Geology Reports⌕ Search

Geology topics

Richard J. Blakely

Publications and source records attributed to Richard J. Blakely.

At least 55 records · Page 3Linked to original sources
USGS2004

Interpretation of the Seattle Uplift, Washington, as a passive-roof duplex

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago.

Washington↗
USGS2004

The Catfish Lake Scarp, Allyn, Washington: Preliminary field data and implications for earthquake hazards posed by the Tacoma fault

The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km across central Puget lowland from Tacoma to western Kitsap County. Tomography implies at least 6 km of post-Eocene uplift to the north of the fault relative to basinal sedimentary rocks to the south. Coastlines north of the Tacoma fault rose about 1100 years ago during a large earthquake. Abrupt uplift up to several meters caused tidal flats at Lynch Cove, North Bay, and Burley Lagoon to turn into forested wetlands and freshwater marshes. South of the fault at Wollochet Bay, Douglas-fir forests sank into the intertidal zone and changed into saltmarsh. Liquefaction features found beneath the marsh at Burley Lagoon point to strong ground shaking at the time of uplift. Recent lidar maps of the area southwest of Allyn, Washington revealed a 4 km long scarp, or two closely spaced en-echelon scarps, which correspond closely to the Tacoma fault gravity and aeromagnetic anomalies. The scarp, named the Catfish Lake scarp, is north-side-up, trends east-west, and clearly displace striae left by a Vashon-age glacier. A trench across the scarp exposed evidence for postglacial folding and reverse slip. No organic material for radiocarbon dating was recovered from the trench. However, relationships in the trench suggest that the folding and faulting is postglacial in age.

Washington↗
USGS2004

The Cottage Lake aeromagnetic lineament: A possible onshore extension of the southern Whidbey Island fault, Washington

The northwest-striking southern Whidbey Island fault zone (SWIF) was mapped previously using borehole data and potential-field anomalies on Whidbey Island and marine seismic surveys beneath surrounding waterways. Abrupt subsidence at a coastal marsh on south-central Whidbey Island suggests that the SWIF experienced a MW 6.5 to 7.0 earthquake about 3000 years ago. Southeast of Whidbey Island, a hypothesized southeastward projection of the SWIF would make landfall between the cities of Seattle and Everett. As part of systematic, ongoing studies by the U.S. Geological Survey, University of Washington, and other earth science organizations to evaluate potentially active faults and other earth hazards throughout the Puget Lowland, we test this hypothesis using aeromagnetic, lidar, and borehole data. Linear, northwest-striking magnetic anomalies traversing the mainland region project southeastward toward the communities of Woodinville and Maltby, Washington. All of these magnetic anomalies are low in amplitude and best illuminated in residual magnetic fields. The most prominent of the residual magnetic anomalies extends at least 16 km, lies approximately on strike with the SWIF on Whidbey Island, and passes near Crystal and Cottage Lakes, about 27 km southeast of downtown Everett. In places, this magnetic anomaly is associated with topographic lineaments, but spectral analysis indicates that the source of the anomaly extends to depths greater than 2 km and cannot be explained entirely by topographic effects. The Alderwood #1 oil exploration well located on strike with the Cottage Lake aeromagnetic lineament shows evidence of deformation over a total depth range of 3000 m; some beds within this interval exhibit intense fracturing and shearing, although deformation within the well can only be constrained as post-early Oligocene and pre-Pleistocene. Boreholes acquired as part of a wastewater tunnel project show evidence of soil disturbance at locations where some topographic and aeromagnetic lineaments cross the tunnel alignment. Some of the disturbance is likely tectonic in origin, although other explanations are possible. Some of the soil disturbance demonstrably predates the 15-13 ka Fraser glaciation of the Puget Lowland; other samples have inconclusive ages and may be younger. Subtle scarps in Pleistocene surfaces are visible on high-resolution lidar topography at a number of locations along the Cottage Lake aeromagnetic lineament. Collectively, the scarps are parallel to the trend of the aeromagnetic lineament and extend a total distance of 18 km. In the field, scarps exhibit 1 to 5 m of north-side-up offset. The scarps provide targets for future paleoseismic trenching studies to test the hypothesis that they have a tectonic origin.

