Search USGS⌕ Search

Geology topics

Brian D. Rodriguez

Publications and source records attributed to Brian D. Rodriguez.

At least 55 records · Page 3Linked to original sources
USGS2003

Magnetotelluric data release for locating the Archean/Proterozoic suture zone, east-central Tooele County, Utah

Many sediment-hosted gold deposits occur along linear trends in the Great Basin. The distribution and genesis of these deposits in the Great Basin is not fully understood. In general, most models agree that regional structures played an important role in the spatial distribution of these deposits (e.g. Arehart and others, 1993; Ilchik and Barton, 1997; Radtke, 1985; Shawe, 1991; Sillitoe and Bonham, 1990; Tosdal, 1998). To investigate crustal structures that may be related to the genesis of gold deposits in the Great Basin, a regional south-north profile of magnetotelluric (MT) soundings was acquired in 2003. Resistivity modeling of the MT data can be used to investigate buried structures or sutures that may have influenced subsequent tectonism, sedimentation, and regional fluid flow. The goal of this survey is to infer the location of the Archean/Proterozoic suture zone in east-central Tooele County, Utah. The purpose of this report is to release the MT sounding data; no interpretation of the data is included.

Open-File Report↗
USGS2002

Magnetotelluric data along the Tangle Lakes profile, Alaska

The 89 km long Tangle Lakes profile of 14 magnetotelluric (MT) stations begins 4 km south of the confluence of Augustana Creek and the Delta River in south central Alaska. The northernmost station, TLM13, is located just south of the Denali Fault in the rugged Clearwater Mountains. The north-south profile crosses the Ampitheater Mountains northwest of the Tangle Lakes area and continues until it terminates in the flatlands south of the Alphabet Hills. The profile intersects the Denali Highway 32km west of Paxson.

Alaska↗
USGS2002

Analysis of magnetotelluric profile data from the Ruby Mountains metamorphic core complex and southern Carlin Trend region, Nevada

We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings are in three east-west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass and Secret Pass latitudes). Two shorter lines cross a prominent east-west structure to the north of the northern profile. MT impedance tensor and vertical magnetic field rotations imply a N-NNE average regional geoelectric strike, similar to surface geologic trends. Model resistivity cross sections were derived using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure, emphasizing the transverse magnetic (TM) mode and vertical magnetic field data. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity-thickness product) from east to west. These increases in conductance are attributed to graphitization caused by Elko-Sevier era compressional shear deformation and possibly by intrusive heating. The resistive crystalline central massifs adjoin the host stratigraphy across crustal-scale, subvertical fault zones. These zones provide electric current pathways to the lower crust for heterogeneous, upper crustal induced current flow. Resistive core complex crust may be steeply bounded under the middle of the neighboring grabens and not deepen at a shallow angle to arbitrary distances to the west. The numerous crustal breaks imaged with MT may contribute to the low effective elastic thickness estimated regionally for the Great Basin and exemplify the mid-crustal, steeply dipping slip zones in which major earthquakes nucleate. An east-west oriented conductor in the crystalline upper crust spans the East Humboldt Range and northern Ruby Mountains. The conductor may be related to an inferred ArcheanProterozoic suture or nearby graphitic metasediments, with possible alteration by middle Tertiary magmatic activity. Lower crustal resistivity everywhere under the profiles is low and appears quasi one-dimensional. It is consistent with a low rock porosity (

