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At least 667 records · Page 37Linked to original sources
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

Turbidite systems: State of the art and future directions

The study of turbidite systems covering a wide range of physical scales has led to confus ion regarding the use of certain key terms and hence a breakdown in communication between workers involved in turbidite research. There are three fundamentally different scales and types of observations derived from the study of outcrop data (ancient systems), high-resolution seismic reflection and side scan sonar data (modern systems), and multichannel seismic reflection data (modern and older buried systems). Despite the variability of scale the same terms are used to describe features that may have little in common. Consequently, turbidite system terminology has become imprecise and even misleading in some cases, thus providing impediments to developing useful predictive models for processes, depositional environments, and lateral and vertical distribution of sand bodies within turbidite systems. To address this concern, we review the principal elements critical to deepwater systems: slump scars, submarine canyons, channels, channel fill deposits, overbank deposits, and lobes and discuss some of their recognition criteria with each different type of data base. Local and regional tectonic setting, relative sea level variations, and bottom current activity are probably the main factors that control size, external geometry, internal stratal configuration, and facies characteristics of both modern and ancient turbidite systems. These factors ultimately control the timing and bounding characteristics between stages of growth of deepwater systems. If comparison of elements from different turbidite deposits using various data types is carried out at similar physical and temporal scales, predictive models eventually may be improved.

Reviews of Geophysics↗
USGS2012

Seismic velocity structure and composition of the continental crust: A global view

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle ( Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m 3 , and the average SiO 2 content is 61.8%.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Mapping playa evaporite minerals and associated sediments in Death Valley, California, with multispectral thermal infrared images

Efflorescent salt crusts and associated sediments in Death Valley, California, were studied with remote-sensing data acquired by the NASA thermal infrared multispectral scanner (TIMS). Nine spectral classes that represent a variety of surface materials were distinguished, including several classes that reflect important aspects of the playa groundwater chemistry and hydrology. Evaporite crusts containing abundant thenardite (sodium sulfate) were mapped along the northern and eastern margins of the Cottonball Basin, areas where the inflow waters are rich in sodium. Gypsum (calcium sulfate) crusts were more common in the Badwater Basin, particularly near springs associated with calcic groundwaters along the western basin margin. Evaporite-rich crusts generally marked areas where groundwater is periodically near the surface and thus able to replenish the crusts though capillary evaporation. Detrital silicate minerals were prevalent in other parts of the salt pan where shallow groundwater does not affect the surface composition. The surface features in Death Valley change in response to climatic variations on several different timescales. For example, salt crusts on low-lying mudflats form and redissolve during seasonal-to-interannual cycles of wetting and desiccation. In contrast, recent flooding and erosion of rough-salt surfaces in Death Valley probably reflect increased regional precipitation spanning several decades. Remote-sensing observations of playas can provide a means for monitoring changes in evaporite facies and for better understanding the associated climatic processes. At present, such studies are limited by the availability of suitable airborne scanner data. However, with the launch of the Earth Observing System (EOS) AM-1 Platform in 1998, multispectral visible/near-infrared and thermal infrared remote-sensing data will become globally available.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Perú form the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belén Schist in northern Chile generally have present-day low 206 Pb/ 204 Pb (<18.0), elevated 207 Pb/ 204 Pb (>15.57), and elevated 208 Pb/ 204 Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher present-day 206 Pb/ 204 Pb values than those of the Early Proterozoic rocks of the Arequipa massif ( 206 Pb/ 204 Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton ( 206 Pb/ 204 Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

central Andes↗
USGS1997

Stratigraphic and structural implications of conodont and detrital zircon U-Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska

New paleontologic and isotopic data from the Emma Creek and Marion Creek schists of the Coldfoot terrane, Arctic Alaska superterrane, central Brooks Range, suggest Devonian and possibly younger ages of deposition for their sedimentary protoliths. Conodonts from marble of the Emma Creek schist, intruded by a roughly 392 Ma orthogneiss, are late Lochkovian (early Early Devonian, between about 408 and 396 Ma) and Silurian to Devonian at two other locations. Spherical to oblong detrital zircons from quartz-mica schist of the overlying Marion Creek schist yield mostly discordant U—Pb data suggestive of provenance ages of 3.0, 2.0–1.8, and 1.5–1.4 Ga; however, several euhedral grains of zircon from Marion Creek quartz-mica schist have concordant U—Pb ages from 370 to 360 Ma. The Marion Creek schist in our study area therefore is at least 26 m.y. younger than the Emma Creek schist. The age data imply that the protolith of the Emma Creek schist is age correlative with Devonian carbonate rocks in the Hammond and North Slope terranes, whereas the Marion Creek schist is age correlative with Upper Devonian and Lower Mississippian clastic sedimentary rocks of the Endicott Group in the Endicott Mountains terrane and shale and carbonate units in the De Long Mountains and Sheenjek River terranes. Consequently, tectonic models restoring the entire Coldfoot terrane beneath partly or wholly coeval rocks of the Hammond, Endicott Mountains, De Long Mountains, and Sheenjek River terranes of the Arctic Alaska superterrane require revision. Alternative reconstructions, including restoration of the Coldfoot terrane inboard of the Endicott Mountains terrane or outboard of the De Long Mountains and Sheenjek River terranes are plausible but require either larger amounts of shortening than previously suggested or indicate problematic facies relations.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Petrogenesis of garnet two-mica granites in the Ruby Mountains, Nevada

