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At least 631 records · Page 35Linked to original sources
USGS2003

Geologic map of the Rusalka Planitia Quadrangle (V-25), Venus

The Rusalka Planitia quadrangle (herein referred to as V-25) occupies an 8.1 million square kilometer swath of lowlands nestled within the eastern highlands of Aphrodite Terra on Venus. The region (25?-0? N., 150?-180? E.) is framed by the crustal plateau Thetis Regio to the southwest, the coronae of the Diana-Dali chasmata complex to the south, and volcanic rise Atla Regio to the west. Regions to the north, and the quadrangle itself, are part of the vast lowlands, which cover four-fifths of the surface of Venus. The often-unspectacular lowlands of Venus are typically lumped together as ridged or regional plains. However, detailed mapping reveals the mode of resurfacing in V-25's lowlands: a mix of corona-related flow fields and local edifice clusters within planitia superimposed on a background of less clearly interpretable extended flow fields, large volcanoes, probable corona fragments, and edifice-flow complexes. The history detailed within the Rusalka Planitia quadrangle is that of the extended evolution of long-wavelength topographic basins in the presence of episodes of extensive corona-related volcanism, pervasive low-intensity small-scale eruptions, and an early phase of regional circumferential shortening centered on central Aphrodite Terra. Structural reactivation both obscures and illuminates the tectonic development of the region. The data are consistent with progressive lithospheric thickening, although the critical lack of an independent temporal marker on Venus severely hampers our ability to test this claim and correlate between localities. Two broad circular basins dominate V-25 geology: northern Rusalka Planitia lies in the southern half of the quadrangle, whereas the smaller Llorona Planitia sits along the northwestern corner of V-25. Similar large topographic basins occur throughout the lowlands of Venus, and gravity data suggest that some basins may represent dynamic topography over mantle downwellings. Both planitiae include coronae and associated lava flows, as well as fields of volcanic shields. Within each basin, the local geologic histories are relatively well constrained; correlations between the planitiae are difficult without making assumptions. The region between the two basins contains large volcanoes, corona fragments, deformation belts, and shield fields embedded within a topographically higher heterogeneous expanse of rolling plains. V-25's most prominent structural grain is a suite of wrinkle ridges that arc around the southwest corner of the quadrangle. A patchy suite of northeast-trending assorted lineaments underlies much of the map area. Although these lineaments originally were narrow fractures, this structural suite appears to have subsequently opened up along extensional troughs near Corpman crater in the southwest corner of the map area and been reactivated as wrinkle ridges at Ran Colles in the middle of the southern boundary of V-25. Nineteen impact craters dot the quadrangle. Craters Yazruk, du Chatelet, and Caccini contribute large geology- obscuring ejecta halos. Crater densities are too low for either relative or absolute age dating. Ten splotches, presumably associated with meteor airbursts, also occur across V-25.

IMAP↗
USGS2000

Map and database of Quaternary faults and folds in Colombia and its offshore regions

As part of the International Lithosphere Program’s “World Map of Major Active Faults,” the U.S. Geological Survey (USGS) is assisting in the compilation of a series of digital maps of Quaternary faults and folds in Western Hemisphere countries. The maps show the locations, ages, and activity rates of major earthquake-related features such as faults and fault-related folds. They are accompanied by databases that describe these features and document current information on their activity in the Quaternary. Top date, the project has published fault and fold maps for Costa Rica (Montero and others, 1998), Panama (Cowan and others, 1998), Venezuela (Audemard and others, 2000), Bolovia/Chile (Lavenu, and others, 2000), and Argentina (Costa and others, 2000). The project is a key part of the Global Seismic Hazards Assessment Program (ILP Project II-0) for the International Decade for Natural Hazard Disaster Reduction.

