Geology topics

Richard J. Blakely

Publications and source records attributed to Richard J. Blakely.

At least 37 records · Page 2Linked to original sources

USGS2014

Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data

The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeast-trending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low reflects the large density contrast between basin fill and the mafic crust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is &sim;6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pull-apart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.

Oregon↗

USGS2013

Structural evolution of the east Sierra Valley system (Owens Valley and vicinity), California: a geologic and geophysical synthesis

The tectonically active East Sierra Valley System (ESVS), which comprises the westernmost part of the Walker Lane-Eastern California Shear Zone, marks the boundary between the highly extended Basin and Range Province and the largely coherent Sierra Nevada-Great Valley microplate (SN-GVm), which is moving relatively NW. The recent history of the ESVS is characterized by oblique extension partitioned between NNW-striking normal and strike-slip faults oriented at an angle to the more northwesterly relative motion of the SN-GVm. Spatially variable extension and right-lateral shear have resulted in a longitudinally segmented valley system composed of diverse geomorphic and structural elements, including a discontinuous series of deep basins detected through analysis of isostatic gravity anomalies. Extension in the ESVS probably began in the middle Miocene in response to initial westward movement of the SN-GVm relative to the Colorado Plateau. At ca. 3-3.5 Ma, the SN-GVm became structurally separated from blocks directly to the east, resulting in significant basin-forming deformation in the ESVS. We propose a structural model that links high-angle normal faulting in the ESVS with coeval low-angle detachment faulting in adjacent areas to the east.

California↗

USGS2012

Crustal magnetic anomalies

This Quadrennial Report departs in format from its predecessors. Earlier reports provided brief descriptions of the entire range of topics on “crustal magnetic anomalies.” Following guidelines established by the AGU, we instead focus our discussion on only a few selected topics. Our abbreviated list of references reflects this restricted focus. A more comprehensive bibliography can be obtained from the authors as noted subsequently. The past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research. Advances in instrumentation, navigation, and data processing techniques resulted in increasing use of two-dimensional anomaly maps rather than profile data. Although maps are commonly used in aeromagnetic studies of continental areas, their use in the ocean basins has been limited until recently. The other important change has been a trend towards increased integration of magnetic anomaly data with gravity, seismic, geochemical, remote sensing, and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data.

Reviews of Geophysics↗

USGS2012

Tectonic setting of the Wooded Island earthquake swarm, eastern Washington

Magnetic anomalies provide insights into the tectonic implications of a swarm of ~1500 shallow (~1 km deep) earthquakes that occurred in 2009 on the Hanford site,Washington. Epicenters were concentrated in a 2 km2 area nearWooded Island in the Columbia River. The largest earthquake (M 3.0) had first motions consistent with slip on a northwest-striking reverse fault. The swarm was accompanied by 35 mm of vertical surface deformation, seen in satellite interferometry (InSAR), interpreted to be caused by ~50 mm of slip on a northwest-striking reverse fault and associated bedding-plane fault in the underlying Columbia River Basalt Group (CRBG). A magnetic anomaly over exposed CRBG at Yakima Ridge 40 km northwest of Wooded Island extends southeastward beyond the ridge to the Columbia River, suggesting that the Yakima Ridge anticline and its associated thrust fault extend southeastward in the subsurface. In map view, the concealed anticline passes through the earthquake swarm and lies parallel to reverse faults determined from first motions and InSAR data. A forward model of the magnetic anomaly near Wooded Island is consistent with uplift of concealed CRBG, with the top surface <200 m below the surface. The earthquake swarm and the thrust and bedding-plane faults modeled from interferometry all fall within the northeastern limb of the faulted anticline. Although fluids may be responsible for triggering the Wooded Island earthquake swarm, the seismic and aseismic deformation are consistent with regional-scale tectonic compression across the concealed Yakima Ridge anticline.

Washington↗

USGS2012

Holocene faulting in the Bellingham forearc basin: Upper-plate deformation at the northern end of the Cascadia subduction zone

The northern Cascadia forearc takes up most of the strain transmitted northward via the Oregon Coast block from the northward-migrating Sierra Nevada block. The north-south contractional strain in the forearc manifests in upper-plate faults active during the Holocene, the northern-most components of which are faults within the Bellingham Basin. The Bellingham Basin is the northern of four basins of the actively deforming northern Cascadia forearc. A set of Holocene faults, Drayton Harbor, Birch Bay, and Sandy Point faults, occur within the Bellingham Basin and can be traced from onshore to offshore using a combination of aeromagnetic lineaments, paleoseismic investigations and scarps identified using LiDAR imagery. With the recognition of such Holocene faults, the northernmost margin of the actively deforming Cascadia forearc extends 60 km north of the previously recognized limit of Holocene forearc deformation. Although to date no Holocene faults are recognized at the northern boundary of the Bellingham Basin, which is 15 km north of the international border, there is no compelling tectonic reason to expect that Holocene faults are limited to south of the international border.