Garnet-two-mica granites of Jurassic (160 ± 3 m.y.) and Cretaceous (83 ± 1.3 m.y.) ages intrude amphibolite facies metasedimentary rocks of Precambrian age and lower Paleozoic sedimentary rocks in the northern part of the Ruby Mountains, Nevada. High initial 87 Sr/ 86 Sr and high values of δ 18 O for minerals suggest a dominantly sedimentary source for the magmas. Total rare-earth contents are low and less fractionated (Ce N /Yb N = 10) than granitic rocks of similar composition in the Sierra Nevada and show a negative Eu anomaly (Eu/Eu* = 0.57). These data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon, sphene, and apatite. Minor euhedral garnets are manganese-rich and magnesium-poor (al 60 sp 34 py 3.6 gr 2.8 ) compared to garnet (al 79 sp 6 py 13 gr 2 ) in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns. Fractionation of magnesium-iron between garnet and biotite, and of sodium-potassium between K-feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures, in the range of 365° to 505°C, whereas oxygen isotope equilibration temperatures for two granites are higher at 480° and 570°C. The compositions of the granites plot near minimum melting compositions in the water-saturated quartz-albite-orthoclase system. Comparison of muscovite plus quartz stability, the water-saturated granite solidus, and experimental garnet-melt equilibria suggests pressures of crystallization no lower than about 3.5 kbar. This pressure is compatible with that estimated from garnet-plagioclase-sillimanite-quartz equilibria for the intruded metasedimentary rocks.

Nevada↗
USGS2012

A seismic refraction survey of the Imperial Valley Region, California

The U.S. Geological Survey conducted an extensive seismic refraction survey in the Imperial Valley region of California in 1979. The Imperial Valley is located in the Salton Trough, an active rift between the Pacific and North American plates. Forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0.5–1 km. More than 1300 recording locations were occupied, and more than 3000 usable seismograms were obtained. We analyzed five profiles using a standard ray‐tracing program, constructed a contour map of reduced travel times from our most widely recorded shot point, and modeled an existing gravity profile across the Salton Trough. Results are itemized: (1) All models have in common a sedimentary layer ( V p = 1.8–5.0 km/s), a “transition zone” ( V p = 5.0–5.65 km/s), a basement ( V p = 5.65 km/s in the Imperial Valley, 5.9 km/s on the bordering mesas), and subbasement ( V p = 7.2 km/s). (2) The sedimentary layer ranges in thickness along the axis of the Salton Trough from 3.7 km (Salton Sea) to 4.8 km (U.S.‐Mexican border). On the bordering mesas it is quite variable in thickness. (3) The “transition” zone is about 1 km thick in most places. In the Imperial Valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement. On the bordering mesas, however, there is a discontinuity at the top of this zone. (4) There are apparently two types of basement. On the bordering mesas, basement is crystalline igneous and metamorphic rocks. In the Imperial Valley, basement is mostly lower‐greenshist‐facies sedimentary rocks, based primarily on the smooth transition in character from sediment to basement arrivals, the low value of basement velocity, and the fact that deep (4 km) wells in the valley penetrate only the upper part of the known Cenozoic stratigraphic column for the Salton Trough. (5) The subbasement, or intermediate crustal layer, ranges in depth along the axis of the Salton Trough from 16 km (Salton Sea) to 10 km (U.S.‐Mexican border). Gravity modeling requires that this layer deepen and/or pinch out beneath the bordering mesas and mountain ranges. Based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the Imperial Valley, the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust. (6) Several structures are seen that affect basement, transition zone, and deeper parts of the sedimentary layer. They include a scarp along the Imperial fault, as much as 1 km down to the northeast, and a scarp passing roughly along the topographic boundary between the Imperial Valley and the bordering mesa to the west, as much as 3½ km down to the east. We interpret the latter scarp to be the suture, or rift boundary, between the older crystalline basement on the mesa and the younger metasedimentary basement in the Imperial Valley. (7) On a contour map of reduced travel time from our most widely recorded shot point, subtle patches of early arrivals among otherwise late arrivals in the central Imperial Valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150°C. Apparently the Salton Trough is a location where new crust is being generated. As the rift opens, mafic intrusive rocks fill it from below as sedimentary rocks fill it from above. Rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth (metasedimentary basement in the Imperial Valley) and thus consolidates the new crust.

