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At least 559 records · Page 31Linked to original sources
USGS1997

Tectonic evolution of the central Brooks Range mountain front: Evidence from the Atigun Gorge region

Atigun Gorge, at the northern front of the eastern Endicott Mountains, contains well-exposed rocks of the upper part of the Endicott Mountains allochthon and rocks of the structurally higher Picnic Creek or Ipnavik River allochthon. These allochthons contain rocks as young as Early Cretaceous (Valanginian) and are separated by a nearly vertical fault zone that contains exotic blocks of Triassic and Jurassic chert and silicified mudstone. Siliceous rocks of this type are not present in the Endicott Mountains allochthon but are characteristic of the Picnic Creek, Ipnavik River, and some of the other allochthons that structurally overlie the Endicott Mountains allochthon in the central and western Brooks Range. These exotic blocks, therefore indicate that structurally higher rocks of either the Picnic Creek or Ipnavik River allochthon were emplaced during the Early Cretaceous and are preserved along the northern flank of the eastern Endicott Mountains. The deformed thickness of this higher allochthon in the subsurface north of the mountains is unknown but probably exceeds 2 kilometers. Similar relations are mapped east of Atigun Gorge in an area of structural transition from the eastern Endicott Mountains into the northern Philip Smith Mountains, which are formed by the parautochthonous North Slope stratigraphic assemblage. The allochthonous rocks at the mountain front are regionally unconformably overlain by proximal Lower Cretaceous (Albian) foredeep conglomerate at the southern flank of the Colville basin, but at Atigun Gorge, the base of these deposits is interpreted as a possible back thrust at a triangle zone. Conglomerate clasts in the foredeep deposits are dominantly chert, mafic igneous rock, and other lithologies characteristic of the Picnic Creek and Ipnavik River allochthons and scattered clasts from the Endicott Mountains allochthon. The conglomerates show that the chert-rich allochthonous rocks and the Endicott Mountains allochthon were emplaced in the north-central Brooks Range by large-scale crustal shortening (>300 km) between the Valanginian and Albian (∼135 to ∼112 Ma). This orogenic event significantly postdates early stages of Brooks Range orogeny but predates later stages of orogeny documented by stratigraphic and apatite fission-track data. These relations reduce the magnitude of shortening inferred at the triangle zone at the Brooks Range mountain front. The outcrop data suggest that some of the strata preserved at a structurally low level north of the mountain front and visible in the seismic data of the Trans-Alaska Crustal Transect (TACT) may consist of clastic sedimentary rocks of the structurally higher Picnic Creek or Ipnavik River allochthon.

Journal of Geophysical Research B: Solid Earth↗
USGS1997

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north-south within the magmatic regions and northwest-southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest-southeast dextral shear of the Walker Lane belt to the east-west extension characteristic of the Cascade arc in central Oregon. Faulting along north-south strikes in the volcanically active areas indicates the east-west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Journal of Geophysical Research B: Solid Earth↗
USGS2010

Organic-matter production and preservation and evolution of anoxia in the Holocene Black Sea

Dating of sediments collected in gravity cores during Leg 1 of the 1988 R/V Knorr expedition to the Black Sea suggests that the onset of water-column anoxia at ∼7.5 ka was virtually synchronous across the basin over a depth range of ∼200–2250 m. A finely laminated, organic carbon (OC) rich sapropel (unit II) was produced as a result of this anoxia. The trigger for increased OC production and development of anoxia was the spillover of saline waters through the Bosporus that probably began at ∼9.0 ka and peaked between ∼7.0 and 5.5 ka. This spillover enhanced vertical mixing and nutrient cycling and caused a short-term (2–3 kyr) burst in surface-water productivity during the early part of unit II deposition. Continued incursion of saline waters enhanced vertical stability and inhibited mixing of nutrients into surface waters, thus limiting primary production and decreasing the OC flux to sediments beginning ∼5.5 ka. Concentration, accumulation rate, and degree of preservation of organic matter all decreased in the upper part of unit II as a result of decreasing productivity, but anoxia persisted throughout most of the water column. The end of unit II sapropel deposition was synchronous across the Black Sea as the result of the first blooms of the coccolith Emiliania huxleyi , which presumably marked an increase in surface-water salinity above 11 and the beginning of unit I deposition. The high coccolith-carbonate fluxes that occurred during deposition of unit I diluted the OC concentration in the sediments, but OC accumulation rates are about the same as those in upper part of unit II.