Washington↗
USGS2003

Field guide to hydrothermal alteration in the White River altered area and in the Osceola Mudflow, Washington

The Cenozoic Cascades arcs of southwestern Washington are the product of long-lived, but discontinuous, magmatism beginning in the Eocene and continuing to the present (for example, Christiansen and Yeats, 1992). This magmatism is the result of subduction of oceanic crust beneath the North American continent. The magmatic rocks are divided into two subparallel, north-trending continental-margin arcs, the Eocene to Pliocene Western Cascades, and the Quaternary High Cascades, which overlies, and is east of, the Western Cascades. Both arcs are calc-alkaline and are characterized by voluminous mafic lava flows (mostly basalt to basaltic andesite compositions) and scattered large stratovolcanoes of mafic andesite to dacite compositions. Silicic volcanism is relatively uncommon. Quartz diorite to granite plutons are exposed in more deeply eroded parts of the Western Cascades Arc (for example, Mount Rainier area and just north of Mt. St. Helens). Hydrothermal alteration is widespread in both Tertiary and Quaternary igneous rocks of the Cascades arcs. Most alteration in the Tertiary Western Cascades Arc resulted from hydrothermal systems associated with small plutons, some of which formed porphyry copper and related deposits, including copper-rich breccia pipes, polymetallic veins, and epithermal gold-silver deposits. Hydrothermal alteration also is present on many Quaternary stratovolcanoes of the High Cascades Arc. On some High Cascades volcanoes, this alteration resulted in severely weakened volcanic edifices that were susceptible to failure and catastrophic landslides. Most notable is the sector collapse of the northeast side of Mount Rainier that occurred about 5,600 yr. B.P. This collapse resulted in formation of the clay-rich Osceola Mudflow that traveled 120 km down valley from Mount Rainier to Puget Sound covering more than 200 km2. This field trip examines several styles and features of hydrothermal alteration related to Cenozoic magmatism in the Cascades arcs. The morning of the trip will examine the White River altered area, which includes high-level alteration related to a large, early Miocene magmatic-hydrothermal system exposed about 10 km east of Enumclaw, Washington. Here, vuggy silica alteration is being quarried for silica and advanced argillic alteration has been prospected for alunite. Clay-filled fractures and sulfide-rich, fine-grained sedimentary rocks of hydrothermal origin locally are enriched in precious metals. Many hydrothermal features common in high-sulfidation gold-silver deposits and in advanced argillic alteration zones overlying porphyry copper deposits (for example, Gustafson and Hunt, 1975; Hedenquist and others, 2000; Sillitoe, 2000) are exposed, although no economic base or precious metal mineralized rock has been discovered to date. The afternoon will be spent examining two exposures of the Osceola Mudflow along the White River. The Osceola Mudflow contains abundant clasts of altered Quaternary rocks from Mount Rainier that show various types of hydrothermal alteration and hydrothermal features. The mudflow matrix contains abundant hydrothermal clay minerals that added cohesiveness to the debris flow and helped allow it to travel much farther down valley than other, noncohesive debris flows from Mount Rainier (Crandell, 1971; Vallance and Scott, 1997). The White River altered area is the subject of ongoing studies by geoscientists from Weyerhaeuser Company and the U.S. Geological Survey (USGS). The generalized descriptions of the geology, geophysics, alteration, and mineralization presented here represent the preliminary results of this study (Ashley and others, 2003). Additional field, geochemical, geochronologic, and geophysical studies are underway. The Osceola Mudflow and other Holocene debris flows from Mount Rainier also are the subject of ongoing studies by the USGS (for example, Breit and others, 2003; John and others, 2003; Plumlee and others, 2003, Sisson and others, 2003; Vallance and others, 2003). Studies of hydrothermal alteration in the Osceola Mudflow are being used to better understand fossil hydrothermal systems on Mount Rainier and potential hazards associated with this alteration.

