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At least 649 records · Page 36Linked to original sources
USGS2021

Geochronologic, isotopic, and geochemical data from pre-Cretaceous plutonic rocks in the Lane Mountain area, San Bernardino County, California

Pre-Cretaceous, predominantly dioritic plutonic rocks in the Lane Mountain area, California, intrude metasedimentary and metavolcanic rocks considered part of the El Paso terrane. New geochronologic (uranium-lead zircon), geochemical, and isotopic data provide a reliable basis for dividing these pre-Cretaceous plutonic rocks into two mappable suites of Permian–Triassic and Late Jurassic ages. The 26 Permian–Triassic samples included in this report have a mean age of ~248 mega-annum (Ma), range in composition from monzodiorite to quartz monzonite and granodiorite, and have a mean initial 87 Sr/ 86 Sr ratio (Sri) of ~0.7045. The 22 Late Jurassic samples have a mean age of ~149 Ma, range in composition from gabbro to granite, and have a mean Sr i of ~0.7055. Accurate mapping of these two plutonic suites and their detailed field relations with the associated metamorphic rocks is essential for resolving the geologic history and regional tectonic significance of the Lane Mountain area. The sub-0.706 Sr i values of both plutonic suites at Lane Mountain are consistent with previous suggestions that the El Paso terrane is allochthonous and did not develop on Precambrian continental lithosphere. Both suites are considered parts of northwest-trending magmatic arcs interpreted to have formed above east-dipping subduction zones along the evolving North American continental margin, and both arcs are interpreted to cross a major east-west-trending boundary between the El Paso terrane and rocks considered part of ancestral North America in the San Bernardino Mountains area to the south. The El Paso terrane thus appears to have been attached to the San Bernardino Mountains area at least since Permian–Triassic time, although the boundary probably has been modified by Cenozoic faulting.

California↗
USGS1988

Precambrian ophiolites of Arabia; a summary of geologic settings, U-Pb geochronology, lead isotope characteristics, and implications for microplate accretion, Kingdom of Saudi Arabia

Disrupted ophiolites occur in linear belts as much as 900 km long between micro plates that collided during the late Proterozoic to form the Arabian Shield. The U-Pb zircon ages and lead-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Microplates of the central and western Arabian Shield are generally thought to represent intra-oceanic island arcs that range in age from about 900 Ma to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. The Ophiolites of the Yanbu suture (northwestern Arabian Shield), dated by U-Pb (zircon) and Sm-Nd (mineral isochron) methods, yield model ages of 740-780 Ma. These are the among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older dates of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that these gabbros are either fore-arc oceanic crust or fragments of oceanic crust on which an arc was built. Gabbro samples from ophiolites of the Bir Umq-Port Sudan suture (westcentral Arabian Shield) yield zircons with ages of 820-870 Ma and ≥1,250 Ma. The 820-870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc setting for the ophiolites. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900-km-long Nabitah mobile belt was dated by the zircon U-Pb method at approximately 830 Ma. This date is within the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture zone, but is approximately 100 million years older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq-Port Sudan suture that was rotated parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data are of three types: 1) lead from the ophiolitic rocks and arc tonalites of the northwestern Arabian Shield and ophiolitic rocks of the Nabitah suture zone is similar to lead in present midocean ridge basalt, 2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and 3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Open-File Report↗
USGS1994

Worldwide Assessment of the Status of Seismic Zonation, Fourth International Forum on Seismic Zonation, Proceedings

We are pleased to provide you with information developed for the Fourth International Forum on Seismic Zonation which will be convened in two locations year in conjunction two major international meetings. The objectives are: 1) to assess the status of seismic zonation in every country of the world, 2) to evaluate the reasons for advances and new initiatives, and 3) to foster continued cooperation. Seismic zonation is the process that leads to risk reduction and sustainability of new development. It is based on the division of a geographic region into smaller areas or zones on the basis of an integrated assessment of the hazard, built, and policy environments of the region. Seismic zonation depends on hazard mapping performed on national/regional, subregional, and urban (i.e., microzonation) scales depending on the particular application. We gratefully acknowledge the written communications of many professionals who responded to our request for information. Also, we acknowledge the use of information contained in five valuable reports (see directories in the Appendices for information on where to obtain copies of the reports): 1. United Nations, 1990, Cooperative Project for Seismic Risk Reduction in the Mediterranean Region (SEISMED), proceedings, Office of the United Nations Disaster Relief Coordinator, Geneva, Switzerland, 3 vols. (Franco Maranzana - Italy/Appendix D). 2. United States Geological Survey, 1992, The Worldwide Earthquake Risk Management (WWERM) Program, Reston, Virginia, 19 p (Paul Thenhaus or S.T. Algermissen - USA/ Appendix D). 3. Instituto Panamericano de Geografia Historia, 1992, Revista Geofisica, Lima, Peru, No. 37, July-December, 234 p (Alberto Giesecke- Peru/Appendix D). 4. Annali di Geofisica, 1992, Global Seismic Hazard Assessment Program (GSHAP) (Special Issue), International Lithosphere Program, Publication 209, Bologna, Italy, 257 p (Domenico Giardini-Italy/Appendix F). 5. International Association of Seismology and Physics of the Earth's Interior and European Seismological Commission, 1993, The Practice of Hazard Assessment, Golden, Colorado, 284 p (Write Bob Engdahl, U.S. Geological Survey, Denver Federal Center, Mail Stop 967, Denver, Colorado 80225, USA).