Paleoceanography and Paleoclimatology↗
USGS2012

Vapor-dominated zones within hydrothermal systems: Evolution and natural state

Three conceptual models illustrate the range of hydrothermal systems in which vapor-dominated conditions are found. The first model (model I) represents a system with an extensive near-vaporstatic vapor-dominated zone and limited liquid throughflow and is analogous to systems such as The Geysers, California. Such systems can evolve within low-permeability barriers without changes in boundary conditions or rock properties, given an adequate supply of heat. Their scarcity in nature may be due to the need for a long-lived, potent heat source and for a low-permeability aureole that remains intact for significant lengths of time. Models II and III represent systems with significant liquid throughflow and include steam-heated discharge features at higher elevations and high-chloride springs at lower elevations, connected to and fed by a single circulation system at depth. In model II, as in model I, the vapor-dominated zone has a near-vaporstatic vertical pressure gradient and is generally underpressured with respect to local hydrostatic pressure. The vapor-dominated zone in model III is quite different, in that phase separation takes place at pressures close to local hydrostatic and the overall pressure gradient is near hydrostatic. A relatively large number of high-temperature systems in regions of moderate to great topographic relief are similar to either model II or model III; however, in most cases there are insufficient data to establish a single preferred model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach terranes along the Trans-Alaska Crustal Transect in the Chugach Mountains and southern Copper River Basin, Alaska

The Trans-Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra-mafic-mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid- to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid-Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite-pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc-derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.

Journal of Geophysical Research Solid Earth↗
USGS2012

Igneous history of the Koyukuk terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U-shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time-stratigraphic units: (1) pre-Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction-related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc-alkaline, and alkaline rocks with chondrite-normalized rare earth element patterns that range from flat (La N /Yb N = 1) to highly light rare earth element enriched (La N /Yb N > 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc-continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid-Cretaceous plutons whose isotopic compositions (Arth et al., this issue ( a )) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc-continent collision.

Journal of Geophysical Research Solid Earth↗
USGS2012

The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of 40 Ar/ 39 Ar thermochronology to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The 40 Ar/ 39 Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The 40 Ar/ 39 Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1°–5°C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman‐Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500°C which reset the K–Ar systems of minerals from Proterozoic rocks. High‐grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman‐Piute batholith at 73±1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were ∼100°C/m.y. between 73 and 70 Ma and 5°–10°C/m.y. from 70 to ∼30 Ma. Between 65 and 25 Ma the entire eastern Mojave Desert underwent a period of cooling at a rate of 2°–10°C/m.y. By 30 Ma, rocks exposed in the Old Woman‐Piute, Marble, Ship, Clipper, and Turtle Mountains were below ∼100°C. The 40 Ar/ 39 Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23±1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at ∼180°C and ∼350°C, respectively. This suggests that the exposed part of the Chemehuevi detachment fault initiated at a dip of 5°–30° or as a series of higher‐angle faults that cut to a depth of 10–12 km and were later rotated to their present dip. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10°–50°C/m.y. between 22 and 15 Ma.

California↗
USGS2010

Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington

A refined northeast Pacific plate-motion model provides a framework for analysis of the Tertiary volcanic and tectonic history of western Oregon and Washington. We examine three possible models for the origin of the allochthonous Paleocene and Eocene oceanic basalt basement of the Coast Range: (1) accretion to the continent of hot spot generated linear seamount chains; (2) accretion of thick oceanic crust and seamounts generated during Farallon-Kula spreading reorganizations between 61 and 48 Ma; and (3) eruption of basalt during oblique rifting of the continental margin as it overrode an active Yellowstone hot spot on the Kula-Farallon ridge. The plate model suggests that microplate rotation and accretion of hot spot generated linear aseismic ridges cannot be easily reconciled with rapid northeast motion of the KuIa and Farallon plates and the well-established paleomagnetic rotations. Following emplacement of the Coast Range basement, changes in the character of forearc, back arc and Cascade arc volcanism correlate with a marked decrease in the rate of Farallon-North America convergence between 43 and and 28 Ma. This slowdown may be responsible for (1) westward stepping of the volcanic arc front from the Challis axis to a Cascade axis at about 42 Ma; (2) a subsequent episode of increased ash flow tuff volcanism and extension in the Cascade arc between 37 and 18 Ma that correlates with the “ignimbrite flare-up” in the Basin and Range; and (3) a period of extensional basaltic and alkalic volcanism and intrusion in the Coast Range between 44 and 28 Ma. Reduction of the convergence rate and westward stepping of the flexure in the subducted slab may have reduced the horizontal compressive stress on the continent, allowing increased injection of magma into the crust, development of large, shallow magma chambers, and the outbreak of extensional volcanism over a large area behind the Farallon-North America subduction zone.

