David W. Rodgers

Subsidence of a volcanic basin by flexure and lower crustal flow: The eastern Snake River Plain, Idaho

The Eastern Snake River Plain (ESRP) is a linear volcanic basin interpreted by many workers to reflect late Cenozoic migration of North America over the Yellowstone hotspot. Thermal subsidence of this volcanic province with respect to Yellowstone has been documented by several workers, but no one has characterized subsidence with respect to the adjacent Basin and Range Province. This paper documents crustal flexure along the northwest edge of the ESRP, uses flexure to model the dimensions of a dense load beneath the basin, and presents evidence in support of density-driven subsidence and lower crustal flow away from the basin. Crustal flexure adjacent to the ESRP is reflected by the attitudes of Mesozoic fold hinges and Neogene volcanic rocks. Fold hinges formed with a subhorizontal plunge and a trend perpendicular to the ESRP but now show a southward plunge near the ESRP of as much as 20°–25°. We present a contour map of equal fold plunges proximal to the ESRP that shows flexure is roughly parallel to and extends 10–20 km north of the average edge of the ESRP. Flexural profiles indicate the minimum amount of ESRP subsidence, with respect to the Basin and Range; subsidence ranges from 4.5 to 8.5 km. The structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models. These models indicate the flexed crust is very weak (flexural parameter of 4–10 km), interpreted to be a result of the high heat flow of the ESRP. Assuming subsidence was induced by emplacement of a dense crustal layer beneath the ESRP, a midcrustal “sill” identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure. New dimensions include a thickness of 17–25 km and a half width of 40–50 km, which place the edge of the sill beneath the edge of the ESRP. The dimensions of the ESRP sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30+ km and because ESRP seismic, gravity, and heat flow data support lower crustal compensation. Density-driven lower crustal flow away from the ESRP is proposed to accommodate subsidence and maintain isostatic equilibrium. Timing of subsidence is constrained by ESRP exploratory wells, where 6.6 Ma rhyolites at a depth of 1.5 km indicate most subsidence occurred prior to their emplacement, and by strong spatial correlations between plunge contours and Quaternary volcanic rift zones. Two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at ∼10 Ma, which provided the heat source for the initial rhyolitic volcanism on the ESRP, and continuing, localized loads from dikes and sills associated with Quaternary basalts. Widespread ∼10 Ma magmatism and subsidence conflicts with simple time-transgressive migration of the Yellowstone hotspot, indicating a need for revision of the hotspot paradigm.

Extension of the Yellowstone plateau, eastern Snake River Plain, and Owyhee plateau

Formation of the late Cenozoic volcanic province comprising the Owyhee plateau, eastern Snake River Plain, and Yellowstone plateau has been accompanied by east-northeast-directed crustal extension. A new vector of 45 mm/yr, N56{degree}E for the migration of silicic volcanism across the volcanic province is calculated. If migration of volcanism reflects west-southwest continental drift over a mantle plume, a zone of crustal extension must separate the volcanic province from the more slowly moving North American craton. Space-time relations of basin fill in the adjacent Basin and Range province provide evidence for a zone of extension, about 125 km wide, coincident with and east of coeval silicic volcanism. Since 16 Ma, the zone of extension has migrated along with silicic volcanism, maintaining its position between the province and the unextended craton.

The growth of fault‐bounded tilt blocks

A series of uniformly tilted fault-bounded blocks is a common feature in actively extending regions, such as the Basin and Range province. If the tilted blocks were produced by rigid “domino-style” rotation, one would predict large voids at either end of a series of these blocks. Using tilt data and a simple flexural calculation, we suggest that much of the apparent rigid behavior could also be produced by internal block deformation. In our model of normal fault growth, isostatic/elastic uplift of the footwall is coupled with hanging wall downdrop within the region between faults, resulting in the appearance of a tilted rigid block. We present tilt data sampled at varying distances from several block-defining faults within the northeast Basin and Range province. Tilt measurements between a series of 30-km spaced block-defining faults are found to be uniform, while tilts between more widely spaced faults exhibit a pattern of tilt that diminishes to zero in less than 30 km. Using a simple flexural calculation for internal block deformation, we show that for this region the patterns of tilt are consistent with a flexural length scale of ∼8–12 km and deflections of 2–4 km. These estimates are compatible with both the lower limit to seismicity and basin depth determined from earthquake and seismic reflection studies.

Bajada formation by monsoonal erosion of a subaerial forebulge, Sultanate of Oman

Abstract A vast, low gradient bajada is present in the interior of northern Oman. With an area of ∼40,000 km2 and longitudinal gradients less than 0.3°, this Late Cenozoic bajada is 10 times larger and 10 times flatter than classical bajadas. The tectonic and climatic setting that favored aerially extensive bajada formation is investigated in this paper. A tectonic model is proposed that involves forebulge development associated with subduction of the Arabian plate beneath the Asian plate. The model identifies the southern Gulf of Oman as a foredeep, the Oman Mountains as a subaerial forebulge, the vast bajada as a deposit that blankets the distal forebulge, and the Umm as Samim sabkha as a backbulge basin deposit. The model is supported by an excellent match between modern topography and a theoretical flexural profile of the Omani lithosphere, by the lateral distribution of Pliocene–Pleistocene sediments, by the inferred Late Miocene to Pleistocene age span of the vast bajada, and by evidence of active surface uplift of the Oman Mountains. A second mechanism, post-orogenic exhumation of the pre-existing Oman Mountains, is likely to have accompanied and perhaps slightly preceded forebulge growth, facilitating erosion and transport of sediment across the forebulge. Evidence for a palaeoclimatic model is provided by previous sedimentological studies and stable isotope data of cements in channel gravels as well as proxy data from ODP Leg 117 (Oman Margin). Transport of coarse-grained sediment across the bajada is interpreted to have occurred mainly during the Late Miocene to Early Pleistocene when a humid-wet climate prevailed across Arabia. High water tables and accompanying cementation of bajada gravels may have aided long-distance transport. The evolution of surficial bajada sediments is correlated with changing climatic and hydrological conditions—from seasonal/perennial flows during the Pliocene (and before) to mainly ephemeral and finally flash flood flows (signifying increasing aridity) during the Pleistocene. These data imply that the Indian Ocean monsoon, represented by rainfall frequency and intensity, had a more northerly range in Arabia in the past compared to the present.

