rocks in eastern California and western Nevada, a disruption previously interpreted as the result of either an originally curving continental margin or tectonic distortion of originally linear trends by large‐scale bending (oroflexural folding) or by crustal‐scale folding related to north‐east‐southwest compression. Main offset on the Coaldale and Excelsior fault zones appears to be late Mesozoic in age. A pre‐mid‐Cretaceous age is indicated because mid‐Cretaceous and younger plutonic rocks are not cut by major strike‐slip faults on line with the fault zones. Major movement is probably younger than the Dunlap Formation, which is dated paleontologically as Early Jurassic but which may also contain rocks as young as Cretaceous. Local reactivation of the faults in the Cenozoic, perhaps under a different stress regime, accounts for offset of Cenozoic rocks. The Coaldale and Excelsior fault zones terminate major northwest trending late Cenozoic and possibly older faults. The Owens Valley‐White Mountain fault system in eastern California and the Furnace Creek fault zone in southeastern California and westernmost Nevada terminate northward at the Coaldale fault zone. A system of south‐east trending right‐lateral and high‐angle faults in western Nevada, including the Bettles Well fault, terminate southward at the Excelsior fault zone. Major movement on northwest trending faults such as the Furnace Creek and Bettles Well fault may be mostly late Cenozoic in age, but present information does not preclude the possibility that these fault zones initiated prior to, and are offset by, the Coaldale and Excelsior fault system. The unusual trends of pre‐Tertiary rocks and of the Coaldale and Excelsior fault zones in eastern California and western Nevada appear to be restricted to the Walker Lane belt, a broad northwest trending structural zone characterized by right‐lateral shear in western Nevada and eastern California. Possibly initial shear produced the northwest trending faults, and a later “kink” in this system produced oroflexural folding and the east‐trending right‐lateral Coaldale and Excelsior fault zones. This “kink” may have resulted from a change in the stress regime and structural complexities where the shear zone obliquely crossed the Paleozoic continental margin.