Open-File Report↗
USGS2002

The Cascadia Subduction Zone and related subduction systems: Seismic structure, intraslab earthquakes and processes, and earthquake hazards

The following report is the principal product of an international workshop titled “Intraslab Earthquakes in the Cascadia Subduction System: Science and Hazards” and was sponsored by the U.S. Geological Survey, the Geological Survey of Canada and the University of Victoria. This meeting was held at the University of Victoria’s Dunsmuir Lodge, Vancouver Island, British Columbia, Canada on September 18–21, 2000 and brought 46 participants from the U.S., Canada, Latin America and Japan. This gathering was organized to bring together active research investigators in the science of subduction and intraslab earthquake hazards. Special emphasis was given to “warm-slab” subduction systems, i.e., those systems involving young oceanic lithosphere subducting at moderate to slow rates, such as the Cascadia system in the U.S. and Canada, and the Nankai system in Japan. All the speakers and poster presenters provided abstracts of their presentations that were a made available in an abstract volume at the workshop. Most of the authors subsequently provided full articles or extended abstracts for this volume on the topics that they discussed at the workshop. Where updated versions were not provided, the original workshop abstracts have been included. By organizing this workshop and assembling this volume, our aim is to provide a global perspective on the science of warm-slab subduction, to thereby advance our understanding of internal slab processes and to use this understanding to improve appraisals of the hazards associated with large intraslab earthquakes in the Cascadia system. These events have been the most frequent and damaging earthquakes in western Washington State over the last century. As if to underscore this fact, just six months after this workshop was held, the magnitude 6.8 Nisqually earthquake occurred on February 28th, 2001 at a depth of about 55 km in the Juan de Fuca slab beneath the southern Puget Sound region of western Washington. The Governor’s Office of the State of Washington estimated damage at more than US$2 billion, making it among the costliest earthquakes in U.S. history.

California, Oregon, Washington↗
USGS2003

Earthquake recordings from the 2002 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS), Washington state

This report describes seismic data obtained during the fourth Seismic Hazard Investigation of Puget Sound (SHIPS) experiment, termed Seattle SHIPS . The experiment was designed to study the influence of the Seattle sedimentary basin on ground shaking during earthquakes. To accomplish this, we deployed seismometers over the basin to record local earthquakes, quarry blasts, and teleseisms during the period of January 26 to May 27, 2002. We plan to analyze the recordings to compute spectral amplitudes at each site, to determine the variability of ground motions over the basin. During the Seattle SHIPS experiment, seismometers were deployed at 87 sites in a 110-km-long east-west line, three north-south lines, and a grid throughout the Seattle urban area (Figure 1). At each of these sites, an L-22, 2-Hz velocity transducer was installed and connected to a REF TEK Digital Acquisition System (DAS), both provided by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) of the Incorporated Research Institutes for Seismology (IRIS). The instruments were installed on January 26 and 27, and were retrieved gradually between April 18 and May 27. All instruments continuously sampled all three components of motion (velocity) at a sample rate of 50 samples/sec. To ensure accurate computations of amplitude, we calibrated the geophones in situ to obtain the instrument responses. In this report, we discuss the acquisition of these data, we describe the processing and merging of these data into 1-hour long traces and into windowed events, we discuss the geophone calibration process and its results, and we display some of the earthquake recordings.

