Fault mechanics: A raging controversy exists in geology and geophysics over the strength of natural faults relative to laboratory friction measurements—are they strong or weak?? Various lines of evidence suggest that faults are weak relative to lab friction, but the cause for this is unclear. This controversy centers on the San Andreas fault, but only very limited samples will be returned from the SAFOD drill site.
I plan to initiate a multidisciplinary research project into the formation of the various fault rocks found around the mechanically enigmatic low-angle normal fault (or detachment faults). Detachment faults are the “weak end-member” of all natural faults: they form at high angles to the maximum principal stress as well as under low differential stress (relative to thrusts or strike-slip faults). This project will aim to characterize the mechanical evolution of two or three low-angle normal faults (including the west Salton detachment fault—see below). Specific methods will include (1) detailed field structural geology to constrain the paleo-stress field around the detachments, (2) structural petrology to characterize the mechanisms of formation of associated fault rocks, (3) stable-isotope and fluid-inclusion analyses to constrain the source and temperature of the fluids that passed through the fault rocks, and (4) coupled structural, chemical, and hydrological modeling to constrain the reasonable fluid fluxes and pressures. Planned collaborators are Jane Selverstone (UNM), Andy Campbell (NMT), and Brian McPherson (NMT).
• Age of the Socorro Magma Body (SMB) and related surface uplift (NSF Tectonics Program). The SMB is the second largest known magma body on Earth: it is a sill about 130 m thick at a depth of 19 km and with a map-view area of ~3400 km2 (so >340 km3 of magma). Surface uplift above the SMB occurs in a bullseye-shaped region with rates of ~2.5 mm/yr at the center. In this project we use river terraces to determine the duration and episodicity of surface uplift related to the SMB. Terraces are deformed both by the SMB and by active normal faults. Terrace ages will be determined using 36Cl depth profiles, and correlated locally and regionally using soils, surface geomorphology, and terrace stratigraphy. This study will also provide information on fault slip rates and on climatic forcing of river terrace formation. Co-PIs are Jolante van Wijk, Fred Phillips and Bruce Harrison. This project forms the core of PhD research of Brad Sion (river terrace work) and part of the PhD work of Rediet Abera and Shuoyu Yao (both geodynamic modeling).
• 3D palinspastic reconstructions of the highly extended Death Valley terrain, from the southern Sierra Nevada to Las Vegas, Nevada (NSF Integrated Earth Systems Program). These reconstructions, to be made at various time steps between ~14 Ma and the present, will serve as input to a forward geodynamic-hydrologic model that predicts surface and groundwater flow during the tectonic evolution of the area. These predictions will be tested against spring water chemistry and subsurface residence times, as well as biological diversity and endemism of spring fish, snails and microbes. NMT Co-PIs include Fred Phillips (lead PI), John Wilson, Jolante van Wijk, along with researchers at UNLV, Pudue, Desert Research Institute and University of the Pacific. This project will support three NMT PhD students and one Postdoctoral Researcher.
• Extensional tectonics of the Great Kavir (Desert), central Iran. Thermochronology (40Ar/39Ar on biotite, muscovite amphibole and K-feldspar multi-domain diffusion modeling) applied to the footwall of a normal-sense detachment fault system shows that large-magnitude extension occurred in this part of Iran in Late Cretaceous time, earlier than exhumation of other known metamorphic core complexes in Iran. Stratigraphic relationships suggest that extension continued to Eocene time. PhD research of Ahmad Malekpour-Alamdarie. Collaboration with Matt Heizler of the New Mexico Bureau of Geology.
• Large-magnitude gravity-slide tectonics, Sawtooth Mountains, New Mexico. Remnant klippen of one or more gravity-driven sheets record motion on an(?) allochthon, originally at least several tens of square kilometers in area, underlain by a zone of soft-sediment deformation ~50-100 m thick. Sediments involved are coarse sandstones to conglomerates of the alluvial apron from a mid-Cenozoic andesitic stratovolcano. We use detailed geologic mapping and cliff-mapping of the klippen (most exposures are on cliffs) in conjunction with structural measurements and outcrop- to thin-section-scale structural observations to determine if sliding occurred in a single or multiple events, catastrophically or slowly, and to determine the direction(s) of sliding, which will help to understand the cause(s) of the gravity-driven tectonics. M.S. thesis of Jeff Dobbins, co-advised by Steve Cather of the New Mexico Bureau of Geology.
• Decollement tectonics of Laramide and Rio Grande Rift age. Paleozoic rocks east of Socorro, in an area of hundreds of square kilometers in size, are detached along Permian evaporite-rich strata (Yeso Fmn.) and display structures mainly consistent with internal extension of the upper plate. Folds in the decollement zone are cut by dikes of ~32 Ma age and both folds and dikes are cut by ramp-flat low-angle faults with younger-on-older stratigraphic separation. These faults controlled local deposition in a volcanic-sedimentary half graben. It appears that the decollements formed in front of (in the footwall of) a Laramide uplift, and were reactivated during Rio Grande rift time. M.S. thesis of Mark Green, co-advised by Steve Cather of the New Mexico Bureau of Geology.
|Created: ( Tuesday, 19 April 2011 15:05 )|
|Last Updated: ( Monday, 22 February 2016 17:41 )|