Anatomy of the southern Cordilleran hingeline, Utah and Nevada, from deep electrical resistivity profiling

  • Wannamaker P
  • Johnston J
  • Stodt J
 et al. 
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Abstract

To address outstanding questions in Mesozoic-Cenozoic structure and present-day deep physicochemical state in the region of the southern Cordilleran hingeline, a detailed, east-west profile of magnetotelluric (MT) soundings 155 km in length was acquired, From these soundings, a resistivity interpretation was produced using an inversion algorithm based on a structural parameterization. In the upper ten kilometers of the transect, the interpretation shows two segments of low resistivity lying beneath allochthonous rocks of the Late Mesozoic, Sevier thrust sheet. Subsequent industry drilling motivated in part by our surveying confirms the existence and position of the eastern subthrust conductor and, more spectacularly, identifies the presence of yet deeper, autochthonous Mesozoic rocks. The conductors cannot be specified uniquely with present public data, because their electrical characteristics appear consistent with Paleozoic, pyrolized graphitic strata of either Late Devonian-Mississippian or Middle Ordovician age, However, the drilling results show that Late Paleozoic and younger rocks lie underthrust much farther west than recognized previously, and perhaps as far west as the Utah-Nevada border. A simple structural interpretation is offered where one underthrust segment of low-resistivity sediments was created originally, but this segment was broken later into two major ones during higher-angle Tertiary extension. For the middle and lower crust, the MT data imply a nearly 1-D resistivity structure of remarkable uniformity across the entire transect. In particular, there occurs a deep low-resistivity layer most pronounced (about 8 ohm-m) in the nominal depth interval of 17.5 to 40 km, The MT data indicate that the layer cannot be confined to a single thin layer in the lower crust but instead represents vertically distributed low resistivity, With temperatures estimated from surface heat flow to range from 550 degrees C to 1050 degrees C with depth in the layer, and with a metaigneous mineralogy of high metamorphic grade assumed, mechanisms to produce the low resistivity can be constrained, The deep layer is thus consistent with H2O-rich brines at its upper levels, fluids of lower H2O activity toward middle levels, and H2O-deficient melting below about 30 km, The marked uniformity of the deep conductive layer across the transect suggests a similar uniformity of deep physicochemical state. However, this is not at odds with recent analyses of heat flow Curie depth, Quaternary extension, and basaltic volcanism. Pre-existing structural fabrics have had no measureable influence on localizing regions of high temperature, fluids and melting in the lower crust, at least averaged over the scale of tens of kilometers. Given its uniformity over a distance of 155 km or more, the depth to the regional deep conductor does not appear related to the distribution of high-temperature geothermal resources.

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Authors

  • Philip E. Wannamaker

  • Jeffery M. Johnston

  • John A. Stodt

  • John R. Booker

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