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At least 361 records · Page 20Linked to original sources

USGS2010

Stratigraphy and Mesozoic–Cenozoic tectonic history of northern Sierra Los Ajos and adjacent areas, Sonora, Mexico

Geologic mapping in the northern Sierra Los Ajos reveals new stratigraphic and structural data relevant to deciphering the Mesozoic–Cenozoic tectonic evolution of the range. The northern Sierra Los Ajos is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata, equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic–Paleozoic sequence is mantled by Upper Cretaceous rocks partly equivalent to the Fort Crittenden and Salero Formations in Arizona, and the Cabullona Group in Sonora, Mexico. Absence of the Upper Jurassic–Lower Cretaceous Bisbee Group below the Upper Cretaceous rocks and above the Proterozoic–Paleozoic rocks indicates that the Sierra Los Ajos was part of the Cananea high, a topographic highland during the Late Jurassic and Early Cretaceous. Deposition of Upper Cretaceous rocks directly on Paleozoic and Proterozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Laramide Cabullona basin during Late Cretaceous time. Basal beds of the Upper Cretaceous sequence are clast-supported conglomerate composed locally of basement (Paleozoic) clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Cabullona basin. The present-day Sierra Los Ajos reaches elevations of greater than 2600 m, and was uplifted during Tertiary basin-and-range extension. Upper Cretaceous rocks are exposed at higher elevations in the northern Sierra Los Ajos and represent an uplifted part of the inverted Cabullona basin. Tertiary uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north-to-northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the headwaters of the San Pedro River basin, an important bi-national water resource in the US-Mexico border region.

Sierra Los Ajos↗

USGS2012

Frictional properties of saponite-rich gouge from a serpentinite-bearing fault zone along the Gokasho-Arashima Tectonic Line, central Japan

We studied a serpentinite-bearing fault zone in Gokasho-Arashima Tectonic Line, Mie Prefecture, central Japan, characterizing its internal structures, mineral assemblage, permeability, and frictional properties. The fault core situated between the serpentinite breccia and the adjacent sedimentary rocks is characterized by a zone locally altered to saponite. The clayey gouge layer separates fault rocks of serpentinite origin containing talc and tremolite from fault rocks of sedimentary origin containing chlorite but no quartz. The minerals that formed within the fault are the products of metasomatic reaction between the serpentinite and the siliceous rocks. Permeability measurements show that serpentinite breccia and fault gouge have permeability of 10 −14 –10 −17 m 2 and 10 −15 –10 −18 m 2 , respectively, at 5–120 MPa confining pressure. Frictional coefficient of the saponite-rich clayey fault gouge ranged between 0.20 and 0.35 under room-dry condition, but was reduced to 0.06–0.12 when saturated with water. The velocity dependence of friction was strongly positive, mostly ranging between 0.005 and 0.006 in terms of a–b values. The governing friction law is not constrained yet, but we find that the saponite-rich gouge possesses an evolutional behavior in the opposite direction to that suggested by the rate and state friction law, in addition to its direct velocity dependence.

Journal of Structural Geology↗

USGS2017

Causes of distal volcano-tectonic seismicity inferred from hydrothermal modeling

Distal volcano-tectonic (dVT) seismicity typically precedes eruption at long-dormant volcanoes by days to years. Precursory dVT seismicity may reflect magma-induced fluid-pressure pulses that intersect critically stressed faults. We explored this hypothesis using an open-source magmatic-hydrothermal code that simulates multiphase fluid and heat transport over the temperature range 0 to 1200 °C. We calculated fluid-pressure changes caused by a small (0.04 km 3 ) intrusion and explored the effects of flow geometry (channelized vs. radial flow), magma devolatilization rates (0–15 kg/s), and intrusion depths (5 and 7.5 km, above and below the brittle-ductile transition). Magma and host-rock permeabilities were key controlling parameters and we tested a wide range of permeability ( k ) and permeability anisotropies ( k h / k v ), including k constant, k ( z ), k ( T ), and k ( z , T , P ) distributions, examining a total of ~ 1600 realizations to explore the relevant parameter space. Propagation of potentially causal pressure changes (Δ P ≥ 0.1 bars) to the mean dVT location (6 km lateral distance, 6 km depth) was favored by channelized fluid flow, high devolatilization rates, and permeabilities similar to those found in geothermal reservoirs ( k ~ 10 − 16 to 10 − 13 m 2 ). For channelized flow, magma-induced thermal pressurization alone can generate cases of ∆ P ≥ 0.1 bars for all permeabilities in the range 10 − 16 to 10 − 13 m 2 , whereas in radial flow regimes thermal pressurization causes ∆ P < 0.1 bars for all permeabilities. Changes in distal fluid pressure occurred before proximal pressure changes given modest anisotropies ( k h / k v ~ 10–100). Invoking k ( z , T , P ) and high, sustained devolatilization rates caused large dynamic fluctuations in k and P in the near-magma environment but had little effect on pressure changes at the distal dVT location. Intrusion below the brittle-ductile transition damps but does not prevent pressure transmission to the dVT site.