Washington↗
USGS2003

Geologic map of the Mound Spring quadrangle, Nye and Clark Counties, Nevada, and Inyo County, California

The Mound Spring quadrangle, the southwestern-most 7.5' quadrangle of the area of the Las Vegas 1:100,000-scale quadrangle, is entirely within the Pahrump Valley, spanning the Nevada/California State line. New geologic mapping of the predominantly Quaternary materials is combined with new studies of gravity and geochronology in this quadrangle. Eleven predominantly fine-grained units are delineated, including playa sediment, dune sand, and deposits associated with several cycles of past groundwater discharge and distal fan sedimentation. These units are intercalated with 5 predominantly coarse-grained alluvial-fan and wash gravel units mainly derived from the Spring Mountains. The gravel units are distinguished on the basis of soil development and associated surficial characteristics. Thermoluminescence and U-series geochronology constrain most of the units to the Holocene and late and middle Pleistocene. Deposits of late Pleistocene groundwater discharge in the northeast part of the quadrangle are associated with a down-to-the-southwest fault zone that is expressed by surface fault scarps and a steep gravity gradient. The gravity field also defines a northwest-trending uplift along the State line, in which the oldest sediments are poorly exposed. About 2 km to the northeast a prominent southwest-facing erosional escarpment is formed by resistant beds in middle Pleistocene fine-grained sediments that dip northeast away from the uplift. These sediments include cycles of groundwater discharge that were probably caused by upwelling of southwesterly groundwater flow that encountered the horst.

Miscellaneous Field Studies Map↗
USGS2001

Active tectonics of the Devils Mountain Fault and related structures, northern Puget Lowland and eastern Strait of Juan de Fuca region, Pacific Northwest

Information from marine high-resolution and conventional seismic-reflection surveys, aeromagnetic mapping, coastal exposures of Pleistocene strata, and lithologic logs of water wells is used to assess the active tectonics of the northern Puget Lowland and eastern Strait of Juan de Fuca region of the Pacific Northwest. These data indicate that the Devils Mountain Fault and the newly recognized Strawberry Point and Utsalady Point faults are active structures and represent potential earthquake sources.

Professional Paper↗
USGS2000

New aeromagnetic data reveal large strike-slip (?) faults in the Northern Willamette Valley, Oregon

High-resolution aeromagnetic data from the northern Willamette Valley, Oregon, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are well defined by the data and appear to extend far beyond their mapped surface traces. The Mount Angel fault, the likely source of the Richter magnitude (M1) 5.6 earthquake in 1993, is at least 55 km long and may be connected in the subsurface with the Gales Creek fault 25 km farther northwest. Northeast of the Mount Angel fault, a 60-km-long, northwest-striking anomaly may represent a previously unrecognized dextral-slip fault beneath the towns of Canby and Molalla. Vertical offsets along the Mount Angel fault increase with depth, indicating a long history of movement for the fault. Dominantly northwest- trending, relatively straight faults, consistent stepover geometries, offset magnetic anomalies and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel-Gales Creek fault zone.

Oregon↗
USGS2000

Summary of geophysical investigations of the Death Valley regional water-flow modeling project, Nevada and California

This report summarizes geophysical investigations in the Amargosa Desert and surrounding areas conducted between 1997 and 2000 in support of the Death Valley Regional Water-Flow Modeling Project of the U.S. Geological Survey. These investigations used both gravity and aeromagnetic data to develop a regional-scale tectonic and lithologic model for the near-surface. Gravity data were inverted in order to estimate the depth to pre-Cenozoic basement throughout the study area. This basement surface is interpreted to be the contact between pre-Cenozoic carbonate rocks and overlying alluvium, an important component of water-flow models. Gravity data also were used to identify locations where basement rocks are anomalously dense, possibly indicating areas resistant to water flow. A new aeromagnetic survey of the central part of the study area was used to map concealed faults and areas underlain by volcanic rocks.

Open-File Report↗
USGS2000

Aeromagnetic Survey of the Amargosa Desert, Nevada and California: A Tool for Understanding Near-Surface Geology and Hydrology