Open-File Report↗
USGS1998

Rotational and accretionary evolution of the Klamath Mountains, California and Oregon, from Devonian to present time

The purpose of this report is to show graphically how the Klamath Mountains grew from a relatively small nucleus in Early Devonian time to its present size while rotating clockwise approximately 110°. This growth occurred by the addition of large tectonic slices of oceanic lithosphere, volcanic arcs, and melange during a sequence of accretionary episodes. The Klamath Mountains province consists of eight lithotectonoic units called terranes, some of which are divided into subterranes. The Eastern Klamath terrane, which was the early Paleozoic nucleus of the province, is divided into the Yreka, Trinity, and Redding subterranes. Through tectonic plate motion, usually involving subduction, the other terranes joined the early Paleozoic nucleus during seven accretionary episodes ranging in age from Early Devonian to Late Jurassic. The active terrane suture is shown for each episode by a bold black line. Much of the western boundary of the Klamath Mountains is marked by the South Fork and correlative faults along which the Klamath terranes overrode the Coast Range rocks during an eighth accretionary episode, forming the South Fork Mountain Schist in Early Cretaceous time.

California, Oregon↗
USGS1998

Map and data for Quaternary faults and folds in New Mexico

The "World Map of Major Active Faults" Task Group is compiling a series of digital maps for the United States and other countries in the Western Hemisphere that show the locations, ages, and activity rates of major earthquake-related features such as faults and fault-related folds; the companion database includes published information on these seismogenic features. The Western Hemisphere effort is sponsored by International Lithosphere Program (ILP) Task Group H-2, whereas the effort to compile a new map and database for the United States is funded by the Earthquake Reduction Program (ERP) through the U.S. Geological Survey. The maps and accompanying databases represent a key contribution to the new Global Seismic Hazards Assessment Program (ILP Task Group II-O) for the International Decade for Natural Disaster Reduction. This compilation, which describes evidence for surface faulting and folding in New Mexico, is the third of many similar State and regional compilations that are planned for the U.S. The compilation for West Texas is available as U.S. Geological Survey Open-File Report 96-002 (Collins and others, 1996 #993) and the compilation for Montana will be released as a Montana Bureau of Mines product (Haller and others, in press #1750).

New Mexico↗
USGS1999

Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

The Cascadia subduction zone is extremely complex in the western Washington region, involving local deformation of the subducting Juan de Fuca plate and complicated block structures in the crust. It has been postulated that the Cascadia subduction zone could be the source for a large thrust earthquake, possibly as large as M9.0. Large intraplate earthquakes from within the subducting Juan de Fuca plate beneath the Puget Sound region have accounted for most of the energy release in this century and future such large earthquakes are expected. Added to these possible hazards is clear evidence for strong crustal deformation events in the Puget Sound region near faults such as the Seattle fault, which passes through the southern Seattle metropolitan area. In order to understand the nature of these individual earthquake sources and their possible interrelationship, we have conducted an extensive seismotectonic study of the region. We have employed P-wave velocity models developed using local earthquake tomography as a key tool in this research. Other information utilized includes geological, paleoseismic, gravity, magnetic, magnetotelluric, deformation, seismicity, focal mechanism and geodetic data. Neotectonic concepts were tested and augmented through use of anelastic (creep) deformation models based on thin-plate, finite-element techniques developed by Peter Bird, UCLA. These programs model anelastic strain rate, stress, and velocity fields for given rheological parameters, variable crust and lithosphere thicknesses, heat flow, and elevation. Known faults in western Washington and the main Cascadia subduction thrust were incorporated in the modeling process. Significant results from the velocity models include delineation of a previously studied arch in the subducting Juan de Fuca plate. The axis of the arch is oriented in the direction of current subduction and asymmetrically deformed due to the effects of a northern buttress mapped in the velocity models. This buttress occurs under the North Cascades region of Washington and under southern Vancouver Island. We find that regional faults zones such as the Devils Mt. and Darrington zones follow the margin of this buttress and the Olympic-Wallowa lineament forms its southern boundary east of the Puget Lowland. Thick, high-velocity, lower-crustal rocks are interpreted to be a mafic/ultramafic wedge occuring just above the subduction thrust. This mafic wedge appears to be jointly deformed with the arch, suggesting strong coupling between the subducting plate and upper plate crust in the Puget Sound region at depths >30 km. Such tectonic coupling is possible if brittle-ductile transition temperatures for mafic/ultramafic rocks on both sides of the thrust are assumed. The deformation models show that dominant north-south compression in the coast ranges of Washington and Oregon is controlled by a highly mafic crust and low heat flow, allowing efficient transmission of margin-parallel shear from Pacific plate interaction with North America. Complex stress patterns which curve around the Puget Sound region require a concentration of northwest-directed shear in the North Cascades of Washington. The preferred model shows that greatest horizontal shortening occurs across the Devils Mt. fault zone and the east end of the Seattle fault.