Journal of Volcanology and Geothermal Research↗

USGS2023

Precaldera mafic magmatism at Long Valley, California: Magma-tectonic siting and incubation of the Great Rhyolite System

The iconic volcanic center at Long Valley has released ∼820 km 3 of rhyolite in at least 110 eruptions. From 2.2 Ma until 0.23 Ma, products were exclusively rhyolitic, and ∼ 700 km 3 were high-silica rhyolite severely depleted in Sr, Ba, and Eu. The rhyolitic interval was preceded by an interval from 3.9 to 2.6 Ma with numerous basalt-andesite-dacite eruptions accompanied by no rhyolite at all. We have now mapped the circumcaldera products of this interval, defined 107 eruptive units, characterized them all chemically and petrographically, and dated many by 40 Ar/ 39 Ar. Here we display and describe them by sector around the caldera, interpret the nature of the transcrustal magma system that eventuated in the 35-km-wide Long Valley granite-rhyolite pluton, and analyze regional tectonic factors that did or did not contribute to siting the system. Nine Miocene (12–6 Ma) eruptive units close to Long Valley were followed by a Pliocene flare-up that released >300 mafic eruptions in a SW–NE swath 170 km long, centered across the later site of Long Valley. The basalts and their fractionates are intraplate alkalic products dominated by a continental lithosphere that had long been fluxed by Mesozoic subduction. Tertiary arc volcanism had not impinged on the area of Long Valley. Volumes estimated for the 107 Neogene precaldera eruptive units (only 40 of which exceeded 0.1 km 3 ) total ∼ 27 km 3 ± 50%—only ∼3% of the subsequent volume of rhyolite erupted. Such a volume of high-silica rhyolite with ultra-low Sr and Eu is not a product of partial melting but requires as proximate parent a leucogranitic crystal mush that is itself the upper level of a long-lived plutonic reservoir that extends to the lower crust. The 27 km 3 of Neogene magma that erupted was a small contingent of the mantle-derived basaltic flux needed to energize (and contribute its fractionated melt to) a 30-km-deep compositionally graded crustal column, which culminated in ∼10,000 km 3 of granitoid mush from which 820 km 3 of Quaternary high-silica rhyolitic melt escaped and erupted. Pliocene basaltic eruptions ceased at ∼2.6 Ma, probably because the basaltic flux intensified sufficiently to render the mushy upper crust impenetrable. The 2.6–2.2 Ma quiescent interval represented culmination of thermal activation of the plutonic column and refinement of its leucogranitic mushy upper layer, from which extreme melts escaped for the next 2 Myr. The Pliocene mafic swath crosses the Sierran rangefront fault zone coincident with a left-stepping extensional reentrant that also began developing at ∼3 Ma. Moreover, Long Valley overlies a dextral offset in the initial Sr-isotope 0.706 line, which may reflect the rifted or attenuated edge of Proterozoic crust and mantle lithosphere. Concatenation of these three influences may account for siting of intensified edge-focused magmatism that produced the great Quaternary pluton.

California↗

USGS2006

Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (<5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of &sim;50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates <0.1 m/kyr (uplift) or <0.4 m/kyr (subsidence) over this 30 kyr period.

Hawai'i↗

USGS2020

Holocene relative sea-level change along the tectonically active Chilean coast

We present a comprehensive relative sea-level (RSL) database for north, central, and south-central Chile (18.5°S – 43.6°S) using a consistent, systematic, and internationally comparable approach. Despite its latitudinal extent, this coastline has received little rigorous or systematic attention and details of its RSL history remain largely unexplored. To address this knowledge gap, we re-evaluate the geological context and age of previously published sea-level indicators, providing 78 index points and 84 marine or terrestrial limiting points spanning from 11 ka to the present day. Many data points were originally collected for research in other fields and have not previously been examined for the information they provide on sea-level change. Additionally, we describe new sea-level data from four sites located between the Gulf of Arauco and Valdivia. By compiling RSL histories for 11 different regions, we summarise current knowledge of Chilean RSL. These histories indicate mid Holocene sea levels above present in all regions, but at highly contrasting elevations from ∼30 m to <5 m. We compare the spatiotemporal distribution of sea-level data points with a suite of glacial isostatic adjustment models and place first-order constraints on the influence of tectonic processes over 10 3 –10 4 year timescales. While seven regions indicate uplift rates <1 m ka −1 , the remaining regions may experience substantially higher rates. In addition to enabling discussion of the factors driving sea-level change, our compilation provides a resource to assist attempts to understand the distribution of archaeological, palaeoclimatic, and palaeoseismic evidence in the coastal zone and highlights directions for future sea-level research in Chile.

