Leslie J. Sonder

Lithospheric gravitational potential energy and past orogenesis: Implications for conditions of initial Basin and Range and Laramide deformation

Gravitational body forces (i.e., buoyancy forces) have come to be seen as critical to the evolution of orogens. Nevertheless, constraining the role of body forces in specific geologic scenarios is made difficult by the substantial number of poorly constrained physical parameters needed to fully relate forces to deformation. By separating the calculation of buoyancy forces from the calculation of the resulting deformation, models based on relatively simple descriptions of the lithosphere can yield geologically useful constraints. Among these are the importance of paleoelevation in driving syn- and postcontractional extension and in localizing contractional strain. Although such phenomena have been considered in more complex models of continental deformation, the simpler analysis presented here clearly establishes first-order limits on lithospheric structures and paleoelevations consistent with buoyancy-driven deformation. In the early Cenozoic Great Basin of the western United States, we show that the low elevations inferred in much of the geologic literature are inconsistent with a body-force origin for observed extensional tectonism. East of the Colorado Plateau, localization of Laramide deformation coincides with pre-Laramide subsidence of the Western Interior seaway. This subsidence prestressed the lithosphere, making the Southern Rocky Mountains the weak link in responding to regional compressional stress.

Vertical axis rotations in the Las Vegas Valley Shear Zone, southern Nevada: Paleomagnetic constraints on kinematics and dynamics of block rotations

Paleomagnetic study of vertical axis rotations at 23 localities along the right-lateral Las Vegas Valley Shear Zone (LVVSZ) in southern Nevada indicates that clockwise rotations generally increase with proximity to the LVVSZ, reaching 100° at the closest localities. Rates of rotation determined at four localities range from l°/m.y. to 12°/m.y. The maximum characteristic size of rotating blocks is 2–4 km, considerably smaller than the dimensions of the zone of deformation associated with the shear zone (∼100 km in length, ∼15 km in half width). Thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone. Instead, the deformation appears to be quasi-continuous when considered on a scale of > ∼10 km. We suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material, mechanically decoupled from deeper parts of the crust, and that the across-strike distribution of rotations is controlled by the rheological properties of the upper crust, the length of the shear zone, and the total amount of offset across the shear zone.

Distribution, accumulation, and fluxes of soil carbon in four monoculture lysimeters at San Dimas Experimental Forest, California

Abstract This research examines how vegetation type controls soil processes involving soil carbon fluxes, accumulation, and transport in a chaparral ecosystem. Carbon concentrations and δ13C values were measured for soil samples collected in 1987 from 1-m depth profiles in four lysimeters in the San Dimas Experimental Forest, southern California, USA. Each lysimeter has sustained a single vegetation type since 1946, the species being Quercus dumosa, Ceanothus crassifolius, Adenostoma fasciculatum, and Pinus coulteri. Archived samples of soil originally used for filling the lysimeters and litter samples from the surface of each lysimeter were analyzed to determine initial and boundary conditions. Although detectable changes in carbon content were limited to the topmost 20 cm of the profiles, variations in δ13C were found to depths of 80 cm, indicating that processing of carbon occurs much deeper than indicated by carbon concentrations alone, underscoring the utility of carbon isotopes in the study of soil carbon dynamics. A one-dimensional model that considers surface carbon input, downward transport, and loss through decomposition is developed to describe the evolution of carbon concentration and stable carbon isotope ratios in the four soil profiles. Comparison of measured and calculated profiles yields estimates of carbon fluxes, turnover rates, and accumulation of soil carbon. A set of physical parameters, including the rate constant of decomposition, downward transport rate of organic carbon, and rate of carbon input from the surface are derived from the model and can be related to the species and environmental conditions. The calculations indicate the importance of species on soil formation and carbon cycles, which is important for understanding the effects of changes in land use on ecosystem processes. Our results also suggest that fire may increase the rate of soil carbon accumulation in a chaparral ecosystem.

Foreland normal fault control on northwest Himalayan thrust front development

In the Trans-Indus Ranges along the western part of the northwest Himalayan thrust front, unconformities, changes in paleocurrent directions, and locally derived conglomerates in synorogenic foreland basin deposits provide evidence for major local deformation at ≥3.5 Ma. The tectonic history of the Trans-Indus Ranges has previously been described in terms of a single episode of major thrusting at ≤1 Ma, thus our work implies that there were two distinct phases of deformation. In conjunction with published evidence in the Salt Range to the east for two phases of deformation (∼6 to 5 Ma, and ∼2.5 Ma to present), this study demonstrates that these two phases of deformation are regionally significant and probably correlative along the entire present-day NW Himalayan thrust front. Reconstruction of possible source areas for the locally derived conglomerates shows that the earlier deformation is probably related to normal faulting. These results suggest that the tectonic evolution of the area along the present-day thrust front is characterized by (1) latest Miocene to early Pliocene formation of north dipping normal fault zones (total throw ≥ 600 m) within the foreland basin, related to syn-orogenic flexure of the Indian plate, and (2) late Pliocene to early Pleistocene initiation of south directed thrusting along the present-day thrust front, related to outward growth of the NW Himalayan thrust wedge. The location of the present-day thrust front appears to be controlled by north dipping normal faults and monoclines that formed during the earlier deformation and subsequently localized structural ramps during later thrusting.