Open-File Report↗
USGS1978

Reconstruction of crustal blocks of California on the basis of initial strontium isotopic compositions of Mesozoic granitic rocks

Initial 87Sr/ 86 Sr was determined for samples of Mesozoic granitic rocks in the vicinity of the Garlock fault zone in California. These data along with similar data from the Sierra Nevada and along the San Andreas fault system permit a reconstruction of basement rocks offset by the Cenozoic lateral faulting along both the San Andreas and Garlock fault systems. The location of the line of initial 87Sr/ 86 Sr = 0.7060 can be related to the edge of the Precambrian continental crust in the western United States. Our model explains the present configuration of the edge of Precambrian continental crust as the result of two stages of rifting that occurred about 1,250 to 800 m.y. ago, during Belt sedimentation, and about 600 to 350 m.y. ago, prior to and during the development of the Cordilleran geosyncline and to left-lateral translation along a locus of disturbance identified in the central Mojave Desert. The variations in Rb, Sr, and initial 87Sr/ 86 Sr of the Mesozoic granitic rocks are interpreted as due to variations in composition and age of the source materials of the granitic rocks. The variations of Rb, Sr, and initial 87Sr/ 86 Sr in Mesozoic granitic rocks, the sedimentation history during the late Precambrian and Paleozoic, and the geographic position of loci of Mesozoic magmatism in the western United States are related to the development of the continental margin and different types of lithosphere during rifting.

Professional Paper↗
USGS1980

Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the Arabian Shield

Rubidium-strontium studies of Precambrian volcanic and plutonic rocks of the Arabian Shield document an early development of the Arabian craton between 900 and 680 m.y. (million years) ago. Geologic studies indicate an island-arc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north or northwest-trending axes. Magmatic trends show consistent variation in both composition and geographic location as a function of age. The oldest units belong to an assemblage of basaltic strata exposed in western Saudi Arabia that yield an age of 1165:!:110 m.y. The oldest andesitic strata studied yield an age of 912:!:76 m.y. The earliest plutonic units are diorite to trondhjemite batholiths that range from 800 to 9,00 m.y. in age and ,occur along the western and southern parts of Saudi Arabia. Younger plutonic units, 680 to 750 m.y. in age, range from quartz diorite to granodiodte and become more abundant in the central and northeastern parts of the Arabian Shield. Initial 'Sr/ 86 Sr ratios for both dioritic groups range from 0.7023 to 0.7030 and average 0.7027. The absence of sialic detritus in sedimentary units and the evidence for an island-arc environment suggest the early development of the Arabian craton at a convergent plate margin between plates of oceanic lithosphere. Active subduction apparently extended from at least 900 m.y. to about 680 m.y. Subsequent to this subduction-related magmatism and tectonism, called the Hijaz tectonic cycle, the Arabian craton was sutured to the late Precambrian African plate in a collisional event. This period of orogeny, represented in Arabia and eastern Africa by the Mozambiquian or Pan-African event, extended from some time before 650 m.y. to at least 540 m.y. and perhaps 520 m.y. B.P. Although the tectonic processes of subduction and continental collision during the 900+ to 500-m.y. period require similar directions of plate convergence, the differences in magmatic and tectonic. styles of Hijaz orogenesis from those of the Pan-African and the temporal break between them in much of the southern part of the Arabian Shield support division into at least two events. As defined by the ages of major plutonic units, the axis of magmatic and tectonic activity migrated eastward or northeastward during the Hijaz cycle, the predominantly dioritic plutonic rocks becoming younger and more siliceous to the east. Granodiorite to granite pl}.1tonism of the Pan-African event, however, shows no geographic bias, being distributed throughout the Arabian Shield. Although the Hijaz diorites and Pan-African granitic rocks exhibit strong contrasts in composition and age differences as great as 250 m.y. in the westernmost parts of the area, the two groups are less distinct compositionally and nearly the same age in the eastern part.