Quaternary Science Reviews↗

USGS2014

The Wallula fault and tectonic framework of south-central Washington, as interpreted from magnetic and gravity anomalies

The Yakima fold and thrust belt (YFTB) in central Washington has accommodated regional, mostly north-directed, deformation of the Cascadia backarc since prior to emplacement of Miocene flood basalt of the Columbia River Basalt Group (CRBG). The YFTB consists of two structural domains. Northern folds of the YFTB strike eastward and terminate at the western margin of a 20-mGal negative gravity anomaly, the Pasco gravity low, straddling the North American continental margin. Southern folds of the YFTB strike southeastward, form part of the Olympic–Wallowa lineament (OWL), and pass south of the Pasco gravity low as the Wallula fault zone. An upper crustal model based on gravity and magnetic anomalies suggests that the Pasco gravity low is caused in part by an 8-km-deep Tertiary basin, the Pasco sub-basin, abutting the continental margin and concealed beneath CRBG. The Pasco sub-basin is crossed by north-northwest-striking magnetic anomalies caused by dikes of the 8.5 Ma Ice Harbor Member of the CRBG. At their northern end, dikes connect with the eastern terminus of the Saddle Mountains thrust of the YFTB. At their southern end, dikes are disrupted by the Wallula fault zone. The episode of NE–SW extension that promoted Ice Harbor dike injection apparently involved strike-slip displacement on the Saddle Mountains and Wallula faults. The amount of lateral shear on the OWL impacts the level of seismic hazard in the Cascadia region. Ice Harbor dikes, as mapped with aeromagnetic data, are dextrally offset by the Wallula fault zone a total of 6.9 km. Assuming that dike offsets are tectonic in origin, the Wallula fault zone has experienced an average dextral shear of 0.8 mm/y since dike emplacement 8.5 Ma, consistent with right-lateral stream offsets observed at other locations along the OWL. Southeastward, the Wallula fault transfers strain to the north-striking Hite fault, the possible location of the M 5.7 Milton-Freewater earthquake in 1936.

Washington↗

USGS2002

Cordilleran-margin quartzites in Baja California – implications for tectonic transport

One of the current controversies in Cordilleran tectonics concerns the position of Baja California prior to ∼300 km of opening of the Gulf of California. Geologic arguments, together with paleomagnetic results from Lower Cretaceous volcanic rocks, suggest that the rocks of the Baja Peninsula formed and evolved along the coast of northwestern Mexico prior to opening of the Gulf. In contrast, paleomagnetic data from Cretaceous–early Tertiary plutonic rocks and clastic strata have been interpreted by some workers to suggest that Baja was located near southern Mexico at approximately 80 Ma. The presence of similar detrital zircon ages in lower Paleozoic quartzites of northeast Baja and in lower Paleozoic strata east of the Gulf, in northwestern Mexico and southwestern US, provides strong support for the northern paleoposition, suggesting that Baja has been transported northward by only ∼300 km

Earth and Planetary Science Letters↗

USGS1997

Tertiary and Quaternary tectonic faulting in southernmost Illinois

Tertiary and/or Quaternary tectonic faulting is documented in three areas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in Alexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alexander County. In the FAFC, faults that strike NE and NNE displace Mounds Gravel (late Miocene to early Pleistocene) and, locally, the Metropolis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. North-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments have not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral strike-slip. The CFZ coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement. Surface expression in Illinois is subtle, but mafic and ultramafic intrusions, hydrothermal alteration and small faults align with the Commerce geophysical lineament. Earthquake foci in Missouri and Illinois lie on or close to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current stress field (principal compressive stress axis E-W to ENE-WSW). Possibly, the stress field changed during Neogene time. Alternatively, high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement.

Illinois↗

USGS2003

Controls of tectonics and sediment source locations on along-strike variations in transgressive deposits on the northern California margin

We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (???80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest-southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0-10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them. ?? 2003 Elsevier Science B.V. All rights reserved.