Washington↗

USGS2012

Subducted seamounts and recent earthquakes beneath the central Cascadia forearc

Bathymetry and magnetic anomalies indicate that a seamount on the Juan de Fuca plate has been subducted beneath the central Cascadia accretionary complex and is now located ∼45 km landward of the deformation front. Passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology, gas hydrate stability, and sedimentation. Based on potential-field data and a new three-dimensional seismic velocity model, we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of Cascadia. More deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along-strike variations in the thickness of the subduction channel, which may affect coupling across the plate boundary.

Geology↗

USGS2012

Miocene magmatism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc

The Middle to Late Miocene Bodie Hills volcanic field is a >700 km 2 , long-lived (∼9 Ma) but episodic eruptive center in the southern segment of the ancestral Cascades arc north of Mono Lake (California, U.S.). It consists of ∼20 major eruptive units, including 4 trachyandesite stratovolcanoes emplaced along the margins of the field, and numerous, more centrally located silicic trachyandesite to rhyolite flow dome complexes. Bodie Hills volcanism was episodic with two peak periods of eruptive activity: an early period ca. 14.7–12.9 Ma that mostly formed trachyandesite stratovolcanoes and a later period between ca. 9.2 and 8.0 Ma dominated by large trachyandesite-dacite dome fields. A final period of small silicic dome emplacement occurred ca. 6 Ma. Aeromagnetic and gravity data suggest that many of the Miocene volcanoes have shallow plutonic roots that extend to depths ≥1–2 km below the surface, and much of the Bodie Hills may be underlain by low-density plutons presumably related to Miocene volcanism. Compositions of Bodie Hills volcanic rocks vary from ∼50 to 78 wt% SiO 2 , although rocks with <55 wt% SiO 2 are rare. They form a high-K calc-alkaline series with pronounced negative Ti-P-Nb-Ta anomalies and high Ba/Nb, Ba/Ta, and La/Nb typical of subduction-related continental margin arcs. Most Bodie Hills rocks are porphyritic, commonly containing 15–35 vol% phenocrysts of plagioclase, pyroxene, and hornblende ± biotite. The oldest eruptive units have the most mafic compositions, but volcanic rocks oscillated between mafic and intermediate to felsic compositions through time. Following a 2 Ma hiatus in volcanism, postsubduction rocks of the ca. 3.6–0.1 Ma, bimodal, high-K Aurora volcanic field erupted unconformably onto rocks of the Miocene Bodie Hills volcanic field. At the latitude of the Bodie Hills, subduction of the Farallon plate is inferred to have ended ca. 10 Ma, evolving to a transform plate margin. However, volcanism in the region continued until 8 Ma without an apparent change in rock composition or style of eruption. Equidimensional, polygenetic volcanoes and the absence of dike swarms suggest a low differential horizontal stress regime throughout the lifespan of the Bodie Hills volcanic field. However, kinematic data for veins and faults in mining districts suggest a change in the stress field from transtensional to extensional approximately coincident with the inferred cessation of subduction. Numerous hydrothermal systems were operative in the Bodie Hills during the Miocene. Several large systems caused alteration of volcaniclastic rocks in areas as large as 30 km 2 , but these altered rocks are mostly devoid of economic mineral concentrations. More structurally focused hydrothermal systems formed large epithermal Au-Ag vein deposits in the Bodie and Aurora mining districts. Economically important hydrothermal systems are temporally related to intermediate to silicic composition domes. Rock types, major and trace element compositions, petrographic characteristics, and volcanic features of the Bodie Hills volcanic field are similar to those of other large Miocene volcanic fields in the southern segment of the ancestral Cascade arc. Relative to other parts of the ancestral arc, especially north of Lake Tahoe in northeastern California, the scarcity of mafic rocks, relatively K-rich calc-alkaline compositions, and abundance of composite dome fields in the Bodie Hills may reflect thicker crust beneath the southern ancestral arc segment. Thicker crust may have inhibited direct ascent and eruption of mafic, mantle-derived magma, instead stalling its ascent in the lower or middle crust, thereby promoting differentiation to silicic compositions and development of porphyritic textures characteristic of the southern ancestral arc segment.