Professional Paper↗
USGS1980

Paleozoic and Mesozoic deformations in the central Sierra Nevada, California

Analysis of structural and stratigraphic data indicates that several periods of regional deformation, consisting of combined folding, faulting, cataclasis, and regional metamorphism, occurred throughout the central Sierra Nevada during Paleozoic and Mesozoic time. The oldest regional deformation occurred alono northward trends during the Devonian and Mississippian periods in most roof pendants containing lower Paleozoic metasedimentary rocks at the center and along the crest of the range. This deformation is expressed in some roof pendants by an angular unconformity separating older thrice-deformed from younger twice-deformed Paleozoic metasedimentary rocks. The first Mesozoic deformation, which consisted of uplift and erosion and was accompanied by the onset of Andean-type volcanism during the Permian and Triassic, is expressed by an angular unconformity in several roof pendants from the Saddlebag Lake to the Mount Morrison areas. This unconformity is defined by Permian and Triassic andesitic to rhyolitic metavolcanic rocks unconformably overlying more intensely deformed Pennsylvanian, Permian(?), and older metasedimentary rocks. A later regional deformation occurred during the Triassic along N. 20?_30? W. trends in Permian and Triassic metavolcanic rocks of the Saddlebag Lake and Mount Dana roof pendants, in upper Paleozoic rocks of the Pine Creek roof pendant, and in the Calaveras Formation of the western metamorphic belt; the roof pendants are crosscut by Upper Triassic granitic rocks of the Lee Vining intrusive epoch. A still later period of Early and Middle Jurassic regional deformation occurred along N. 30?-60? E. trends in upper Paleozoic rocks of the Calaveras Formation of the western metamorphic belt. A further period of deformation was the Late Jurassic Nevadan orogeny, which occurred along N. 20?_40? W. trends in Upper Jurassic rocks of the western metamorphic belt that are crosscut by Upper Jurassic granitic rocks of the Yosemite intrusive epoch. Structures of similar age occur in intensely deformed oceanic-lithospheric and syntectonic plutonic rocks of the lower Kings River area, in Jurassic metavolcanic rocks of the Ritter Range roof pendant, and in Triassic metasedimentary rocks of the Mineral King roof pendant. The final Mesozoic deformation occurred along N. 50?-80? W. trends in both high-country roof pendants and the lower Kings River area; structures of this generation are crosscut by relatively undeformed Upper Cretaceous granitic rocks of the Cathedral Range intrusive epoch.

Professional Paper↗
USGS2012

State of the Earth’s cryosphere at the beginning of the 21st century: Glaciers, global snow cover, floating ice, and permafrost and periglacial environments

This chapter is the tenth in a series of 11 book-length chapters, collectively referred to as “this volume,” in the series U.S. Geological Survey Professional Paper 1386, Satellite Image Atlas of Glaciers of the World. In the other 10 chapters, each of which concerns a specific glacierized region of Earth, the authors used remotely sensed images, primarily from the Landsat 1, 2, and 3 series of spacecraft, in order to analyze that glacierized region and to monitor changes in its glaciers. Landsat images, acquired primarily during the period 1972 through 1981, were used by an international team of glaciologists and other scientists to study the various glacierized regions and (or) to discuss related glaciological topics. In each glacierized region, the present distribution of glaciers within its geographic area is compared, wherever possible, with historical information about their past areal extent. The atlas provides an accurate regional inventory of the areal extent of glacier ice on our planet during the 1970s as part of an expanding international scientific effort to measure global environmental change on the Earth’s surface. However, this chapter differs from the other 10 in its discussion of observed changes in all four elements of the Earth’s cryosphere (glaciers, snow cover, floating ice, and permafrost) in the context of documented changes in all components of the Earth System. Human impact on the planet at the beginning of the 21st century is pervasive. The focus of Chapter A is on changes in the cryosphere and the importance of long-term monitoring by a variety of sensors carried on Earth-orbiting satellites or by a ground-based network of observatories in the case of permafrost. The chapter consists of five parts. The first part provides an introduction to the Earth System, including the interrelationships of the geosphere (cryosphere, hydrosphere, lithosphere, and atmosphere), the biosphere, climate processes, biogeochemical cycles, and the critically important hydrologic cycle, in which glacier ice is the second largest reservoir of water after the oceans. The second part assesses the state of glaciers in all of the glacierized regions of the planet, primarily as drawn in the other 10 chapters. It includes sections on ice cores and the climate record they contain, volumetric changes in glaciers, harnessing spaceborne sensors to measure changes in glaciers, and related topics. The third part summarizes trends in global snow cover. The fourth part summarizes long-term changes in area and thickness of floating ice, including polar sea ice and freshwater (lake and river) ice. The fifth part assesses the loss of permafrost and changes in periglacial environments at high latitudes and high altitudes.

