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At least 577 records · Page 32Linked to original sources
USGS2023

Future marsh evolution due to tidal changes induced by human adaptation to sea level rise

With sea level rise threatening coastal development, decision-makers are beginning to act by modifying shorelines. Previous research has shown that hardening or softening shorelines may change the tidal range under future sea level rise. Tidal range can also be changed by natural factors. Coastal marshes, which humans increasingly depend on for shoreline protection, are ecologically sensitive to tidal range. Therefore, it is critical to examine how changes in tidal range could influence marsh processes. A marsh accretion model was used to investigate the ecological response of a San Francisco Bay, California, USA marsh to multiple tidal range scenarios and sea level rise from 2010 to 2100. The scenarios include a baseline scenario with no shoreline modifications in the estuary, a shoreline hardening scenario that amplifies the tidal range, and 14 tidal range scenarios as a sensitivity analysis that span tidal amplification and reduction of the baseline scenario. The modeling results expose key tradeoffs to consider when planning for sea level rise. Compared to the baseline, the hardening scenario shows minor differences. However, further tidal amplification prolongs marsh survival but decreases Sarcocornia pacifica cover, an important species for certain threatened wildlife and an effective attenuator of wave energy. Conversely, tidal reduction precipitates marsh drowning but shows gains in Sarcocornia pacifica cover. These mixed impacts of tidal amplification and reduction shown by the model indicate potential tradeoffs in relation to marsh survival, habitat characteristics, and shoreline protection. This study suggests the need for a cross-sectoral, regional approach to sea level rise adaptation.

California↗
USGS2024

Evolution of a lake margin recorded in the Sutton Island member of the Murray formation, Gale crater, Mars

This study uses data from the Mars Science Laboratory Curiosity rover to document the facies of the Sutton Island member of the Murray formation, interpret paleoenvironments, and establish key stratigraphic transitions at Gale crater. Two facies associations were identified: Facies Association 1 (FA1) and Facies Association 2 (FA2). Individual facies in FA1 include planar-laminated mudstone with minor intervals of planar sandstone, ripple cross-laminated sandstone, cross-stratified sandstone, and alternating laminated sandstone and mudstone. Meter-thick packages of planar-laminated mudstone in FA1 are interpreted to represent deposition in low-energy ponded environments along the lake margin. Straight- and curve-crested ripple cross-laminated facies are interpreted to represent current-influenced deposition. Cross-stratified sandstone facies consist of dm-thick sets that represent deposition in distal channels. Intercalated mm-scale mudstone and sandstone laminae represent waning flow conditions and possible channel abandonment. Facies in FA1 collectively represent deposition in a distal delta plain. FA2 is comprised of planar-laminated mudstone with minor sandstone and is interpreted to represent deposition in a lacustrine-basin setting by suspension settling linked to density flows. FA1 transitions upward into FA2, defining a rapid transgression substantial enough to facilitate the deposition of distal lake facies above delta plain facies. The abrupt transition from FA2 back to FA1 deltaic deposits is suggestive of forced regression. Facies in FA1 and FA2 are consistent with the prevalence of aqueous environments recorded in other Murray formation members and extend our understanding of the dynamic sedimentary processes that characterized ancient lacustrine systems at Gale crater.

Journal of Geophysical Research - Planets↗
USGS2024

Formation and evolution of the Pacific-North American (San Andreas) plate boundary: Constraints from the crustal architecture of northern California

The northward migration of the Mendocino triple junction (MTJ) drives a fundamental plate boundary transformation from convergence to translation; producing a series of strike-slip faults, that become the San Andreas plate boundary. We find that the 3-D structure of the Pacific plate lithosphere in the vicinity of the MTJ controls the location of San Andreas plate boundary formation. At the time of initiation of the Pacific-North America plate boundary (∼30 Ma), the sequential interaction with the western margin of North America of the Pioneer Fracture Zone, soon followed by the Mendocino Fracture Zone, led to the capture of a small segment of partially subducted Farallon lithosphere by the Pacific plate, termed the Pioneer Fragment (PF). Since that time, the PF has translated with the Pacific Plate along the western margin of North America. Recently developed, high-resolution seismic-tomographic imagery of northern California indicates that (a) the PF is extant, occupying the western half of the slab window, immediately south of the MTJ; (b) the eastern edge of the PF lies beneath the newly forming Maacama fault system, which develops to become the locus for the primary plate boundary structure after approximately 6–10 Ma; and (c) the location of the translating PF adjacent to the asthenosphere of the slab window generates a shear zone within and below the crust that develops into the plate boundary faults. As a result, the San Andreas plate boundary forms interior to the western margin of North America, rather than at its western edge.

California↗
USGS2024

Zircon constraints on the eruptive sequence and magma evolution of rhyolites at South Sister volcano, Oregon

We present 230 Th- 238 U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated 40 Ar/ 39 Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these 40 Ar/ 39 Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down-temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano.