California, Nevada↗

USGS2012

The western limits of the Seattle fault zone and its interaction with the Olympic Peninsula, Washington

We present evidence that the Seattle fault zone of Washington State extends to the west edge of the Puget Lowland and is kinemati-cally linked to active faults that border the Olympic Massif, including the Saddle Moun-tain deformation zone. Newly acquired high-resolution seismic reflection and marine magnetic data suggest that the Seattle fault zone extends west beyond the Seattle Basin to form a >100-km-long active fault zone. We provide evidence for a strain transfer zone, expressed as a broad set of faults and folds connecting the Seattle and Saddle Mountain deformation zones near Hood Canal. This connection provides an explanation for the apparent synchroneity of M7 earthquakes on the two fault systems ~1100 yr ago. We redeﬁ ne the boundary of the Tacoma Basin to include the previously termed Dewatto basin and show that the Tacoma fault, the southern part of which is a backthrust of the Seattle fault zone, links with a previously unidentiﬁ ed fault along the western margin of the Seattle uplift. We model this north-south fault, termed the Dewatto fault, along the western margin of the Seattle uplift as a low-angle thrust that initiated with exhu-mation of the Olympic Massif and today accommodates north-directed motion. The Tacoma and Dewatto faults likely control both the southern and western boundaries of the Seattle uplift. The inferred strain trans-fer zone linking the Seattle fault zone and Saddle Mountain deformation zone deﬁ nes the northern margin of the Tacoma Basin, and the Saddle Mountain deformation zone forms the northwestern boundary of the Tacoma Basin. Our observations and model suggest that the western portions of the Seattle fault zone and Tacoma fault are com-plex, require temporal variations in principal strain directions, and cannot be modeled as a simple thrust and/or backthrust system.

Washington↗

USGS2011

Geomagnetic Workshop

A workshop on geomagnetism, sponsored by the Geologic Division of the U.S. Geological Survey, was held in the Denver West Office Complex in Golden, Colorado, April 13–15, 1982. There were 90 registered participants from government agencies, academic institutions, and industry. This effort stemmed from the realization that geomagnetism, once a small but coherent discipline, has now expanded into numerous areas of the geosciences, yet those doing research in these specialties seldom make contact outside their area of immediate interest. The impetus for this event came from the members of a committee formed to review the geomagnetic activities within the U.S. Geological Survey. They observed that the level of communication between the various elements of this now diverse discipline was inadequate, not only within their organization but also between federal agencies, academia, and the private sector. While the desire was to cover as much of geomagnetism as possible, it was necessary for a workshop of reasonable size and length to exclude some important areas of the subject: magnetic reversal chronology, studies of the externally produced variations, and most aspects of internal induction. The plan was to give emphasis to some of the newer areas: those which have recently seen a high level of activity and those with increasing activity abroad compared to that in the United States. The purpose was to evaluate the status and problems in selected areas with an eye to those whose emphasis might produce fruitful results in the next decade.

Eos, Transactions, American Geophysical Union↗

USGS2011

A tunnel runs through it — An inside view of the Tualatin Mountains, Oregon

The Tualatin Mountains form a northwest-striking ridge about 350 m high that separates Portland, Oregon, from the cities of the Tualatin Valley to the west. Known informally as the Portland Hills, the ridge is a late Cenozoic anticline, bounded by reverse faults that dip toward the anticlinal axis. The anticline is a broad, open fold consisting chiefly of Miocene Columbia River Basalt Group, with remnants of Miocene-Pliocene Troutdale Formation and Pleistocene basalt of the Boring Volcanic Field on the flanks of the anticline. Anticlinal structures similar to the Tualatin Mountains are characteristic of the northern Willamette Valley, where the structures accommodate margin-parallel shortening of the Cascadia fore arc. Global Positioning System (GPS) results indicate that the shortening is due to the northward motion of Oregon at several millimeters per year with respect to stable North America. Some of the uplifts may contain active faults, but the structures are poorly exposed and are overlain by thick Pleistocene loess and Missoula flood deposits. Between 1993 and 1998, construction of the 3-mile-long (4500-m-long) TriMet MAX Light Rail tunnel through the Tualatin Mountains provided an unusual opportunity to investigate the geological structure and history of the Tualatin Mountains. This report is a collaborative effort among the tunnel geologists and the U.S. Geological Survey (USGS) to document the geologic story and quantify late Cenozoic and Quaternary deformation rates of the Tualatin Mountains.