Professional Paper↗
USGS2026

Forecasting volcanic activity in Germany—A multi-criteria approach

Igneous activity, including shallow intrusions and volcanism, has the potential to disrupt underground critical infrastructure. Notably, future underground infrastructure projects like high-level radioactive waste repositories must be sited in areas of extremely low disruption probability by igneous activity. In Germany, according to the Repository Site Selection Act of 2017 (Standortauswahlgesetz, or StandAG), areas in which Quaternary volcanism is either present or future volcanic activity is expected within the next 1 million years (m.y.) must be excluded from the site selection process. Although the locations of regions with Quaternary volcanism are reasonably well known in Germany, forecasting potential igneous activity at intraplate volcanic fields is challenging, as many processes and their interactions control the spatial distribution of volcanic centers. Here, a semi-quantitative, multi-criteria approach is proposed for a regional evaluation of the relative potential of future igneous activity in Germany. A variety of geoscientific indicators are used, including seismic anomalies in Earth’s mantle, gravity data, tectonic activity, sutures, ground motion, earthquakes, mantle degassing centers, and geochronological data of volcanic rocks. The indicators describe the sequence of processes from potential melt generation in Earth’s mantle, through ascent and accumulation of melt within the lithosphere, to eruption at Earth’s surface. In total, 15 out of 30 proposed geoscientific indicators are selected and quantified using 20 total assigned parameters. Defined threshold values are used to spatially delimit relevant parameter properties to focus on areas with higher potential of future magmatic activity. To consider uncertainties of parameters and their underlying processes, which are usually more spatially extensive below ground, buffer zones are defined in which values of relevance decrease with increasing distance from the initial lateral shape of a parameter. Normalized parameters are combined into an index, whose spatial value distribution is used to differentiate the relative potential of future igneous activity (within the next 1 m.y.). The sensitivity of the results is shown by varying the weighting factors for the relevant parameters in country-wide index maps. Thereby, profiles illustrate the distribution of the resulting index values and respective index fractions of various parameters. Different index maps for the relative potential of future igneous activity are presented and can be used for hazard assessments.

Professional Paper↗
USGS1976

Metamorphism and plutonism around the middle and south forks of the Feather River, California

The area around the Middle and South Forks of the Feather River provides information on metamorphic and igneous processes that bear on the origin of andesitic and granitic magmas in general and on the variation of their potassium content in particular. In the north, the area joins the Pulga and Bucks Lake quadrangles studied previously. Tectonically, this area is situated in the southern part of an arcuate segment of the Nevadan orogenic belt in the northwestern Sierra Nevada. The oldest rocks are metamorphosed calcalkaline island-arc-type andesite, dacite, and sodarhyolite with interbedded tuff layers (the Franklin Canyon Formation), all probably correlative with Devonian rocks in the Klamath Mountains. Younger rocks form a sequence of volcanic, volcaniclastic, and sedimentary rocks including some limestone (The Horseshoe Bend Formation), probably Permian in age. All the volcanic and sedimentary rocks were folded and recrystallized to the greenschist facies during the Nevadan (Jurassic) orogeny and were invaded by monzotonalitic magmas shortly thereafter. A second lineation and metamorphism to the epidote-amphibolite facies developed in a narrow zone around the plutons. In light of the concept of plate tectonics, it is suggested that the early (Devonian?) island-arc-type andesite, dacite, and sodarhyolite (the Franklin Canyon Formation) were derived from the mantle above a Benioff zone by partial melting of peridotite in hydrous conditions. The water was probably derived from an oceanic plate descending to the mantle. Later (Permian?) magmas were mainly basaltic; some discontinuous layers of potassium-rich rhyolite indicate a change into anhydrous conditions and a deeper level of magma generation. The plutonic magmas that invaded the metamorphic rocks at the end of the Jurassic may contain material from the mantle, the subducted oceanic lithosphere, and the downfolded metamorphic rocks. The ratio of partial melts from these three sources may have changed with time, giving rise to the diversity in composition of magmas.

California↗
USGS2004

Geologic Map of the Woodland Quadrangle, Clark and Cowlitz Counties, Washington

The Woodland 7.5' quadrangle is situated in the Puget-Willamette Lowland approximately 50 km north of Portland, Oregon (fig. 1). The lowland, which extends from Puget Sound into west-central Oregon, is a complex structural and topographic trough that lies between the Coast Range and the Cascade Range. Since late Eocene time, the Cascade Range has been the locus of an active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. The Coast Range occupies the forearc position within the Cascadia arc-trench system and consists of a complex assemblage of Eocene to Miocene volcanic and marine sedimentary rocks. The Woodland quadrangle lies at the northern edge of the Portland Basin, a roughly 2000-km2 topographic and structural depression that is the northernmost of several sediment-filled structural basins, which collectively constitute the Willamette Valley segment of the Puget-Willamette Lowland (Beeson and others, 1989; Swanson and others, 1993; Yeats and others, 1996). The Portland Basin is approximately 70 km long and 30 km wide; its long dimension is oriented northwest. Its northern boundary coincides, in part, with the lower Lewis River, which flows westward through the center of the quadrangle. The Lewis drains a large area in the southern Washington Cascade Range, including the southern flank of Mount St. Helens approximately 25 km upstream from the quadrangle, and joins the Columbia River about 6 km south of Woodland (fig. 1). Northwest of Woodland, the Columbia River exits the broad floodplain of the Portland Basin and flows northward through a relatively narrow bedrock valley at an elevation near sea level. The flanks of the Portland Basin consist of Eocene through Miocene volcanic and sedimentary rocks that rise to elevations exceeding 2000 ft (610 m). Seismic-reflection profiles (L.M. Liberty, written commun., 2003) and lithologic logs of water wells (Swanson and others, 1993; Mabey and Madin, 1995) indicate that as much as 550 m of late Miocene and younger sediments have accumulated in the deepest part of the basin near Vancouver. Most of this basin-fill material was carried in from the east by the Columbia River but sediment deposited by streams draining the adjacent highlands are locally important. The Portland Basin has been interpreted as a pull-apart basin located in the releasing stepover between two en echelon, northwest-striking, right-lateral fault zones (Beeson and others, 1985, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). These fault zones are thought to reflect regional transpression and dextral shear within the forearc in response to oblique subduction of the Pacific Plate along the Cascadia Subduction Zone (Pezzopane and Weldon, 1993; Wells and others, 1998). The southwestern margin of the Portland Basin is a well-defined topographic break along the base of the Tualatin Mountains, an asymmetric anticlinal ridge that is bounded on its northeast flank by the Portland Hills Fault Zone (Balsillie and Benson, 1971; Beeson and others, 1989; Blakely and others, 1995), which is probably an active structure (Wong and others, 2001; Liberty and others, 2003). The nature of the corresponding northeastern margin of the basin is less clear, but a poorly defined and partially buried dextral extensional fault zone has been hypothesized from topography, microseismicity, potential field-anomalies, and reconnaissance geologic mapping (Beeson and others, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). Another dextral structure may control the north-northwest-trending reach of the Columbia River between Portland and Longview (Blakely and others, 1995; Evarts, 2002; Evarts and others, 2002). This map is a contribution to a U.S. Geological Survey program designed to improve the geologic database for the Portland Basin part of the Pacific Northwest urban corridor,