Washington↗
USGS2000

Data for Quaternary faults in western Montana

The "World Map of Major Active Faults" Task Group is compiling published fault data, developing a digital database of the fault data, and preparing a series of maps for the United States and other countries in the western Hemisphere. The data is intended to portray the locations, ages, and activity rates of major earthquake-related features such as faults, folds, and liquefaction features that have geologic evidence of Quaternary (1.6 Ma) deformation. The Western Hemisphere effort is sponsored by International Lithosphere Program (ILP) Task Group II-2; the data compilation, database, and map for the United States is funded largely by the National Earthquake Hazard Reduction Program (NEHRP) through the U.S. Geological Survey. The ILP effort in the Western Hemisphere is coordinated by Michael N. Machette, the digital database is designed and managed Kathleen M. Haller, and map data are digitized and manipulated by Richard L. Dart. In addition to meeting the goals of the Task Group II-2, this effort represents a key contribution to the new Global Seismic Hazards Assessment Program (ILP Task Group II-0) for the International Decade for Natural Disaster Reduction. This compilation, which documents the published data on Quaternary surface faulting in western Montana, is one of many similar state or regional compilations that are planned for the project. Compilations for Arizona (Pearthree, 1998 #2945), Colorado (Widmann and others, 1998 #3441), New Mexico (Machette and others, 1998), and West Texas (Collins and others, 1996 #993) are currently available and the compilation for features east of the Rocky Mountain front will be available in early 2000 (Crone and Wheeler, in press). All are primarily a catalog of data that includes a variety of geographic, geologic, and paleoseismologic parameters for known or assumed Quaternary faults. These data compilations, the digital database, and the companion maps summarize the published information on known tectonic features and present the information in an internally consistent format. The compilations will be available in digital database format on the WorldWide Web in the near future, which will greatly improve their utility. Release of data for individual states and regions within the United States in this text-based format was necessary because of the time required to develop the national database.

Montana↗
USGS2004

Location, Age, and Tectonic Significance of the Western Idaho Suture Zone (WISZ)

The Western Idaho Suture Zone (WISZ) represents the boundary between crust overlying Proterozoic North American lithosphere and Late Paleozoic and Mesozoic intraoceanic crust accreted during Cretaceous time. Highly deformed plutons constituted of both arc and sialic components intrude the WISZ and in places are thrust over the accreted terranes. Pronounced variations in Sr, Nd, and O isotope ratios and in major and trace element composition occur across the suture zone in Mesozoic plutons. The WISZ is located by an abrupt west to east increase in initial 87Sr/86Sr ratios, traceable for over 300 km from eastern Washington near Clarkston, east along the Clearwater River thorough a bend to the south of about 110? from Orofino Creek to Harpster, and extending south-southwest to near Ola, Idaho, where Columbia River basalts conceal its extension to the south. K-Ar and 40Ar/39Ar apparent ages of hornblende and biotite from Jurassic and Early Cretaceous plutons in the accreted terranes are highly discordant within about 10 km of the WISZ, exhibiting patterns of thermal loss caused by deformation, subsequent batholith intrusion, and rapid rise of the continental margin. Major crustal movements within the WISZ commenced after about 135 Ma, but much of the displacement may have been largely vertical, during and following emplacement of batholith-scale silicic magmas. Deformation continued until at least 85 Ma and probably until 74 Ma, progressing from south to north.

Open-File Report↗
USGS2006

Magnetotelluric survey to locate the Archean/Proterozoic suture zone north of Wells, Nevada

It is important to know whether major mining districts in the Northern Nevada Gold Province are underlain by rocks of the Archean Wyoming craton, which are known to contain orogenic gold deposits, or by accreted rocks of the Paleoproterozoic Mojave province. It is also important to know the location and orientation of the Archean/Proterozoic suture zone between these provinces as well as major basement structures within these terranes because they may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The Archean was the main gold-mineralization period, and Archean lode-gold deposits were formed at mid-crustal depths along major shear zones. The nature of the crystalline basement below the Northern Nevada Gold Province and the location of major faults within it are relevant to Rodinian reconstructions, crustal development, and ore deposit models (e.g., Hofstra and Cline, 2000; Grauch and others, 2003). According to Whitmeyer and Karlstrom (2004), the Archean cratons of the northwestern United States and Canada had stabilized as continental lithosphere by 2.5 Ga, and were rifted and assembled into a large continental mass by 1.8 Ga, to which the 1.73-1.68 Ga Mohave province was accreted by 1.65 Ga. The Archean/Proterozoic suture zone has a west-southwest strike where it is exposed (Reed, 1993) at the eastern Utah and southwestern Wyoming border (Cheyenne Belt) where it is characterized by an up to 7-km-thick mylonite zone (Smithson and Boyd, 1998). In the Great Basin, the strike of the Archean/Proterozoic suture zone is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and basin fill. East-west and southwest-northeast strikes for the Archean/Proterozoic suture zone have been inferred based on Sr, Nd, and Pb isotopic compositions of granitoid intrusions (Tosdal and others, 2000). To better constrain the location and strike of the Archean/Proterozoic suture zone below cover, three regional north-south magnetotelluric (MT) sounding profiles were acquired in western Utah and northeastern Nevada (Williams and Rodriguez, 2003; 2004; 2005), and one east-west MT sounding profile (fig. 1) MT sounding profile was acquired in northeastern Nevada. Resistivity modeling of the MT data can be used to investigate buried structures or sutures that may have influenced subsequent regional fluid flow and localized mineralization. The purpose of this report is to release the MT sounding data collected along the east-west profile in northeastern Nevada; no interpretation of the data is included.