Oregon, Washington↗
USGS2010

The evolution of forearc structures along an oblique convergent margin, central Aleutian Arc

Multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central Aleutian Ridge. Since at least late Miocene time this sector of the ridge has been obliquely underthrust 30° west of orthogonal convergence by the northwestward converging Pacific plate at a rate of 80–90 km/m.y. Our data indicate that prior to late Eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits; the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits. Beginning in latest Miocene or earliest Pliocene time, a zone of outer-arc structural highs and a forearc basin began to form. Formation of these companion intraarc structures may be linked to the late Neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the Aleutian Trench. Initial structures of the zone of outer-arc highs formed as the thickening wedge underran, compressively deformed, and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust. Forearc basin strata ponded arcward of the elevating zone of outer-arc highs. However, most younger structures of the zone of outer-arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge. Oblique convergence created a major right-lateral shear zone (the Hawley Ridge shear zone) that longitudinally disrupted the zone of outer-arc highs, truncating the seaward flank of the forearc basin and shearing the southern limb of Hawley Ridge, an exceptionally large antiformal outer-arc high structure. Slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes. Uplift of Hawley Ridge may be related to the thickening of the arc massif by westward directed basement duplexes. In addition, the forearc is disrupted by structures transverse to the margin that occur where unusually high-stress accumulations have resulted in the rupture of repeated great earthquakes. It is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central Aleutian Ridge. Great structural complexity, including the close juxtaposition of coeval structures recording compression, extension, differential vertical movements, and strike-slip displacement, should be expected, even within areas of generally kindred tectonostratigraphic terranes.

Alaska↗
USGS2024

Concepts and evolution of urban hydrology.

Urbanization and climate change are exacerbating the flood risk and ecosystem degradation in urban catchments, with traditional stormwater management systems often overwhelmed. In this Review, we discuss changes in urban hydrology and approaches to stormwater management. Roughly 90% of rainfall on impervious surfaces and drainage infrastructure becomes run-off, enhancing rainfall export away from cities and leading to local water scarcity and downstream flooding and pollution. Projected increases in urban populations (68% in cities by 2050) and rainfall intensity (~12% in the 10-year and 50-year recurrence interval intensity, under 1.5 °C warming) will exacerbate these issues. Transforming stormwater systems is thus urgently needed, to mitigate flood risk and also to address community desires for environmental protection and enhanced water security. Opportunities include rain gardens and other nature-based stormwater control measures (which restore natural flows and offer other ecosystem services), smart sensor monitoring networks and real-time management (which sustain natural flow regimes, mitigate flood risk and protect ecosystem services) and stormwater harvesting (to avoid local water scarcity). Community acceptance of stormwater harvesting is as high as 96% and stormwater is a substantial resource, with volumes often exceeding demand in some parts of the world. Delivering additional transformations globally requires research into strategies to incentivize engagement and investment, and policies to guide governance of decentralized networks.

Nature Reviews Earth & Environment↗
USGS2025

Understanding the evolution of scoria cone morphology using multivariate models

Scoria cones are the most abundant type of volcano in the Solar System. They occur in every tectonic setting and often overlap with human populations, yet our ability to provide complete geochronology within volcanic fields remains limited. Appropriate geochronology underpins the reconstruction of size-frequency distribution and is a key input for robust volcanic hazard assessment. Morphometric data have long been used to estimate relative ages of scoria cones; however, they have only shown promise at single volcanic fields and simple cones with homogenous pyroclastics. Here, we present a new global inventory of dated scoria cones ( n = 572) from 71 volcanic fields formed under diverse magmatic, tectonic and climatic regimes, and build data-driven age models for dating scoria cones using easily accessible morphometric, reflectance and climatic variables. Our models suggest chemical composition of ascending magma may influence the initial scoria cone morphology which is then gradually modified by erosion over time.