Oregon↗

USGS2010

High-resolution seismic reflection imaging of growth folding and shallow faults beneath the Southern Puget Lowland, Washington State

Marine seismic reflection data from southern Puget Sound, Washington, were collected to investigate the nature of shallow structures associated with the Tacoma fault zone and the Olympia structure. Growth folding and probable Holocene surface deformation were imaged within the Tacoma fault zone beneath Case and Carr Inlets. Shallow faults near potential field anomalies associated with the Olympia structure were imaged beneath Budd and Eld Inlets. Beneath Case Inlet, the Tacoma fault zone includes an &sim;350-m wide section of south-dipping strata forming the upper part of a fold (kink band) coincident with the southern edge of an uplifted shoreline terrace. An &sim;2 m change in the depth of the water bottom, onlapping postglacial sediments, and increasing stratal dips with increasing depth are consistent with late Pleistocene to Holocene postglacial growth folding above a blind fault. Geologic data across a topographic lineament on nearby land indicate recent uplift of late Holocene age. Profiles acquired in Carr Inlet 10 km to the east of Case Inlet showed late Pleistocene or Holocene faulting at one location with &sim;3 to 4 m of vertical displacement, south side up. North of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad Tacoma fault zone. Seismic reflection profiles across part of the Olympia structure beneath southern Puget Sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding. Directly beneath one of these faults, a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw, suggesting strike-slip motion. Deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset, consistent with strike-slip faulting. These faults and folds indicate that the Tacoma fault and Olympia structure include active structures with probable postglacial motion.

Washington↗

USGS2010

Overview of geomagnetism and paleomagnetism, 1983–1986

Michael Fuller, author of a similar overview four years ago, concluded that “...the next quadrennium promises to be a most interesting one, with plenty to keep us busy.” The 11 review papers that follow support his prediction; research in geomagnetism and paleomagnetism indeed has flourished over the last four years. In this brief overview, I will attempt to summarize what I perceive as important advances during the last quadrennium and to predict where research might lead during the next four years. These observations are seen through the eyes of a very specialized geophysicist; another author undoubtedly would see the past and future much differently.

Reviews of Geophysics↗

USGS2010

Marine magnetic anomalies

Marine magnetic data have been available for many years from all of the world's oceans, and their contribution to marine geophysics and geology is profound. These data, for example, have allowed charting the age of the ocean floor, reconstruction of the geologic history of the major ocean basins, development of a Cenozoic and Mesozoic timescale of geomagnetic reversals, and speculation on the processes of sea‐floor spreading. Research on these and similar problems actively continued during this quadrennial, but here we discuss only a few topics in which we believe the most significant advances have been made during the last four years: the source of marine magnetic anomalies, the geomagnetic time‐scale, high‐amplitude anomalies, and studies of back‐arc basins. Source of Anomalies Studies of magnetic surveys over continental areas can often be constrained by the magnetic properties of samples collected from below the survey. Until recently, model studies of marine magnetic anomalies have enjoyed the luxury of an inaccessible source. Except for samples scraped from the ocean floor by dredging [e.g., Irving , 1970] and inferences drawn from magnetic studies of ophiolite sequences [e.g., Vine and Moores , 1972], very little information was available for constraining studies of marine magnetic anomalies.

Reviews of Geophysics↗

USGS2010

Preliminary atlas of active shallow tectonic deformation in the Puget Lowland, Washington

This atlas presents an up-to-date map compilation of the geological and geophysical observations that underpin interpretations of active, surface-deforming faults in the Puget Lowland, Washington. Shallow lowland faults are mapped where observations of deformation from paleoseismic, seismic-reflection, and potential-field investigations converge. Together, results from these studies strengthen the identification and characterization of regional faults and show that as many as a dozen shallow faults have been active during the Holocene. The suite of maps presented in our atlas identifies sites that have evidence of deformation attributed to these shallow faults. For example, the paleoseismic-investigations map shows where coseismic surface rupture and deformation produced geomorphic scarps and deformed shorelines. Other maps compile results of seismic-reflection and potential-field studies that demonstrate evidence of deformation along suspected fault structures in the subsurface. Summary maps show the fault traces derived from, and draped over, the datasets presented in the preceding maps. Overall, the atlas provides map users with a visual overview of the observations and interpretations that support the existence of active, shallow faults beneath the densely populated Puget Lowland.