Open-File Report↗
USGS2007

Platinum-group elements in sills of the Jurassic Ferrar Large Igneous Province from Northern Victoria Land, Antarctica

Platinum-group element (PGE) abundances were analysed in basaltic andesites and andesites from sills of the Ferrar Large Igneous Province in northern Victoria Land. The strongly fractionated primitive mantle-normalised PGE- patterns show enrichment of the Pt-PGE over the Ir-PGE. The single element abundances exhibit good correlations with the degree of differentiation of the distinctly evolved samples and are interpreted to result mainly from low-pressure in-situ differentiation after magma emplacement in the upper crust. Compared to tholeiitic rocks from other magmatic provinces, only the Ferrar rocks exhibit coupled enrichment of Pd, Pt and Cu even in most evolved samples. The decrease of Pt and Pd in some of the more evolved samples does not necessarily signify sulphide fractionation, but may indicate the formation of other PGE-compounds. The inferred sulphur-undersaturated conditions during differentiation are in agreement with the high degrees of partial melting as well as the refractory nature of the proposed subcontinental lithospheric mantle source beneath this magmatic province

Open-File Report↗
USGS2007

Thermochronologic constraints on the tectonic evolution of the western Antarctic Peninsula in late Mesozoic and Cenozoic times

West of the Antarctic Peninsula, oceanic lithosphere of the Phoenix plate has been subducted below the Antarctic plate. Subduction has ceased successively from south to north over the last 65 Myr. An influence of this evolution on the segmentation of the crust in the Antarctic plate is disputed. Opposing scenarios consider effects of ridge crest – trench interactions with the subduction zone or differences in slip along a basal detachment in the overriding plate. Fission track (FT) analyses on apatites and zircons may detect thermochronologic patterns to test these hypotheses. While existing data concentrate on accretionary processes in Palmer Land, new data extend information to the northern part of the Antarctic Peninsula. Zircons from different geological units over wide areas of the Antarctic Peninsula yield fission track ages between 90 and 80 Ma, indicating a uniform regional cooling episode. Apatite FT ages obtained so far show considerable regional variability

Open-File Report↗
USGS2007

Crustal architecture of the oblique-slip conjugate margins of George V Land and southeast Australia

A conceptual, lithospheric-scale cross-section of the conjugate, oblique-slip margins of George V Land, East Antarctica, and southeast Australia (Otway Basin) has been constructed based on the integration of seismic and sample data. This cross-section is characterised by asymmetry in width and thickness, and depth-dependent crustal extension at breakup in the latest Maastrichtian. The broad Antarctic margin (~360 km apparent rift width) developed on thick crust (~42 km) of the Antarctic craton, whereas the narrow Otway margin (~220 km) developed on the thinner crust (~31 km) of the Ross–Delamerian Orogen. The shallow basement (velocities ~5.5 km.s-1) and the deep continental crust (velocities >6.4 km.s-1) appear to be largely absent across the central rift, while the mid-crustal, probably granitic layer (velocities ~6 km.s-1) is preserved. Comparison with published numerical models suggests that the shallow basement and deep crust may have been removed by simple shear, whereas the mid-crust has been ductilely deformed.