Communications Earth & Environment↗
USGS2003

Impact of structural and autocyclic basin-floor topography on the depositional evolution of the deep-water Valparaiso forearc basin, central Chile

The Valparaiso Basin constitutes a unique and prominent deep-water forearc basin underlying a 40-km by 60-km mid-slope terrace at 2.5-km water depth on the central Chile margin. Seismic-reflection data, collected as part of the CONDOR investigation, image a 3-3.5-km thick sediment succession that fills a smoothly sagged, margin-parallel, elongated trough at the base of the upper slope. In response to underthrusting of the Juan Ferna??ndez Ridge on the Nazca plate, the basin fill is increasingly deformed in the seaward direction above seaward-vergent outer forearc compressional highs. Syn-depositional growth of a large, margin-parallel monoclinal high in conjunction with sagging of the inner trough of the basin created stratal geometries similar to those observed in forearc basins bordered by large accretionary prisms. Margin-parallel compressional ridges diverted turbidity currents along the basin axis and exerted a direct control on sediment depositional processes. As structural depressions became buried, transverse input from point sources on the adjacent upper slope formed complex fan systems with sediment waves characterising the overbank environment, common on many Pleistocene turbidite systems. Mass failure as a result of local topographic inversion formed a prominent mass-flow deposit, and ultimately resulted in canyon formation and hence a new focused point source feeding the basin. The Valparaiso Basin is presently filled to the spill point of the outer forearc highs, causing headward erosion of incipient canyons into the basin fill and allowing bypass of sediment to the Chile Trench. Age estimates that are constrained by subduction-related syn-depositional deformation of the upper 700-800m of the basin fill suggest that glacio-eustatic sea-level lowstands, in conjunction with accelerated denudation rates, within the past 350 ka may have contributed to the increase in simultaneously active point sources along the upper slope as well as an increased complexity of proximal depositional facies.

Basin Research↗
USGS2023

Bedrock erosion by debris flows at Chalk Cliffs, Colorado, USA: Implications for bedrock channel evolution

Debris flow erosion into bedrock helps to set the pace of mountain denudation, but there are few empirical observations of this process. We studied the effects of debris flows on bedrock erosion using Structure-From-Motion photogrammetry and multiple real-time monitoring measurements. We found that the distribution of bedrock erosion across the channel cross-section could be generalized as an exponentially decreasing function of height above the channel thalweg. Using this empirical function, we simulated the erosion at a cross-section after the theoretical passage of a migrating knickpoint effectively matching the upstream pre-knickpoint cross-sectional shape to the downstream post-knickpoint cross-sectional shape via debris-flow bedrock erosion.

Colorado↗
USGS2012

Coastal evolution and sediment budget at the mouth of the Columbia River, USA

The coastal morphology of the Mouth of the Columbia River (MCR) has changed dramatically over the past century. Since the construction of jetties in 1914 and 1917, the inlet deepened and stabilized, the outer ebb delta migrated northward and offshore several kilometers, and the adjacent shorelines to the north and south prograded several hundreds of meters. Recently, high rates of erosion along these adjacent shorelines affecting parklands have raised questions about the cause of the erosion. The purpose of the sediment budget analysis presented here is to address these questions by quantitatively examining the links between the morphology change of the shoreline and the ebb delta and historical changes in local and regional sediment supply.

Oregon, Washington↗
USGS2012

Modeling nearshore morphological evolution at seasonal scale

A process-based model is compared with field measurements to test and improve our ability to predict nearshore morphological change at seasonal time scales. The field experiment, along the dissipative beaches adjacent to Grays Harbor, Washington USA, successfully captured the transition between the high-energy erosive conditions of winter and the low-energy beach-building conditions typical of summer. The experiment documented shoreline progradation on the order of 20 m and as much as 175 m of onshore bar migration. Significant alongshore variability was observed in the morphological response of the sandbars over a 4 km reach of coast. A detailed sensitivity analysis suggests that the model results are more sensitive to adjusting the sediment transport associated with asymmetric oscillatory wave motions than to adjusting the transport due to mean currents. Initial results suggest that alongshore variations in the initial bathymetry are partially responsible for the observed alongshore variable morphological response during the experiment. Copyright ASCE 2006.

Conference Paper↗
USGS2012

Geologic framework of the long bay inner shelf: implications for coastal evolution in South Carolina

The inner continental shelf off northern South Carolina is a sediment-limited environment characterized by extensive hardground areas, where coastal plain strata and ancient channel-fill deposits are exposed at the sea floor. Holocene sand is concentrated in large shoals associated with active tidal inlets, an isolated shore-detached sand body, and a widespread series of low-relief sand ridges. The regional geologic framework is a strong control on the production, movement and deposition of sediment. High-resolution geologic mapping of the sea floor supports conceptual models indicative of net southwestward sediment transport along the coast.

South Carolina↗