A high-resolution aeromagnetic survey of the Amargosa Desert and surrounding areas provides insights into the buried geology of this structurally complex region. The survey covers an area of approximately 7,700 km2 (2,970 mi2), extending from Beatty, Nevada, to south of Shoshone, California, and includes parts of the Nevada Test Site and Death Valley National Park. Aeromagnetic flight lines were oriented east-west, spaced 400 m (0.25 mi) apart, and flown at an altitude of 150 m (500 ft) above terrain, or as low as permitted by safety considerations. Characteristic magnetic anomalies occur over volcanic terranes, such as Yucca Mountain and the Greenwater Range, and over Proterozoic basement rocks, such as Bare Mountain and the Black Mountains. Linear magnetic anomalies caused by offsets of volcanic rocks permit detailed mapping of shallow faults in volcanic terranes. Of particular interest are subtle anomalies that overlie alluvial deposits at Devils Hole and Pahrump Valley. Alignments of springs along magnetic anomalies at these locales suggest that these anomalies are caused by faults that cut the alluvium, displace magnetic rocks at depth, and eventually influence ground-water flow. Linear magnetic anomalies over the Funeral Mountains appear to coincide with a prominent set of north-northeast-striking faults that cut the Precambrian Stirling Quartzite, rocks that are typically nonmagnetic. The position and orientation of these anomalies with respect to springs north of Furnace Creek suggest that the faults may act as conduits for the flow of water from the north into Death Valley, but the mineralogical cause of the anomalies is unknown.

Open-File Report↗
USGS1999

Cenozoic basins of the Death Valley extended terrane as reflected in regional-scale gravity anomalies

The contrast in density between basin-filling deposits and underlying basement rocks in the Basin and Range province produces a distinctive pattern of gravity anomalies that roughly correlates with basin-and-range topography. This is especially apparent in the Death Valley extended terrane, where minima in residual gravity closely correspond to well-known extensional basins, such as Owens Valley, Saline Valley, Pahrump Valley, and Death Valley itself. A three-dimensional calculation of the thickness and shape of Cenozoic deposits throughout the Basin and Range based on digital gravity, geologic, and topographic information is here interpreted in terms of the depths and shapes of extensional basins in the Death Valley region. In most basins, the gravity inversion predicts a much more complex basement than might be assumed from the surface topography. Death Valley itself is, to first order, a two-dimensional feature; as reflected by the topography of the bounding ranges down to 400–800 m below the valley floor, it is a long, narrow, and continuous valley along its entire length of 200 km. This apparent uniformity, however, belies a complex basement surface beneath the Cenozoic cover as revealed by the gravity inversion. The floor of Death Valley is pockmarked with four deep (3–5 km), steep-sided depressions that may have formed as relatively small pull-apart structures superimposed on the more uniform extension that created the twodimensional aspects of Death Valley. Lateral dimensions of these subbasins suggest a minimum of 10 km of northwest extension across Death Valley. If a regionally continuous detachment surface lies beneath Death Valley, it must lie deeper than the subbasins, that is, below a depth of 3–5 km. Similar deep, steep-sided subbasins are seen elsewhere in the Death Valley extended terrane, notably at Saline Valley, Panamint Valley, Pahrump Valley, the Amargosa Desert, and Fremont Valley, but are noticeably absent from the Mojave Desert block south of the Garlock fault zone where extensional rates are lower. The floor of the caldera complex at the southwest Nevada volcanic field is predicted by the gravity inversion to be extraordinarily deep (>8 km) and rugged. Deep, linear basins that branch out of the caldera complex may reflect modification of extensional strain due to massive injections of magma into the middle and upper crust.

California, Nevada↗
USGS1996

Airborne hunt for faults in the Portland-Vancouver area

Geologic hazards in the Portland-Vancouver area include faults entirely hidden by river sediments, vegetation, and urban development. A recent aerial geophysical survey revealed patterns in the Earth's magnetic field that confirm the existence of a previously suspected fault running through Portland. It also indicated that this fault may pose a significant seismic threat. This discovery has enabled the residents of the populous area to better prepare for future earthquakes.

Fact Sheet↗
USGS1993

Geyser's magma chamber, California: constraints from gravity data, density measurements, and well information

A new crustal model based on isostatic residual gravity, geologic mapping, well information, and density measurements shows that the high-gradient parts of the residual gravity anomaly can be explained in terms of lithologic variations within the upper 7 km of the crust, consistent with the upper-crustal framework of the area. This conclusion does not rule out the presence of a magma chamber at lower crustal depths; the broad aspects of the gravity anomaly support the presence of low-density partial melting at 15 to 20 km depth, consistent with magnetotelluric soundings and other geophysical measurements.

Conference Paper↗