Open-File Report↗
USGS2009

Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

Reconstructions of Phanerozoic tectonic plates can be closely constrained by lithologic correlations across conjugate margins by paleontologic information, by correlation of orogenic belts, by paleomagnetic location of continents, and by ocean floor magmatic stripes. In contrast, Proterozoic reconstructions are hindered by the lack of some of these tools or the lack of their precision. To overcome some of these difficulties, this report focuses on a different method of reconstruction, namely the use of the shape of continents to assemble the supercontinent of Rodinia, much like a jigsaw puzzle. Compared to the vast amount of information available for Phanerozoic systems, such a limited approach for Proterozoic rocks, may seem suspect. However, using the assembly of the southern continents (South America, Africa, India, Arabia, Antarctica, and Australia) as an example, a very tight fit of the continents is apparent and illustrates the power of the jigsaw puzzle method. This report focuses on Neoproterozoic rocks, which are shown on two new detailed geologic maps that constitute the backbone of the study. The report also describes the Neoproterozoic, but younger or older rocks are not discussed or not discussed in detail. The Neoproterozoic continents and continental margins are identified based on the distribution of continental-margin sedimentary and magmatic rocks that define the break-up margins of Rodinia. These Neoproterozoic continental exposures, as well as critical Neo- and Meso-Neoproterozoic tectonic features shown on the two new map compilations, are used to reconstruct the Mesoproterozoic supercontinent of Rodinia. This approach differs from the common approach of using fold belts to define structural features deemed important in the Rodinian reconstruction. Fold belts are difficult to date, and many are significantly younger than the time frame considered here (1,200 to 850 Ma). Identifying Neoproterozoic continental margins, which are primarily extensional in origin, supports recognition of the Neoproterozoic fragmentation pattern of Rodinia and outlines the major continental masses that, prior to the breakup, formed the supercontinent. Using this pattern, Rodinia can be assembled by fitting the pieces together. Evidence for Neoproterozoic margins is fragmentary. The most apparent margins are marked by miogeoclinal deposits (passive-margin deposits). The margins can also be outlined by the distribution of continental-margin magmatic-arc rocks, by juvenile ocean-floor rocks, or by the presence of continent-ward extending aulacogens. Most of the continental margins described here are Neoproterozoic, and some had an older history suggesting that they were major, long-lived lithospheric flaws. In particular, the western margin of North America appears to have existed for at least 1,470 Ma and to have been reactivated many times in the Neoproterozoic and Phanerozoic. The inheritance of trends from the Mesoproterozoic by the Neoproterozoic is particularly evident along the eastern United States, where a similarity of Mesoproterozoic (Grenville) and Neoproterozoic trends, as well as Paleozoic or Mesozoic trends, is evident. The model of Rodinia presented here is based on both geologic and paleomagnetic information. Geologic evidence is based on the distribution and shape of Neoproterozoic continents and on assembling these continents so as to match the shape, history, and scale of adjoining margins. The proposed model places the Laurasian continents?Baltica, Greenland, and Laurentia?west of the South American continents (Amazonia, Rio de La Plata, and Sa? Francisco). This assembly is indicated by conjugate pairs of Grenville-age rocks on the east side of Laurentia and on the west side of South America. In the model, predominantly late Neoproterozoic magmatic-arc rocks follow the trend of the Grenville rocks. The boundary between South America and Africa is interpreted as the site of a Wilson cycle