Open-File Report↗
USGS2002

Resistivity structure across the Humboldt River basin, north-central Nevada

Magnetotelluric data collected along five profiles show deep resistivity structures beneath the Battle Mountain-Eureka and Carlin gold trends in north-central Nevada, which appear consistent with tectonic breaks in the crust that possibly served as channels for hydrothermal fluids. It seems likely that gold deposits along these linear trends were, therefore, controlled by deep regional crustal fault systems. Two-dimensional resistivity modeling of the magnetotelluric data generally show resistive (30 to 1,000 ohm-m) crustal blocks broken by sub-vertical, two-dimensional, conductive (1 to 10 ohmm) zones that are indicative of large-scale crustal fault zones. These inferred fault zones are regional in scale, trend northeast-southwest, north-south, and northwest-southeast, and extend to mid-crustal (20 km) depths. The conductors are about 2- to 15-km wide, extend from about 1 to 4 km below the surface to about 20 km depth, and show two-dimensional electrical structure. By connecting the locations of similar trending conductors together, individual regional crustal fault zones within the upper crust can be inferred that range from about 4- to 10-km wide and about 30- to 150-km long. One of these crustal fault zones coincides with the Battle Mountain-Eureka mineral trend. The interpreted electrical property sections also show regional changes in the resistive crust from south to north. Most of the subsurface in the upper 20 km beneath Reese River Valley and southern Boulder Valley are underlain by rock that is generally more conductive than the subsurface beneath Kelly Creek Basin and northern Boulder Valley. This suggests that either elevated-temperature or high-salinity fluids, alteration, or carbonaceous rocks are more pervasive in the more conductive area (Battle Mountain Heat-Flow High), which implies that the crust beneath these valleys is either more fractured or has more carbonaceous rocks than in the area surveyed along the 41st parallel.

Nevada↗
USGS2002

Magnetotelluric data in the middle Rio Grande basin, Albuquerque volcanoes, New Mexico

The population in the Albuquerque-Santa Fe region of New Mexico is rapidly growing. The Santa Fe Group aquifer in the Middle Rio Grande Basin is the main source of municipal water for the greater Albuquerque metropolitan area. The capacity of this aquifer is more limited than previously thought (Thorn et al., 1993). The Middle Rio Grande Basin, as defined hydrologically and used here, is the area within the Rio Grande Valley extending from Cochiti Dam downstream to the community of San Acacia (Figure 1). Because approximately 600,000 people (40 percent of the population of New Mexico) live in the study area (Bartolino, 1999), water shortfalls could have serious consequences. Future growth and land management in the region depends on accurate assessment and protection of the region’s groundwater resources. An important issue in defining the ground water resources is a better understanding of the hydrogeology of the Santa Fe Group and the other sedimentary deposits that fill the Rio Grande rift.

New Mexico↗
USGS2001

Deep regional resistivity structure across the Battle Mountain-Eureka and Carlin trends, north-central Nevada

Magnetotelluric data collected along four, regional scale, southwest-to-northeast profiles show deep resistivity structures beneath the Battle Mountain-Eureka and Carlin gold trends in north-central Nevada, which appear consistent with tectonic breaks in the crust that possibly served as channels for hydrothermal fluids. It seems likely that gold deposits along these linear trends were, therefore, controlled by deep regional crustal fault systems. Two-dimensional resistivity modeling of the magnetotelluric data generally show resistive (30 to 1,000 ohm-m) crustal blocks broken by narrow, sub-vertical, two-dimensional, conductive (1 to 10 ohm-m) zones that are indicative of large-scale crustal fault zones. These inferred fault zones are regional in scale, trend southeast-to-northwest, and extend to mid-crustal (20 km) depths. The conductors are about 3 to 15 km wide, extend from 1 to 8 km below the surface to about 20 km depth, and show two- dimensional electrical structure with general north to northwesterly strikes. From connecting the locations of the conductors together, a single regional crustal fault zone can be inferred that is about 10 km wide within the upper crust and about 150-km long. It coincides with the Battle Mountain-Eureka mineral trend. The images also show regional changes in the resistive crust from north to south. Most of Reese River Valley and Boulder Valley are underlain by a thick (20 km) southwest-to-northeast section of conductive (1 to 10 ohm-m) rock, suggesting that high-temperature fluids are more pervasive in this area (Battle Mountain Heat-Flow High), which implies that the crust beneath these valleys is more fractured than in the areas surveyed to the south.

Nevada↗