Scientific Investigations Map↗
USGS2004

Geologic Map of the Saint Helens Quadrangle, Columbia County, Oregon, and Clark and Cowlitz Counties, Washington

The Saint Helens 7.5' quadrangle is situated in the Puget-Willamette Lowland approximately 35 km north Portland, Oregon. The lowland, which extends from Puget Sound into west-central Oregon, is a complex structural and topographic trough that lies between the Coast Range and the Cascade Range. Since late Eocene time, Cascade Range has been the locus of a discontinuously active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. The Coast Range occupies the forearc position within the Cascadia arc-trench system and consists of a complex assemblage of Eocene to Miocene volcanic and marine sedimentary rocks. The Saint Helens quadrangle lies in the northern part of the Portland Basin, a roughly 2000-km2 topographic and structural depression. It is the northernmost of several sediment-filled structural basins that collectively constitute the Willamette Valley segment of the Puget-Willamette Lowland (Beeson and others, 1989; Swanson and others, 1993; Yeats and others, 1996). The rhomboidal basin is approximately 70 km long and 30 km wide, with its long dimension oriented northwest. The Columbia River flows west and north through the Portland Basin at an elevation near sea level and exits through a confined bedrock valley less than 2.5 km wide about 16 km north of Saint Helens. The flanks of the basin consist of Eocene through Miocene volcanic and sedimentary rocks that rise to elevations exceeding 2000 ft (610 m). Seismic-reflection profiles (L.M. Liberty, written commun., 2003) and lithologic logs of water wells (Swanson and others, 1993; Mabey and Madin, 1995) indicate that as much as 550 m of late Miocene and younger sediments have accumulated in the deepest part of the basin near Vancouver. Most of this basin-fill material was carried in from the east by the Columbia River but contributions from streams draining the adjacent highlands are locally important. The Portland Basin has been interpreted as a pull-apart basin located in the releasing stepover between two echelon, northwest-striking, right-lateral fault zones (Beeson and others, 1985, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). These fault zones are thought to reflect regional transpression and dextral shear within the forearc in response to oblique subduction along the Cascadia Subduction Zone Pezzopane and Weldon, 1993; Wells and others, 1998). The southwestern margin of the Portland Basin is a well-defined topographic break along the base of the Tualatin Mountains, an asymmetric anticlinal ridge that is bounded its northeast flank by the Portland Hills Fault Zone (Balsillie and Benson, 1971; Beeson and others, 1989; Blakely and others, 1995), which is probably an active structure (Wong and others, 2001; Liberty and others, 2003). The nature of the corresponding northeastern margin of the basin is less clear, but a poorly defined and partially buried dextral extensional fault zone has been hypothesized from topography, microseismicity, potential fieldanomalies, and reconnaissance geologic mapping (Beeson and others, 1989; Beeson and Tolan, 1990; Yelin and Patton, 1991; Blakely and others, 1995). Another dextral structure, the Kalama Structural Zone of Evarts (2002), may underlie the north-northwest-trending reach of the Columbia River north of Woodland (Blakely and others, 1995). This map is a contribution to a U.S. Geological Survey (USGS) program designed to improve the geologic database for the Portland Basin region of the Pacific Northwest urban corridor, the populated forearc region of western Washington and Oregon. Better and more detailed information on the bedrock and surficial geology of the basin and its surrounding area is needed to refine assessments of seismic risk (Yelin and Patton, 1991; Bott and Wong, 1993), ground-failure hazards (Madin and Wang, 1999; Wegmann and Walsh, 2001) and resource availability in this rapid