Open-File Report↗
USGS2010

Seismicity of the Earth 1900–2010: Caribbean plate and vicinity

Extensive diversity of tectonic regimes characterizes the perimeter of the Caribbean plate, involving no fewer than four major adjacent plates (North America, South America, Nazca, and Cocos). Inclined zones of deep earthquakes (Wadati-Benioff zones), deep ocean trenches, and arcs of volcanoes clearly indicate subduction of oceanic lithosphere along the Central American and Atlantic Ocean margins of the Caribbean plate, while shallow seismicity and focal mechanisms of major shocks in Guatemala, northern Venezuela, and the Cayman Ridge and Cayman Trench indicate transform fault and pull-apart basin tectonics. The depth profile panels on this map portray earthquakes that extend from the Middle America Trench axis in the west to depths as great as 300 km beneath Guatemala, and from the Lesser Antilles Trench axis in the east to depths of approximately 200 km beneath Guadeloupe and the northeast Caribbean. In contrast, seismicity along the segments of the Caribbean plate margins from Guatemala to Hispaniola and from Trinidad to western Venezuela is indicative of transform fault tectonics.

Open-File Report↗
USGS2010

Seismicity of the Earth 1900-2007, Nazca Plate and South America

The South American arc extends over 7,000 km, from the Chilean triple junction offshore of southern Chile to its intersection with the Panama fracture zone, offshore the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their decent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 mm/yr in the south to approximately 70mm/yr in the north.

Open-File Report↗
USGS2013

Seismicity of the Earth 1900–2010 Himalaya and vicinity

Seismicity in the Himalaya region predominantly results from the collision of the India and Eurasia continental plates, which are converging at a relative rate of 40–50 mm/yr. Northward underthrusting of India beneath Eurasia generates numerous earthquakes and consequently makes this area one of the most seismically hazardous regions on Earth. The surface expression of the plate boundary is marked by the foothills of the north-south trending Sulaiman Range in the west, the Indo-Burmese Arc in the east, and the east-west trending Himalaya Front in the north of India. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Further north again, the Tian Shan is a seismically active intra-continental mountain belt defined by a series of east-west trending thrust faults thought to be related to the broad footprint of the India-Eurasia collision. Tectonics in northern India are dominated by motion along the Main Frontal Thrust and associated thrust faults of the India-Eurasia plate boundary, which have resulted in a series of large and devastating earthquakes in (and prior to) the 20th century. The Tibetan Plateau to the north of the main plate boundary is a broad region of uplift associated with the India-Eurasia collision, and is cut by a series of generally east-west trending strike-slip faults. These include the Kunlun, Haiyuan, and the Altyn Tagh faults, all of which are left-lateral structures, and the Kara-Koram right-lateral fault. Throughout the plateau, thrust faults accommodate the north-south compressional component of crustal shortening associated with the ongoing collision of India and Eurasia, while strike-slip and normal faults accommodate east-west extension. To the east, The Longmen Shan thrust belt marks the eastern margin of the Tibetan Plateau separating the complex tectonics of the plateau region from the relatively undeformed Sichuan Basin. Further south, the left-lateral Xiangshuihe-Xiaojiiang, right-lateral Red River and right-lateral Sagaing strike-slip fault systems accommodate deformation along the eastern margin of the India plate. Deep earthquakes have also occurred in the Indo-Burmese Arc region, thought to be an expression of eastward-directed subduction of the India plate, though whether subduction is ongoing is still debated.

