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Results for “Northwest Geology”

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At least 19 recordsLinked to original sources
USGS2006

Geophysical Investigations of the Smoke Creek Desert and their Geologic Implications, Northwest Nevada and Northeast California

The Smoke Creek Desert is a large basin about 100 km (60 mi) north of Reno near the California-Nevada border, situated along the northernmost parts of the Walker Lane Belt, a physiographic region defined by diverse topographic expression consisting of northweststriking topographic features and strike-slip faulting. Because geologic and geophysical framework studies play an important role in understanding the hydrogeology of the Smoke Creek Desert, a geophysical effort was undertaken to help determine basin geometry, infer structural features, and estimate depth to basement. In the northernmost parts of the Smoke Creek Desert basin, along Squaw Creek Valley, geophysical data indicate that the basin is shallow and that granitic rocks are buried at shallow depths throughout the valley. These granitic rocks are faulted and fractured and presumably permeable, and thus may influence ground-water resources in this area. The Smoke Creek Desert basin itself is composed of three large oval sub-basins, all of which reach depths to basement of up to about 2 km (1.2 mi). In the central and southern parts of the Smoke Creek Desert basin, magnetic anomalies form three separate and narrow EW-striking features. These features consist of high-amplitude short-wavelength magnetic anomalies and probably reflect Tertiary basalt buried at shallow depth. In the central part of the Smoke Creek Desert basin a prominent EW-striking gravity and magnetic prominence extends from the western margin of the basin to the central part of the basin. Along this ridge, probably composed of Tertiary basalt, overlying unconsolidated basin-fill deposits are relatively thin (< 400 m). The central part of the Smoke Creek Desert basin is also characterized by the Mid-valley fault, a continuous geologic and geophysical feature striking NS and at least 18-km long, possibly connecting with faults mapped in the Terraced Hills and continuing southward to Pyramid Lake. The Mid-valley fault may represent a lateral (east-west) barrier to ground-water flow. In addition, the Mid-valley fault may also be a conduit for along-strike (north-south) ground-water flow, channeling flow to the southernmost parts of the basin and the discharge areas north of Sand Pass.

Open-File Report↗
USGS2004

Surficial geologic map of the Northwest Memphis quadrangle, Shelby County, Tennessee, and Crittenden County, Arkansas

The depiction of geology on this map is designed to aid in urban planning and analysis of potential damage in the event of strong earthquake motion. The geologic map by itself does not analyze potential earthquake damage, but is designed to be used by seismologists who perform such analyses. The nature of geologic materials to a degree determines the severity of damage to infrastructure sustained during a strong earthquake.

Scientific Investigations Map↗
USGS1981

Northwest margin of California continental borderland: Marine geology and tectonic evolution

The northwest margin of the California continental borderland consists mainly of two northwest-trending pre-Neogene lithologic belts blanketed by Miocene and younger strata. These belts, which are lithologically and structurally correlated with the Franciscan Complex and Great Valley sequence of northern California, are interpreted to represent facies corresponding to the subduction complex and forearc-basin deposits of a late Mesozoic and Paleogene continental-margin arc-trench system. The outer belt, which forms the acoustic basement underlying the continental slope and Patton Ridge, is characterized by: (1) moderately high compressional velocities (5.1 to 6.2 km/sec), (2) discordant and discontinuous seismic reflectors, and (3) nonfossiliferous and diverse rock types t at range in metamorphic grade from zeolite-bearing arkosic sandstone to blueschist. Landward, the inner Great Valley sequence belt underlies Santa Rosa-Cortes Ridge and Santa Cruz and San Nicolas Basins; in contrast to the outer belt, this belt is characterized by: (1) intermediate compressional velocities (4.5 to 4.6 km/sec), (2) concordant and relatively continuous reflectors, and (3) a thick turbidite sequence of relatively undeformed and unmetamorphosed fossiliferous Cretaceous and Paleogene strata. The inner belt overlies a 6.6-km/sec layer that is probably oceanic crust, a relation consistent with the hypothesis that trapped old oceanic crust underlies the forearc basins along parts of some convergent margins. Well-developed wrench-fault structures in overlying Miocene strata record a transition from Paleogene subduction to Neogene transform faulting. Related events include widespread andesitic to dacitic volcanism and concurrent uplift and erosion of Miocene and older rocks. The proximity of these Miocene silicic volcanic rocks to the paleotrench (< 20 km) is best explained as resulting from passage of the southward-migrating Rivera ridge-fault-trench triple junction along the margin. On the basis of the age of the adjacent sea floor, passage of this triple junction occurred about 17 m.y. ago, a date that corresponds closely to the inception of volcanism and uplift in the northwest margin of the borderland. A widespread unconformity that separates highly deformed lower and lower middle iocene strata from relatively undeformed younger strata is assumed to mark the subsequent onset of wrench faulting. The timing of volcanism and uplift, and of the slightly younger wrenching event, closely matches the plate-tectonic model of Atwater and Molnar if the northwest margin of the borderland is restored to an earlier tectonic setting off Baja California, approximately 260 km southeast of its present position.

California↗