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USGS2006

Geologic map of the Yacolt quadrangle, Clark County, Washington

The Yacolt 7.5' quadrangle is situated in the foothills of the western Cascade Range of southwestern Washington approximately 35 km northeast of Portland, Oregon. Since late Eocene time, the Cascade Range has been the locus of an active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. Volcanic and shallow-level intrusive rocks emplaced early in the history of the arc underlie most of the Yacolt quadrangle, forming a dissected and partly glaciated terrain with elevations between 250 and 2180 ft (75 and 665 m). The bedrock surface slopes irregularly but steeply to the southwest, forming the eastern margin of the Portland Basin, and weakly consolidated Miocene and younger basin-fill sediments lap up against the bedrock terrain in the southern part of the map area. A deep canyon, carved by the East Fork Lewis River that flows westward out of the Cascade Range, separates Yacolt and Bells Mountains, the two highest points in the quadrangle. Just west of the quadrangle, the river departs from its narrow bedrock channel and enters a wide alluvial floodplain. Bedrock of the Yacolt quadrangle consists of near-horizontal strata of Oligocene volcanic and volcaniclastic rocks that comprise early products of the Cascade volcanic arc. Basalt and basaltic andesite flows predominate. Most were emplaced on the flanks of a large mafic shield volcano and are interfingered with crudely bedded sections of volcanic breccia of probable lahar origin and a variety of well bedded epiclastic sedimentary rocks. At Yacolt Mountain, the volcanogenic rocks are intruded by a body of Miocene quartz diorite that is compositionally distinct from any volcanic rocks in the map area. The town of Yacolt sits in a north-northwest-trending valley apparently formed within a major fault zone. Several times during the Pleistocene, mountain glaciers moved down the Lewis River valley and spread southward into the map area. The largest glacier(s) covered the entire map area north of the East Fork Lewis River except for the summit of Yacolt Mountain. As the ice receded, it left behind a sculpted bedrock topography thickly mantled by drift, and deposited outwash in the fault-bounded valley at Yacolt and along the East Fork Lewis River valley. This map is a contribution to a program designed to improve geologic knowledge of the Portland Basin region of the Pacific Northwest urban corridor, the densely populated Cascadia forearc region of western Washington and Oregon. More detailed information on the bedrock and surficial geology of the basin and its surrounding area is necessary to refine assessments of seismic risk, ground-failure hazards and resource availability in this rapidly growing region.

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USGS2006

Geologic map of the Lacamas Creek quadrangle, Clark County, Washington

The Lacamas Creek 7.5 minute quadrangle is in southwestern Washington, approximately 25 km northeast of Portland, Oregon, along the eastern margin of the Portland Basin, which is part of the Puget-Willamette Lowland that separates the Cascade Range from the Oregon Coast Range. Since late Eocene time, the Cascade Range has been the locus of an episodically active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. Lava flows that erupted early in the history of the arc underlie the eastern half of the Lacamas Creek quadrangle, forming a dissected terrain, with elevations as high as 2050 ft (625 m), that slopes irregularly but steeply to the southwest. These basalt and basaltic andesite flows erupted in early Oligocene time from one or more vents located outside the map area. The flows dip gently (less than 5 degrees) west to southwest. In the western part of the map area, volcanic bedrock is unconformably overlain by middle Miocene to early Pleistocene(?) sediments that accumulated as the Portland Basin subsided. These sediments consist mostly of detritus carried into the Portland Basin by the ancestral Columbia River. Northwest-striking faults offset the Paleogene basin floor as well as the lower part of the basin fill. In middle Pleistocene time, basalt and basaltic andesite erupted from three small volcanoes in the southern half of the map area. These vents are in the northern part of the Boring volcanic field, which comprises several dozen late Pliocene and younger monogenetic volcanoes scattered throughout the greater Portland region. In latest Pleistocene time, the Missoula floods of glacial-outburst origin inundated the Portland Basin. The floods deposited poorly sorted gravels in the southwestern part of the Lacamas Creek quadrangle that grade northward into finer grained sediments. This map is a contribution to a program designed to improve geologic knowledge of the Portland Basin region of the Pacific Northwest urban corridor, the densely populated Cascadia forearc region of western Washington and Oregon. More detailed information on the bedrock and surficial geology of the basin and its surrounding area is necessary to refine assessments of seismic risk, ground-failure hazards and resource availability in this rapidly growing region.