Tectonic controls of metamorphic field gradients

Abstract We investigate the effects of horizontal variations in tectonic factors (heat production and rate, amount, and duration of erosion) on the magnitudes and gradients of pressure (P), temperature (T), and cooling ages observed in mountain belts. Horizontal gradients of P, T, and cooling ages are largely controlled by two dimensionless parameters, (1) the horizontal length scale, λ′, over which erosion, erosion rate or erosion duration varies, and (2) the Peclet number, Pe, which describes the relative rates of advection and diffusion of heat. In contrast, the magnitudes of observed P and T depend strongly on the degree of thickening and original burial depth, as well as Pe, and, since horizontal heat conduction can usually be ignored, do not depend strongly on λ′. P and T gradients generally increase with decreasing λ′ and Pe. For calculations in which either, but not both, erosion rate or erosion duration is horizontally constant, P and T gradients are moderate in magnitude and increase in the direction of increasing total erosion. In contrast, calculations with horizontally constant total erosion (but varying erosion rate and erosion duration) show small temperature and pressure gradients; pressures and temperatures can increase in the same or in opposite directions, depending on the magnitude of λ′. Calculations with horizontally varying crustal heat production show small but non-zero temperature gradients and negligible pressure gradients. These results indicate that knowledge of temperature, pressure and cooling age gradients in individual mountain belts can constrain fundamental quantities such as magnitude and horizontal gradient of the exhumation rate and the average shear strain rate occurring during exhumation. On the basis of their metamorphic field gradients, most mountain belts can be divided into one of three groups. The first and largest group shows strongly positive P and T gradients. Precambrian orogenic areas comprise the second group and have small T gradients (

Rheologic control of buoyancy‐driven extension of the Rio Grande rift

Extensional deformation in the western United States, particularly in the Basin and Range, has frequently been ascribed to buoyancy forces arising from horizontal density contrasts. The Rio Grande rift is subject to buoyancy forces 2-3 times larger than those in the tectonically active northern Basin and Range, suggesting that it should be extending considerably faster than the northern Basin and Range, yet paradoxically is extending more slowly. Using a thin viscous sheet model, we test the hypothesis that regions surrounding the Rio Grande rift, which have rheologically stronger lithosphere, resist deformation and reduce the rate of extensional deformation in the Rio Grande rift to values consistent with observations. We find that these regions must have lithospheric strength at least twice that of the Rio Grande rift. This modest contrast in lithospheric strength is consistent with independent estimates derived from observations of heat flow, seismicity, and crustal thickness.

Ductile shear zones as counterflow boundaries in pseudoplastic fluids: Discussion and theory

Abstract A model presented by Talbot (1999) attempts to use the shapes of S- or J-shaped structures in ductile shear zones experiencing simple shear to infer information about rock rheology, specifically the value of the stress exponent n in a power-law rheology. The model is incorrect, so cannot yield any insight into the deformation or rheology of shear zones. The shape of S- or J-shaped structures is more likely a function of variation across the shear zone of water fugacity, grain size, or other weakening mechanisms.

Testing a Novel Method for Initializing Air Parcel Back Trajectories in Precipitating Clouds Using Reanalysis Data

AbstractLagrangian air parcel tracking is a powerful tool for estimating vapor source locations, particularly for isotope hydrology applications. Identified vapor source regions may be sensitive to the distribution of altitudes at which back trajectories are initiated. Ideally, those initial altitudes should reflect the altitudes where precipitation forms. This paper introduces a novel method for estimating these heights from reanalysis data and an air parcel lofting routine, which is referred to as the “Reanalysis” method. Using Barrow, Alaska (now known as Utqiaġvik), as a test site, the study compares the distribution of air parcel initiation heights and vapor source conditions from back trajectories initiated at 1) heights determined by the Reanalysis method and 2) heights acquired from 35-GHz vertically resolved cloud radar, termed the “Cloud Radar” method. Only 2 of the 70 events failed to produce condensation at any elevation. The distribution of air parcels generated by each method was compared on...