Oregon↗
USGS2026

Structural evolution and slip rate variations through time of the Puente-Hills blind-thrust fault beneath Los Angeles: Implications for seismic hazard and folding kinematics

Using seismic reflection profiles, historical well logging data, and luminescence and radiocarbon ages, we determine a Pleistocene-Holocene slip history for the central, Santa Fe Springs segment of the Puente Hills blind-thrust fault (PHT), a major seismogenic fault situated beneath the urbanized Los Angeles metropolitan region. We analyze the geometry of correlative stratigraphic units in the forelimb and backlimb of the overlying growth-fold, the Santa Fe Springs anticline, and determine the uplift of seven age-correlative markers. Uplift measurements are converted to thrust displacements on the underlying PHT using a structural method laid out by Don et al. (2022, https://doi.org/10.1785/0120220048 ) that accounts for the geometry of the fault. These data indicate that deep thrust displacement on the PHT is partially consumed updip in the creation of a hanging-wall fault-bend fold, with forelimb growth strata recording <80% of the slip documented within the backlimb. Chronological data from growth strata yield age constraints for folding and faulting on the underlying PHT, providing a detailed incremental slip history derived from both the forelimb and backlimb folding for seven discrete growth horizons, spanning the past 1.4 million years. The resulting six incremental slip rates demonstrate that fault slip has varied through time from the middle Pleistocene to the Holocene. Moreover, these results reveal synchronous acceleration of both the central, Santa Fe Springs and western, Los Angeles segments of the fault system since late Pleistocene time (after 200 ka) and slip rates of greater than 2 mm/yr on the downdip, backlimb, fault-ramp below the anticline.

California↗
USGS2026

Impacts of transient critical zone evolution on landslide susceptibility and hazard Across the Utuado batholith, Puerto Rico

Landscapes in humid climates often experience rapid chemical and mechanical weathering that can change groundwater fluxes, drainage pathways, and frictional strength via alterations in subsurface mineral composition and matrix porosity. These changes to hillslope critical zone structure and strength should control landslide susceptibility, size, and failure style. Here we test the impact of differential weathering on landslide failure processes by exploiting a natural experiment along the dioritic Utuado batholith in Puerto Rico's Cordillera Central, where a sharp weathering gradient follows an upstream-propagating knickzone into an uplifted shoreline platform that was affected by Hurricane Maria in 2017. Through a combination of landslide mapping, field geophysical and geotechnical surveys, topographic analysis, calibrated stream profile inversion, 3D back-analysis of hillslope strength, and debris-flow inundation assessments, we show that differential weathering induces changes to critical zone structure and consequently landslide susceptibility and hazard. Our mapping reveals that the hurricane-induced landslides preferentially cluster along less weathered hillslopes despite a marked increase in frictional strength. Jointly inverted seismic refraction and electrical resistivity profiles show that the frictionally weak, heavily weathered hillslopes are highly porous and dry, which we posit acts to stabilize slopes through limiting deeper hydrologic connectivity that can produce shallow water tables with destabilizing lateral seepage forces during storms. Debris-flow inundation mapping and analysis reveals that major contributors to debris-flow growth, channel steepness and landslide coalescence, scale linearly with hillslope friction angle. This leads to higher-mobility debris flows along knickzone reaches, which disproportionately affects communities at the base of the plateau.

Puerto Rico↗
USGS2012

Evolution of the caldera‐forming eruption at Crater Lake, Oregon, indicated by component analysis of lithic fragments

Crater Lake caldera (8 × 10 km), formed 6845 years B. P. ( 14 C age) during the climactic eruption of the volcanic edifice known as Mount Mazama, is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems. Our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of Mount Mazama. Initial activity was from a single vent that produced a rhyodacite pumice fall from a Plinian column. Altered preexisting volcanic rocks are the predominant lithic type in the Plinian deposit, and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin. The Wineglass Welded Tuff lies atop the Plinian deposit and contains a higher proportion of fresh volcanic rocks, suggesting enlargement of the single vent by slumping of its walls. This same vent enlargement caused the Plinian eruption column to collapse and feed valley‐hugging pyroclastic flows that deposited the Wineglass Welded Tuff. When enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported, Mount Mazama collapsed to form the caldera, while highly energetic pyroclastic flows produced the climactic ignimbrite. A lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the Wineglass Welded Tuff and contains predominantly altered volcanic rocks of deeper origin, accompanied by minor granitoids from the magma chamber walls. Azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls, indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system. Systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption. Our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence, such as Crater Lake, nevertheless may form by syneruptive subsidence along ring fractures.

Oregon↗
USGS2010

Chemical evolution of groundwater near a sinkhole lake, northern Florida: 1. Flow patterns, age of groundwater, and influence of lakewater leakage

Leakage from sinkhole lakes significantly influences recharge to the Upper Floridan aquifer in poorly confined sediments in northern Florida. Environmental isotopes (oxygen 18, deuterium, and tritium), chlorofluorocarbons (CFCs: CFC-11, CCl 3 F; CFC-12, CCl 2 F 2 ; and CFC-113, C 2 Cl 3 F 3 ), and solute tracers were used to investigate groundwater flow patterns near Lake Barco, a seepage lake in a mantled karst setting in northern Florida. Stable isotope data indicated that the groundwater downgradient from the lake contained 11–67% lake water leakage, with a limit of detection of lake water in groundwater of 4.3%. The mixing fractions of lake water leakage, which passed through organic-rich sediments in the lake bottom, were directly proportional to the observed methane concentrations and increased with depth in the groundwater flow system. In aerobic groundwater upgradient from Lake Barco, CFC-modeled recharge dates ranged from 1987 near the water table to the mid 1970s for water collected at a depth of 30 m below the water table. CFC-modeled recharge dates (based on CFC-12) for anaerobic groundwater downgradient from the lake ranged from the late 1950s to the mid 1970s and were consistent with tritium data. CFC-modeled recharge dates based on CFC-11 indicated preferential microbial degradation in anoxic waters. Vertical hydraulic conductivities, calculated using CFC-12 modeled recharge dates and Darcy's law, were 0.17, 0.033, and 0.019 m/d for the surficial aquifer, intermediate confining unit, and lake sediments, respectively. These conductivities agreed closely with those used in the calibration of a three-dimensional groundwater flow model for transient and steady state flow conditions.

Florida↗
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↗