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USGS2008

Geologic map of the Camas Quadrangle, Clark County, Washington, and Multnomah County, Oregon

The Camas 7.5' quadrangle is in southwestern Washington and northwestern Oregon approximately 20 km east of Portland. The map area, bisected by the Columbia River, lies on the eastern margin of the Portland Basin, which is part of the Puget-Willamette Lowland that separates the Cascade Range from the Oregon Coast Range. Since late Eocene time, the Cascade Range has been the locus of an episodically active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. Bedrock consists largely of basalt and basaltic andesite flows that erupted during late Oligocene time from one or more vents located outside the map area. These rocks crop out only north of the Columbia River: at the base of Prune Hill in Camas, where they dip southward at about 5°; and east of Lacamas Creek, where they dip to the southeast at 15 to 30°. The volcanic bedrock is unconformably overlain by Neogene sediments that accumulated as the Portland Basin subsided. In the Camas quadrangle, most of these sediments consist of basaltic hyaloclastic debris generated in the volcanic arc to the east and carried into the Portland Basin by the ancestral Columbia River. The dominant structures in the map area are northwest-striking dextral strike-slip faults that offset the Paleogene basin floor as well as the lower part of the basin fill. The Oligocene rocks at Prune Hill and to the east were uplifted in late Pliocene to early Pleistocene time within a restraining bend along one of these dextral faults. In Pleistocene time, basaltic andesite flows issued from a volcano centered on the west side of Prune Hill; another flow entered the map area from the east. These flows are part of the Boring volcanic field, which comprises several dozen late Pliocene and younger monogenetic volcanoes scattered throughout the greater Portland region. In latest Pleistocene time, the Missoula floods of glacial-outburst origin inundated the Portland Basin. The floods deposited huge bars of poorly sorted gravel in the lee of Prune Hill and west of the Sandy River. Volcanic debris from Mount Hood form a prominent delta at the mouth of the Sandy River. This map is a contribution to a program designed to improve geologic knowledge of the Portland Basin region of the Pacific Northwest urban corridor, the densely populated Cascadia forearc region of western Washington and Oregon. More detailed information on the bedrock and surficial geology of the basin and its surrounding area is necessary to refine assessments of seismic risk, ground-failure hazards and resource availability in this rapidly growing region.

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USGS2010

Geologic map of the Artemis Chasma quadrangle (V-48), Venus

Artemis, named for the Greek goddess of the hunt, represents an approximately 2,600 km diameter circular feature on Venus, and it may represent the largest circular structure in our solar system. Artemis, which lies between the rugged highlands of Aphrodite Terra to the north and relatively smooth lowlands to the south, includes an interior topographic high surrounded by the 2,100-km-diameter, 25- to 200-km-wide, 1- to 2-km-deep circular trough, called Artemis Chasma, and an outer rise that grades outward into the surrounding lowland. Although several other chasmata exist in the area and globally, other chasmata have generally linear trends that lack the distinctive circular pattern of Artemis Chasma. The enigmatic nature of Artemis has perplexed researchers since Artemis Chasma was first identified in Pioneer Venus data. Although Venus' surface abounds with circular to quasi-circular features at a variety of scales, including from smallest to largest diameter features: small shield edifices (>1 km), large volcanic edifices (100-1,000 km), impact craters (1-270 km), coronae (60-1,010 km), volcanic rises and crustal plateaus (~1,500-2,500 km), Artemis defies classification into any of these groups. Artemis dwarfs Venus' largest impact crater, Mead (~280 km diameter); Artemis also lacks the basin topography, multiple ring structures, and central peak expected for large impact basins. Topographically, Artemis resembles some Venusian coronae; however Artemis is an order of magnitude larger than the average corona (200 km) and about twice the size of Heng-O Corona (which is 1,010 km in diameter), the largest of Venusian coronae. In map view Artemis' size and shape resemble volcanic rises and crustal plateaus; however, both of these classes of features differ topographically from Artemis. Volcanic rises and crustal plateaus form broad domical regions, and steep-sided regions with flat tops, respectively; furthermore, neither rises nor plateaus include circular troughs. So although it seems clear what Artemis is not, there is little consensus about what Artemis is, much less how Artemis formed. Debate during the past decade has resulted in the proposal of at least four hypotheses for Artemis' formation. The first (herein referred to as H1) is that Artemis Chasma represents a zone of northwest-directed convergence and subduction. The second hypothesis (herein referred to as H2) is that Artemis consists of a composite structure with a part of its interior region marking the exposure of deformed ductile deep-crustal rocks analogous to a terrestrial metamorphic core complex. The third (herein referred to as H3) is that Artemis reflects the surface expression of an ancient (>3.5 Ga) huge bolide impact event on cold strong lithosphere. The fourth hypothesis (herein referred to as H4) is that Artemis marks the surface expression of a deep mantle plume. Each of these hypotheses holds different implications for Venus geodynamics and evolution processes, and for terrestrial planet processes in general. Viability of H1 would provide support that terrestrial-like plate-tectonic processes once occurred on Earth's sister planet. The feasibility of H2 would require high values of crustal extension and therefore imply that significant horizontal displacements occurred on Venus-displacement that may or may not be related to terrestrial-like plate-tectonic processes. The possibility of H3 would suggest that Venus' surface is extremely old, and that Venus has experienced very little dynamic activity for the last 3.5 billion years or more; this would further imply that Venus is essentially tectonically dead, and has been for most of its history. This view contrasts strongly with studies that highlight a rich history of Venus including activity at least as young as 750 million years ago, and quite likely up to the present. If H4 has credibility, then Artemis could provide clues to cooling mechanisms of Earth's sister planet. Each of these hypotheses

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