Himalaya and vicinity↗
USGS2013

Seismicity of the Earth 1900–2010 Middle East and vicinity

No fewer than four major tectonic plates (Arabia, Eurasia, India, and Africa) and one smaller tectonic block (Anatolia) are responsible for seismicity and tectonics in the Middle East and surrounding region. Geologic development of the region is a consequence of a number of first-order plate tectonic processes that include subduction, large-scale transform faulting, compressional mountain building, and crustal extension. In the east, tectonics are dominated by the collision of the India plate with Eurasia, driving the uplift of the Himalaya, Karakorum, Pamir and Hindu Kush mountain ranges. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting, resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Off the south coasts of Pakistan and Iran, the Makran trench is the surface expression of active subduction of the Arabia plate beneath Eurasia. Northwest of this subduction zone, collision between the two plates forms the approximately 1,500-km-long fold and thrust belts of the Zagros Mountains, which cross the whole of western Iran and extend into northeastern Iraq. Tectonics in the eastern Mediterranean region are dominated by complex interactions between the Africa, Arabia, and Eurasia plates, and the Anatolia block. Dominant structures in this region include: the Red Sea Rift, the spreading center between the Africa and Arabia plates; the Dead Sea Transform, a major strike-slip fault, also accommodating Africa-Arabia relative motions; the North Anatolia Fault, a right-lateral strike-slip structure in northern Turkey accommodating much of the translational motion of the Anatolia block westwards with respect to Eurasia and Africa; and the Cyprian Arc, a convergent boundary between the Africa plate to the south, and Anatolia Block to the north.

Open-File Report↗
USGS2014

Seismicity of the Earth 1900-2013 offshore British Columbia-southeastern Alaska and vicinity

The tectonics of the Pacific margin of North America between Vancouver Island and south-central Alaska are dominated by the northwest motion of the Pacific plate with respect to the North America plate at a velocity of approximately 50 mm/yr. In the south of this mapped region, convergence between the northern extent of the Juan de Fuca plate (also known as the Explorer microplate) and North America plate dominate. North from the Explorer, Pacific, and North America plate triple junction, Pacific:North America motion is accommodated along the ~650-km-long Queen Charlotte fault system. Offshore of Haida Gwaii and to the southwest, the obliquity of the Pacific:North America plate motion vector creates a transpressional regime, and a complex mixture of strike-slip and convergent (underthrusting) tectonics. North of the Haida Gwaii islands, plate motion is roughly parallel to the plate boundary, resulting in almost pure dextral strike-slip motion along the Queen Charlotte fault. To the north, the Queen Charlotte fault splits into multiple structures, continuing offshore of southwestern Alaska as the Fairweather fault, and branching east into the Chatham Strait and Denali faults through the interior of Alaska. The plate boundary north and west of the Fairweather fault ultimately continues as the Alaska-Aleutians subduction zone, where Pacific plate lithosphere subducts beneath the North America plate at the Aleutians Trench. The transition is complex, and involves intraplate structures such as the Transition fault. The Pacific margin offshore British Columbia is one of the most active seismic zones in North America and has hosted a number of large earthquakes historically.

Alaska, British Columbia↗
USGS2015

Seismicity of the Earth 1900-2013, seismotectonics of South America (Nazca Plate Region)

The South American arc extends over 7,000 kilometers (km), from the Chilean margin triple junction offshore of southern Chile, to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 millimeters/year (mm/yr) in the south, to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.

Open-File Report↗
USGS2019

Geochronologic, isotopic, and geochemical data from igneous rocks in the Lane Mountain area, San Bernardino County, California

We present new geochronologic, isotopic, and geochemical data for selected igneous rocks in the Lane Mountain area, California. We determined SHRIMP-RG U-Pb zircon ages for the following units: (1) Larrea complex (~253 Ma and ~149–146 Ma); (2) Daisy granodiorite (~151 Ma); (3) Jack Spring quartz monzonite (~85–82 Ma); (4) unnamed porphyritic dikes and stocks (~80–73 Ma); and (5) Lane Mountain volcanics (~22 Ma). These results confirm and refine previous age determinations for the Larrea complex, Jack Spring quartz monzonite, and Lane Mountain volcanics, and provide the first reliable ages for the Daisy granodiorite and the unnamed porphyritic rocks. Sri of the plutonic rocks increases from ~0.704 to ~0.705–0.706 to ~0.708–0.709 for samples dated as ~250, ~150, and ~85–70 Ma, respectively, which indicates progressively greater involvement of Precambrian continental lithosphere in magma generation through time. The Sri and geochemical data suggest that the most significant change in magmatic source occurred between ~150 and 85 Ma.

California↗