California↗
USGS2012

Mineralization, alteration, and hydrothermal metamorphism of the ophiolite-hosted Turner-Albright sulfide deposit, southwestern Oregon

The Turner-Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents, similar to sulfide-depositing hot springs on modern spreading centers. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyalo-clastite altered the rock to quartz + Fe-chlorite and quartz + sericite + Fe-chlorite. Intense alteration resulted in replacement of the protolith by quartz + sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low p H and Mg and high Fe. Sulfide δ 534 S values average 4.7‰ and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide δ 34 S and the difference between sulfide and contemporaneous seawater sulfate δ 34 S values are similar to ophiolite-hosted sulfide deposits in Cyprus. Chlorite-rich hydrothermally altered basalt has δ 18 O values of 3.0–6.8‰ due to high-temperature (250°–350°C) reaction with a seawater-derived ore fluid. Quartz in silicified basalt has higher δ 18 O values of 12.3–15.5‰, probably reflecting lower-temperature recrystallization of amorphous silica formed during initial alteration of basalt. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore-transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite-pumpellyite facies metamorphism. Exchange with altered seawater increased the whole-rock δ 18 O of the basalts to values of 9.4–11.2‰.

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

Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)-enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the “within-plate basalt” fields of trace element discriminant diagrams; none have trace-element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

Journal of Geophysical Research Solid Earth↗
USGS2012

Chronologic and isotopic framework for early Proterozoic crustal evolution in the eastern Mojave Desert region, SE California

The Early Proterozoic geologic evolution of the eastern Mojave Desert region, as defined by characteristics of its supracrustal rocks, granitoids, metamorphism, structural history, and Pb and Nd isotopic signature, contrasts sharply with other Proterozoic provinces of the southwestern United States. The oldest supracrustal rocks of the Mojave Desert region contain zircons over 2.0 Ga, corroborating Nd isotopic evidence for a much older crust here than elsewhere in the southwestern United States. Granitoids widely emplaced within these supracrustal rocks range from 1.76 to 1.64 Ga. The earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 Ma in a migmatite-producing event that we term (informally) the Ivanpah orogeny. Subsequent granitoids, emplaced from 1.69 to 1.67 Ga, were voluminous along a north trending belt in the middle of the Mojave province. Younger plutons were emplaced at about 1.66 Ga in several places and at about 1.64 Ga along the extreme southern part of the province. Commonalities between the Proterozoic evolutions of the Mojave and Arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed; the data only suggest that the juxtaposition occurred between about 1.76 and 1.64 Ga.

Journal of Geophysical Research↗
USGS2010

Significance of Klamath rocks between the Franciscan Complex and Coast Range ophiolite, northern California

Small fault‐bounded slabs of low‐grade (prehnite‐pumpellyite‐bearing) slate, metagraywacke, and greenstone occur between the Coast Range ophiolite and South Fork Mountain Schist for at least 60 km south of the Klamath Mountains, northern California. The metagraywacke slabs differ from typical Franciscan Complex metagraywacke to the west by the absence of blueschist‐facies minerals and the abundance of quartz and chert clasts, and they differ from sandstone and mudstone of the Great Valley sequence to the east by the presence of a penetrative cleavage and low‐grade metamorphism. The metasedimentary rock in the slabs is petrographically identical to the Jurassic Galice Formation, which occurs along the western and southern margins of the Klamath Mountains. Upward drag along a west dipping reverse fault best explains the present structural position of the slabs and is consistent with available geological and geophysical data. These data imply that the Coast Range ophiolite structurally overlies Sierran‐Klamath basement at least locally.

Oregon, Washington↗
USGS2010

Attenuation of the Coast Range ophiolite by extensional faulting and nature of the Coast Range "thrust," California

The late Mesozoic Coast Range ophiolite and Great Valley sequence in California were juxtaposed against the Franciscan Complex during a long tectonic history that included imbricate thrust faulting, low‐angle detachment, and high‐angle reverse faulting. Many low‐angle faults previously mapped as thrusts invariably juxtapose younger over older rocks, suggesting a normal sense of offset. We infer that serpentinite melange that is present structurally beneath the Coast Range ophiolite formed above the subduction zone during convergence and was subsequently faulted and further attenuated with upper plate rocks concurrent with extension. Franciscan blueschist‐facies rock is inferred to have been transported from depth to higher structural levels concurrent with underplating and extensional unroofing in the upper plate. The present juxta‐position of the Coast Range ophiolite and Great Valley sequence with Franciscan rocks is commonly controlled by Neogene high‐angle faults. We propose that the term Coast Range thrust is no longer appropriate and that the name should be changed to Coast Range fault.