Open-File Report↗

USGS2010

Porphyry copper deposit model

This report contains a revised descriptive model of porphyry copper deposits (PCDs), the world's largest source (about 60 percent) and resource (about 65 percent) of copper and a major source of molybdenum, gold and silver. Despite relatively low grades (average 0.44 percent copper in 2008), PCDs have significant economic and societal impacts due to their large size (commonly hundreds of millions to billions of metric tons), long mine lives (decades), and high production rates (billions of kilograms of copper per year). The revised model describes the geotectonic setting of PCDs, and provides extensive regional- to deposit-scale descriptions and illustrations of geological, geochemical, geophysical, and geoenvironmental characteristics. Current genetic theories are reviewed and evaluated, knowledge gaps are identified, and a variety of exploration and assessment guides are presented. A summary is included for users seeking overviews of specific topics.

Scientific Investigations Report↗

USGS2009

Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift

The Saddle Mountain fault, first recognized in the early 1970s, is now well mapped in the Hoodsport area, southeastern Olympic Peninsula (northwestern United States), on the basis of light detection and ranging (LIDAR) surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate that the Saddle Mountain fault produced a M W 6.5–7.0 earthquake 1000–1300 yr ago, likely contemporaneous with the M W 7.5 Seattle fault earthquake 1100 yr ago and with a variety of other fault and landslide activity over a wide region of the Olympic Peninsula and Puget Lowland. This near synchroneity suggests that the Saddle Mountain and Seattle fault may be kinematically linked. Aeromagnetic anomalies and LIDAR topographic scarps define an en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km, from 6 km southwest of Lake Cushman northward to the latitude of the Seattle fault. A magnetic survey over Price Lake using a nonmagnetic canoe illuminated two east-dipping reverse faults with 20 m of vertical offset at 30 m depth associated with 2–4 m of vertical displacement at the topographic surface. Analysis of regional aeromagnetic data indicates that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the northward terminus of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain and nearby Frigid Creek and Canyon River faults, all active in late Holocene time, reflects a >45-km-long zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this view, the Seattle fault and Saddle Mountain deformation zone form the boundaries of the northward-advancing Seattle uplift.

Washington↗

USGS2008

Finding concealed active faults: Extending the southern Whidbey Island fault across the Puget Lowland, Washington

The southern Whidbey Island fault zone (SWIF), as previously mapped using borehole data, potential field anomalies, and marine seismic reflection surveys, consists of three subparallel, northwest trending strands extending ∼100 km from near Vancouver Island to the northern Puget Lowland. East of Puget Sound, the SWIF makes landfall between the cities of Seattle and Everett but is concealed beneath a thick mantle of young glacial deposits and vegetation. A ∼20‐km‐wide, northwest trending swath of subparallel, low‐amplitude aeromagnetic anomalies crosses this region of the Puget Lowland and is on strike with the SWIF. The most prominent aeromagnetic anomaly, the Cottage Lake lineament, extends at least 18 km and lies approximately on strike with the SWIF on Whidbey Island. Subtle scarps and topographic lineaments on Pleistocene surfaces, visible on high‐resolution lidar topography at a number of locations along the SWIF, lie on or near these magnetic anomalies. In the field, scarps exhibit northeast‐side‐up and vertical relief of 1 to 5 m. Excavations across several lidar scarps lying on or near magnetic anomalies show evidence for multiple folding and faulting events since deglaciation, most likely above buried reverse/oblique faults. Excavations in areas away from magnetic anomalies do not show evidence of tectonic deformation. In total, paleoseismological evidence suggests that the SWIF produced at least four earthquakes since deglaciation about 16,400 years ago, the most recent less than 2700 years ago.

Washington↗

USGS2008

Geologic map of the Hidden Hills Ranch Quadrangle, Clark County, Nevada

The prominent structural feature in the Pahrump Valley is the right-lateral Pahrump-Stewart Valley fault system (PSV) (Anderson and others, 1995b), which is the central segment of the State Line fault system as defined by Blakely and others (1998). The main trace of this fault system, as shown in figure 1A, strikes northwest along the California-Nevada state line through Pahrump and Stewart Valleys (Anderson and others, 1995b). Hoffard (1991) describes three separate segments of the PSV within the Pahrump Valley (fig. 1B). The East Nopah fault zone (ENFZ) is a narrow band of right lateral, strike-slip faults along the eastern front of the Nopah Range. The Pahrump Valley fault zone (PVFZ) is a wider band of right-oblique-slip faults through the center of the Pahrump Valley and extending northwest into the Stewart Valley fault zone (SVFZ). The West Spring Mountains fault zone (WSMFZ) is a band of high-angle normal faults that strike north along the western front of the Spring Mountains and may represent a splay of the PVFZ.

Nevada↗