Constraining rates of thrusting and erosion : Insights from kinematic thermal modeling

We present a thermal model of a simple thrust system that can be used to determine thrust rates and erosion rates from low-temperature thermochronology. Unlike previous models, this model incorporates the effects of erosion both during and after thrusting. In particular, we examine the modeled evolving thermal structure and pressure-temperature-time evolution of hanging-wall rocks that undergo fault-bend folding due to transport over a blind footwall ramp. In all cases, rocks cool as they move over the footwall ramp, potentially providing a common pinpoint for determining thrust rates. In the simplest case, low-temperature thermochronology of minerals that pass through their closure temperature over the ramp will yield details on thrust kinematics (thrust rate, timing of initiation, and duration of thrusting). Additional cooling ages of a more comprehensive sample suite can capture cooling due to erosion. In these latter cases, model results can place limitations on erosion rates.

A fixed sublithospheric source for the late Neogene track of the Yellowstone hotspot: Implications of the Heise and Picabo volcanic fields

The Heise and Picabo volcanic fields of eastern Idaho are part of the more extensive time-transgressive Yellowstone-Snake River Plain hotspot track. Calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt, so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern Snake River Plain along with some limited geophysical data. A 1.5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields. Using these new caldera locations, we calculate an extension-adjusted rate of 2.35 cm/yr for the North American plate over the last 6.66 m.y. and a velocity of 2.30 cm/yr over the 10.27 m.y. Recalculation of a previously determined plate velocity-based migration of the deformation field surrounding the eastern Snake River Plain yields an extension-adjusted rate of 2.38 ± 0.21 cm/yr. These migration rates all fall within the previously published range of North American plate velocities of 2.2 ± 0.8 cm/yr, 2.4 cm/yr, and 2.68 ± 0.78 cm/yr based on a global hot spot reference frame. The consistency of these rates suggest that over the last 10 m.y., the Yellowstone hot spot is fixed with respect to the motion of the North American plate and therefore consistent with a classical deep-sourced hotspot model.

Contemporary tectonic motion of the eastern Snake River Plain: A campaign global positioning system study

A comparison of precision campaign GPS data from 1995 and 2004 from ten benchmarks on the eastern Snake River Plain (eSRP) has revealed that the province moved 2.8 ± 0.3 mm/yr to the SW (232.4 ± 6.3°) relative to a fixed North American reference frame. The benchmarks had no measurable displacement relative to one another at the resolution of the GPS during the nine-year study, evidence that the province moves as a rigid, non-extending block. This scenario is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes. However, an additional small component of intra-plain extension must also be invoked to account for the observed NW-trending volcanic rift zones that transect the eSRP. We suggest that intra-plain extension is too slow (<1 mm/yr) to measure using our campaign GPS methods, but may be sufficient over millennial time scales to accommodate rift zone formation. Slower velocities measured on three benchmarks within the neighboring Basin and Range ‘seismic parabola’ are consistent with this region serving as a zone of detachment between the North American craton and the faster-moving eSRP.

Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through lidar topographic analysis

High-resolution lidar data reveal a prominent latest Pleistocene–Holocene scarp on the Sawtooth fault (central Idaho, United States). The fault scarp marks 55–65 km of the range front, and may comprise two segments. The scarp is 4–9 m high in latest Pleistocene glacial landforms (11–14 ka) and 2–3 m high in Holocene alluvial landforms, implying 2–3 postglacial rupture events. Patterns of fault scarp continuity, coupled with existing gravity data, suggest that active faulting may have migrated northward during Pleistocene time. Detailed comparisons of raw lidar digital elevation models (DEMs), bare-earth lidar DEMs, and field surveys indicate that the bare-earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult. The documentation of Holocene motion on the Sawtooth fault demonstrates that ENE-directed extension extends across central Idaho, and that the fault contributes to seismic hazards.

Neutron Log Measurement of Moisture in Unsaturated Basalt: Progress and Problems

The research described here examines whether neutron logs can be used in unsaturated basalt to quantitatively estimate moisture content, and, in tandem with other wireline logs, to determine the relative contribution to neutron log response by minerals containing chemically bound H. Our results show that it should be possible to quantitatively correlate neutron log response to the amount of both bound and unbound H regardless of saturated or unsaturated conditions. Such a correlation is possible only if bulk density, saturated porosity and neutron log response are already known in saturated conditions. It is not yet known, however, what the exact form of this correlation should be. To evaluate the candidate correlation equations, we compare measured permeability in unsaturated basalts with the calculated H content derived from these correlations. As the result of this evaluation, we identify intervals of clay-free impermeable, unsaturated basalt with apparently high H content. The neutron log response in these intervals may be caused by localized variations of the neutron slowing-down length, L s , due to low concentration of H in the formation and borehole environment. When L s is greater than the source-detector spacing of a neutron logging tool, the neutron log response can invert, resulting in potentially faulty interpretation of neutron logs. These measurement uncertainties in unsaturated basalts might be overcome by using a neutron logging tool with both a higher-flux neutron source and increased source-detector spacing, or a neutron tool with several detectors placed at different spacing from the source.