California↗
USGS2014

Igneous activity in the Comstock District, Nevada

The oldest igneous rocks in the Comstock District are amphibolites probably derived from basalts and of Triassic age. These are intruded by pre‐Tertiary quartz monzonlte and by granodiorite of Sierran facies, the latter not being exposed on the surface but found on mine‐dumps. Igneous activity recorded mainly in volcanic rocks was almost continuous throughout the Tertiary. Its products, in order of age, were as follows: Eocene—Rhyolitic flows aggregating a few hundred feet in thickness: One intrusive plug of rhyolite occurs. Eocene or Miocene—Hornblende‐auglte andesite intrusive into the rhyolite, type locality American Ravine. ©1938. American Geophysical Union. All Rights Reserved.

Nevada↗
USGS2014

Petrology and structure of the Moa Chromite district, Oriente Province, Cuba

Chromite deposits In the Moa district, on the north coast of Oriente Province, Cuba, form one facies of the ultramafic complex in which they occur. Two planar structures, one a compositional layering due to variations in the relative proportions of olivine, pyroxene, plagioclase feldspar, and chromite, and the other a foliation caused by the orientation of pyroxene crystals in peridotite, are in general parallel, but diverge in a few places. Dips are fairly constant, being moderate to the northwest over most of the area, but sharp reversals occur immediately adjacent to irregularities in the chromite deposits. Two prominent joint sets are present: (1) Cross joints striking parallel to the strike of the foliation and dipping approximately at right angles to it; and (2) longitudinal joints striking parallel to the direction of dip of the foliation and nearly vertical. Gabbro dikes follow the first set, chrysotlle veins the second. Within the ore bodies dikes are commonly coarser‐grained, larger, and far more abundant than in peridotite; they enclose coarse, angular fragments of chromite. ©1947. American Geophysical Union. All Rights Reserved.

Eos, Transactions, American Geophysical Union↗
USGS1988

Granulite fades Nd-isotopic homogenization in the Lewisian complex of northwest Scotland

A published Sm–Nd whole-rock isochron of 2,920 ± 50 Myr, obtained from a wide range of lithologies in the Lewisian complex of north-west Scotland, was interpreted 1 as the time of protolith formation. This date is ∼ 260 Myr older than estimates for the timing of high-grade metamorphism in the complex at ∼ 2,660 Myr 2'3 , and this period is considered to represent the duration of the Lewisian crustal accretion-differentiation superevent (CADS) 4 . Here we give new Sm-Nd data, obtained specifically from granulite facies tonalitic gneisses, that yield a date of 2,600 ±155 Myr. Although depleted-mantle model ages ( t DM suggest >200 Myr of premetamorphic crustal residence, the regression date and its associated initial Nd-isotopic parameters demonstrate Nd-isotopic homogenization during the high-grade event, as well as the probability of general rare-earth-element (REE) mobility. Models for selective element depletion in the complex have previously assumed REE immobility since 2,920 Myr, but the data presented here suggest that a reappraisal of the depletion mechanism is required.

Nature↗
USGS1988

Geochemical and climatic effects of increased marine organic carbon burial at the Cenomanian/Turonian boundary

Perhaps the most significant event in the Cretaceous record of the carbon isotope composition of carbonate 1,2 , other than the 1–2.5 ‰ negative shift in the carbon isotope composition of calcareous plankton at the Cretaceous/Tertiary boundary 3 , is the rapid global positive excursion of ∼ 2 ‰ ( 13 C enrichment) which took place between ∼ 91.5 Myr and 90.3 Myr (late Cenomanian to earliest Turonian (C/T boundary event)) 1,4,5 . This excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon (TDC) reservoir resulting from an increase in rate of burial of 13 C-depleted organic carbon, which coincided with a major global rise in sea level 5 during the so-called C/T oceanic anoxic event (OAE) 6 . Here we present new data, from nine localities, which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the C/T boundary 7,8 is nearly synchronous with that for carbonate and is widespread throughout the Tethys and Atlantic basins (Fig. 1), as well as in more high-latitude epicontinental seas. The postulated increase in the rate of burial of organic carbon may have had a significant effect on CO 2 and O 2 concentrations in the oceans and atmosphere, and consequent effects on global climate and sedimentary facies.

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