Magnetotelluric transect of Long Valley caldera: Resistivity cross‐section, structural implications, and the limits of a 2-D analysis

Abstract

Twenty-four magnetotelluric (MT) soundings have been collected in an east-west profile across the center of Long Valley caldera. The average station spacing is approximately I km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures. Additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the MT line. Our MT data set reveals numerous resistivity structures which illuminate the evolution and present state of the Long Valley system. Many of these have been quantified through two-dimensional (2-D) finite element modeling emphasizing the transverse magnetic (TM) mode. Important structural components include low-resistivity layers 0.5-1.5 km deep under the eastern half of the caldera, beneath the axial graben of the resurgent dome, and under the west caldera moat. Most of this layering appears to lie in post-caldera Early Rhyolite tuffs, and the uppermost unwelded Bishop Tuff. These rhyolite units have been observed to be porous and highly altered and to commonly contain Pleistocene intercalated lacustrine clays. The remainder and majority of the Bishop Tuff appears highlyresistive. A low resistivity layer also occurs below the axial graben near the base of the Bishop Tuff (1.5 km). Hydrothermal fluids or alteration in precaldera volcanic strata or, less likely, carbonaceous metasediments may be the cause of this. Resistive, probably crystalline basement at high levels is apparent beneath the center of resurgence. Low resistivities are modeled at a depth around 5 km below the entire west moat and central graben and may represent a zone of hydrothermal fluids released from magma crystallization, with potential magmatic contributions at greater depths. The correspondence between this low resistivity and teleseismic delay and low density zones found in other studies is quite striking. A subtle anomaly in the transverse electric (TE) mode impedance is weakly suggestive of a midcrustal conductive axis centered beneath the central graben and resurgent dome. However, it cannot be simulated by two-dimensional transverse electric calculations and requires a full three-dimensional evaluation to ensure that the anomaly does not represent resistivity complexity in just the upper few kilometers. A fundamental, caldera-wide 3-D effect is documented by comparison of observed and computed TE impedance and vertical magnetic field data. The abrupt termination of conductive caldera sediments less than 10 km north and south of our profile greatly depresses the observed TE apparent resistivity and vertical magnetic field relative to the model calculations for periods greater than 0.3 s for the central and eastern caldera. Analysis of the TE mode data also suggests that a similar finite-strike effect lies in the response at periods greater than 3 s due to the mid-crustal west moat conductor. The TM mode measurements are judged to also contain some large-scale departure from the ...

Three‐dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics

“…Multiple EM and MT studies of Long Valley Caldera have imaged the subsurface down to 5 km [Hermance et al, 1984[Hermance et al, , 1988Park and Torres-Verdin, 1988;Wannamaker et al, 1991;Sorey et al, 1991; [Sorey et al, 1991] shaded by temperature, where hot water upflows near well 44-16 at ∼220 ∘ C (dark orange) and flows east to Casa Diablo (CD) (∼170 ∘ C) and farther east toward Crowley Lake (CL) (∼50 ∘ C). MT stations from Wannamaker et al [1991] are indicated by purple triangles, and black triangles represent MT stations collected by the U.S. Geological Survey in 2014 and 2015. Geologic features are RD, resurgent dome; MG, medial graben; MM, Mammoth Mountain; IC, Inyo Craters.…”

2-D Versus 3-D Magnetotelluric Data Interpretation

“…Our objective in this section is to determine the limits on the modes, period and spatial ranges that can be used during the 2-D interpretation of 3-D data. Perhaps the most accepted paradigm in the interpretation of 3-D data is emphasis on the TM mode in the presence of 3-D conductive structures and the TE-mode when the 3-D structure is resistive (Ting and Hohmann, 1981;Wannamaker et al, 1984aWannamaker et al, , b, 1991Berdichevsky, 1999). This approach allows the determination of reasonable geological models in many cases, although it leads to incorrect interpretations in some situations.…”

2-D Versus 3-D Magnetotelluric Data Interpretation

“…The magnetotelluric study of the Long Valley caldera magmatic system by Wannamaker et al (1991) along an E-W profile is a good example of an MT profile crossing a 3-D conductive structure. The observed TE apparent resistivities and the vertical magnetic field were greatly depressed in comparison with the calculated 2-D model responses.…”

“…Our objective in this section is to determine the limits on the modes, period and spatial ranges that can be used during the 2-D interpretation of 3-D data. Perhaps the most 2-D VERSUS 3-D MAGNETOTELLURIC DATA INTERPRETATION accepted paradigm in the interpretation of 3-D data is emphasis on the TM mode in the presence of 3-D conductive structures and the TE-mode when the 3-D structure is resistive (Ting and Hohmann, 1981;Wannamaker et al, 1984aWannamaker et al, , b, 1991Berdichevsky, 1999). This approach allows the determination of reasonable geological models in many cases, although it leads to incorrect interpretations in some situations.…”

“…The problems and limitations of 2-D magnetotelluric interpretation of 3-D data have been a recurrent topic since the development of the first numerical codes to solve the multi-dimensional magnetotelluric problem (Weidelt, 1975;Jones and Vozoff, 1978;Kaufman and Keller, 1981;Ting and Hohmann, 1981;Hermance, 1982;Jones, 1983;Park et al, 1983;Wannamaker et al, 1984aWannamaker et al, , b, 1991Berdichevsky et al, 1998;Berdichevsky, 1999;Wannamaker, 1999). Additionally, several other authors have addressed some specific points (i.e.…”