Brian P. Wernicke

Modes of extensional tectonics

Although hundreds of papers have been devoted to the geometric and kinematic analysis of compressional tectonic regimes, surprisingly little has been written about the details of large-scale strain in extended areas. We attempt, by means of quantitative theoretical analysis guided by real geological examples, to establish some ground rules for interpreting extensional phenomena. We have found that large, very low-angle normal faults dominate highly extended terranes, and that both listric and planar normal faults are common components of their hanging walls. The very low-angle normal faults may have displacements from a few kilometres up to several tens of kilometres and we regard their hanging walls as extensional allochthons, analogous (but with opposite sense of movement) to thrust-fault allochthons. Differential tilt between imbricate fault blocks suggests listric geometry at depth, whereas uniformly tilted blocks are more likely to be bounded by planar faults. The tilt direction of imbricate normal-fault blocks within large extensional allochthons is commonly away from the transport direction of these sheets, but in many cases tilts are in the same direction as transport, thus limiting the usefulness of the direction of tilting as a transport indicator. The presence of chaos structure, a structural style widely recognised in the Basin and Range Province, implies large scale simple shear on very low-angle normal faults and does not necessarily form as a result of listric faulting.

On the role of isostasy in the evolution of normal fault systems

The footwalls of west-dipping normal faults that separate the west-central Colorado Plateau from the Basin and Range province record at least 5-7 km, and perhaps as much as 15-20 km, of west-side-up Neogene uplift, with an axis just 10-20 km west of undeformed plateau strata. The uplift is expressed as folding and steep faulting in pre-Tertiary cratonic and disconformably overlying Neogene strata, forming a basement-cored anticline and coincident topographic high on the western margin of the plateau. The authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin. Profound, isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains, calling into question several assumptions fundamental to existing models of the evolution of normal fault systems.

Cenozoic evolution of Neotethys and implications for the causes of plate motions

Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations and new estimates of crustal shortening in Iran define the Cenozoic history of the Neotethyan ocean between Arabia and Eurasia. The new constraints indicate that Arabia-Eurasia convergence has been fairly constant at 2 to 3 cm/yr since 56 Ma with slowing of Africa-Eurasia motion to <1 cm/yr near 25 Ma, coeval with the opening of the Red Sea. Ocean closure occurred no later than 10 Ma, and could have occurred prior to this time only if a large amount of continental lithosphere was subducted, suggesting that slowing of Africa significantly predated the Arabia-Eurasia collision. These kinematics imply that Africas disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern Eurasia slowed its motion. The slow, steady rate of northward subduction since 56 Ma contrasts with strongly variable rates of magma production in the Urumieh-Dokhtar arc, implying magma production rate in continental arcs is not linked to subduction rate.

Near-field investigations of the Landers earthquake sequence, April to July 1992.

The Landers earthquake, which had a moment magnitude (Mw) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its Mw 6.1 preshock and Mw 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

Dating topography of the Sierra Nevada, California, using apatite (U–Th)/He ages

The upward motion of rock masses relative to the Earths surface has been documented for most of the main mountain belts using thermochronological and petrological techniques. More fundamental to the physical processes of mountain building, however, is the motion of the Earths surface itself, which remains elusive. Here we describe a technique for estimating the age of topographic relief by mapping the low-temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium, thorium and helium contents. The technique exploits horizontal variations in temperature in the shallow crust that result from range-normal river drainages,, because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges. Our results from the Sierra Nevada, California, indicate that two of the modern transverse drainages, the Kings and the San Joaquin, had developed deep canyons by the Late Cretaceous period, suggesting that the high topography of the range is ∼50–60 million years older than generally thought.

Contemporary strain rates in the northern Basin and Range province from GPS data

We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north-south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr^(−1) oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr^(−1) also oriented N47°W, slower than most previous geodetic estimates of SNGV-NA relative motion, and nearly 7° counterclockwise from the direction of P-NA relative plate motion. We estimate CGB-CP relative motion of 2.8 ± 0.2 mm yr^(−1) oriented N84 ± 5°W, consistent with roughly east-west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr^(−1) concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV-CGB relative motion of 9.3 ± 0.2 mm yr^(−1) oriented N37 ± 2°W, essentially parallel to P-NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr^(−1) across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr^(−1) in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr^(−1) across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV-CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr^(−1) extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east-west extension rate of 10 ± 1 nstr yr^(−1) across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range-bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second-order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second-order pattern.

Comparison of geodetic and geologic data from the Wasatch region, Utah, and implications for the spectral character of Earth deformation at periods of 10 to 10 million years

The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows, ranging from the last millennium up to 10 Myr. The three easternmost GPS sites of the Basin and Range Geodetic Network (BARGEN) at this latitude define a ∼130-km-wide region spanning three major normal faults extending east-west at a total rate of 2.7 ± 0.4 mm/yr, with an average regional strain rate estimated to be 21 ± 4 nstrain/yr, about twice the Basin and Range average. On the Wasatch fault, the vertical component of the geologic displacement rate is 1.7 ± 0.5 mm/yr since 6 ka, <0.6 mm/yr since 130 ka, and 0.5–0.7 mm/yr since 10 Ma. However, it appears likely that at the longest timescale, rates slowed over time, from 1.0 to 1.4 mm/yr between 10 and 6 Ma to 0.2 to 0.3 mm/yr since 6 Ma. The cumulative vertical displacement record across all three faults also shows time-variable strain release ranging from 2 to 4 mm/yr since 10 ka to <1 mm/yr averaged over the past 130 kyr. Conventional earthquake recurrence models (“Reid-type” behavior) would require an accordingly large variation in strain accumulation or loading rate on a 10-kyr timescale, for which there appears to be no obvious geophysical explanation. Alternatively, seismic strain release, given a wide range of plausible constitutive behaviors for frictional sliding, may be clustered on the 10-kyr timescale, resulting in the high Holocene rates, with comparatively low, uniform strain accumulation rates on the 100-kyr timescale (“Wallace-type” behavior). The latter alternative, combined with observations at the million-year timescale and the likelihood of a significant contribution of postseismic transients, implies maxima of spectral amplitude in the velocity field at periods of ∼10 Myr (variations in tectonic loading), ∼10 kyr (clustered strain release), and of 100 years (postseismic transients). If so, measurements of strain accumulation and strain release may be strongly timescale-dependent for any given fault system.

Origin of High Mountains in the Continents: The Southern Sierra Nevada

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

An animated tectonic reconstruction of southwestern North America since 36 Ma

We present tectonic reconstructions and an accompanying animation of deformation across the North America–Pacific plate boundary since 36 Ma. Intraplate deformation of southwestern North America was obtained through synthesis of kinematic data (amount, timing, and direction of displacement) along three main transects through the northern (40°N), central (36°N– 37°N), and southern (34°N) portions of the Basin and Range province. We combined these transects with first-order plate boundary constraints from the San Andreas fault and other areas west of the Basin and Range. Extension and strike-slip deformation in all areas were sequentially restored over 2 m.y. (0–18 Ma) to 6 m.y. (18–36 Ma) time intervals using a script written for the ArcGIS program. Regions where the kinematics are known constrain adjacent areas where the kinematics are not well defined. The process of sequential restoration highlighted misalignments, overlaps, or large gaps in each incremental step, particularly in the areas between data transects, which remain problematic. Hence, the value of the reconstructions lies primarily in highlighting questions that might not otherwise be recognized, and thus they should be viewed more as a tool for investigation than as a final product. The new sequential reconstructions show that compatible slip along the entire northsouth extent of the inland right-lateral shear zone from the Gulf of California to the northern Walker Lane is supported by available data and that the east limit of active shear has migrated westward with respect to North America since ca. 10 Ma. The reconstructions also highlight new problems regarding strain-compatible extension east and west of the Sierra Nevada– Great Valley block and strain-compatible deformation between southern Arizona and the Mexican Basin and Range. Our results show ~235 km of extension oriented ~N78°W in both the northern (50% extension) and central (200% extension) parts of the Basin and Range. Following the initiation of east-west to southwest-northeast extension at 15–25 Ma (depending on longitude), a significant portion of right-lateral shear associated with the growing Pacific– North America transform jumped into the continent at 10–12 Ma, totaling ~100 km oriented N25°W, for an average of ~1 cm/yr since that time.

Lithospheric extension near Lake Mead, Nevada: A model for ductile flow in the lower crust

Small variations in gravity anomalies and topographic elevation observed in areas that have undergone highly variable amounts of upper crustal thinning can be satisfactorily explained by ductile flow of lower crustal material under the proper conditions. In this study we examine the boundary between the unextended Colorado Plateau and a strongly extended domain in the Basin and Range Province in the Lake Mead (Nevada) region. Bouguer gravity and topography data suggest that both present and preextensional variations in crustal thickness between the unextended and extended regions are small. Analytic channel flow models show that viscosities required for ductile flow in a lower crustal channel to reduce discontinuities in crustal thickness associated with variable amounts of extension are highly dependent on the channel thickness and on the length scale of flow required. Finite element modeling of Newtonian flow and power law creep shows that flow over the length scale of the eastern Basin and Range (500 km or more) corresponding to upper crustal extension by a factor of 1.4–3 over 10 m.y. requires effective viscosities less than 10^(18)–10^(20) Pa s for ductile channels 10–25 km thick. Flow over shorter length scales (150 km) may be accommodated with effective viscosities as high as 10^(21) Pa s. Modeling suggests that these effective viscosities may be sustained by lower crustal material deforming at laboratory-derived power law creep rates. The longer-scale flow may require elevated crustal temperatures (more than 700°C), depending on the composition and material properties assumed. Under the boundary conditions assumed in this study the linear viscous flow models yield a satisfactory approximation to deformation by power law creep. This work suggests that flow in the lower crust may be a viable mechanism for producing small variations in total crustal thickness between strongly extended and less extended regions, and thereby explaining the relative uniformity in gravity and topography between such regions.