Marine Geology↗

USGS1998

Collision-induced tectonism along the northwestern margin of the Indian subcontinent as recorded in the Upper Paleocene to Middle Eocene strata of central Pakistan (Kirthar and Sulaiman Ranges)

Outcrop data from the Upper Paleocene to Middle Eocene Ghazij Formation of central Pakistan provide information about the depositional environments, source areas, and paleogeographic and tectonic settings along the northwestern margin of the Indian subcontinent during the closing of the Tethys Ocean. In this region, in the lower part of the exposed stratigraphic sequence, are various marine carbonate-shelf deposits (Jurassic to Upper Paleocene). Overlying these strata is the Ghazij, which consists of marine mudstone (lower part), paralic sandstone and mudstone (middle part), and terrestrial mudstone and conglomerate (upper part). Petrographic examination of sandstone samples from the middle and upper parts reveals that rock fragments of the underlying carbonate-shelf deposits are dominant; also present are volcanic rock fragments and chromite grains. Paleocurrent measurements from the middle and upper parts suggest that source areas were located northwest of the study area. We postulate that the source areas were uplifted by the collision of the subcontinent with a landmass during the final stages of the closing of the Tethys Ocean. Middle Eocene carbonate-shelf deposits that overlie the Ghazij record a return to marine conditions prior to the Miocene to Pleistocene sediment influx denoting the main collision with Eurasia.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2003

Tectonics of the Qinling (Central China): Tectonostratigraphy, geochronology, and deformation history

The Qinling orogen preserves a record of late mid-Proterozoic to Cenozoic tectonism in central China. High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) assembled the Yangtze craton, including the lower Qinling unit, into Rodinia during the ~1.0 Ga Grenvillian orogeny. The lower Qinling unit then rifted from the Yangtze craton at ~0.7 Ga. Subsequent intra-oceanic arc formation at ~470-490 Ma was followed by accretion of the lower Qinling unit first to the intra-oceanic arc and then to the Sino-Korea craton. Subduction then imprinted a ~400 Ma Andean-type magmatic arc onto all units north of the northern Liuling unit. Oblique subduction created Silurian-Devonian WNW-trending, sinistral transpressive wrench zones (e.g., Lo-Nan, Shang-Dan), and Late Permian-Early Triassic subduction reactivated them in dextral transpression (Lo-Nan, Shang-Xiang, Shang-Dan) and subducted the northern edge of the Yangtze craton. Exhumation of the cratonal edge formed the Wudang metamorphic core complex during dominantly pure shear crustal extension at ~230-235 Ma. Post-collisional south-directed shortening continued through the Early Jurassic. Cretaceous reactivation of the Qinling orogen started with NW-SE sinistral transtension, coeval with large-scale Early Cretaceous crustal extension and sinistral transtension in the northern Dabie Shan; it presumably resulted from the combined effects of the Siberia-Mongolia-Sino-Korean and Lhasa-West Burma-Qiangtang-Indochina collisions and Pacific subduction. Regional dextral wrenching was active within a NE-SW extensional regime between ~60 and 100 Ma. An Early Cretaceous Andean-type continental magmatic arc, with widespread Early Cretaceous magmatism and back-arc extension, was overprinted by shortening related to the collision of Yangtze-Indochina Block with the West Philippines Block. Strike-slip and normal faults associated with Eocene half-graben basins record Paleogene NNE-SSW contraction and WNW-ESE extension. The Neogene(?) is characterized by normal faults and NNE-trending sub-horizontal extension. Pleistocene(?)-Quaternary NW-SE extension and NE-SW contraction comprises sinistral strike-slip faults and is part of the NW-SE extension imposed across eastern Asia by the India-Asia collision.

Tectonophysics↗

USGS2000

Stratigraphy, geochemistry and tectonic significance of the Oligocene magmatic rocks of western Oaxaca, southern Mexico

In Western Oaxaca, Tertiary magmatic activity is represented by extensive plutons along the continental margin and volcanic sequences in the inland region. K-Ar age determinations reported previously and in the present work indicate that these rocks correspond to a relatively broad arc in this region that was active mainly during the Oligocene (~ 35 to ~ 25 Ma). In the northern sector of western Oaxaca (Huajuapan-Monte Verde-Yanhuitlan), the volcanic suite comprises principally basaltic andesite to andesitic lavas, overlying minor silicic to intermediate volcaniclastic rocks (epiclastic deposits, ash fall tuffs, ignimbrites) that were deposited in the lacustrine-fluvial environment. The southern sector of the volcanic zone includes the Tlaxiaco-Laguna de Guadalupe region and consists of intermediate to silicic pyroclastic and epiclastic deposits, with silicic ash fall tuffs and ignimbrites. In both sectors, numerous andesitic to dacitic hypabyssal intrusions (stocks and dikes) were emplaced at different levels of the sequence. The granitoids of the coastal plutonic belt are generally more differentiated than the volcanic rocks that predominate in the northern sector and vary in composition from granite to granodiorite. The studied rocks show large-ion lithophile element (LILE) enrichment (K, Rb, Ba, Th) relative to high-field-strength (HFS) elements (Nb, Ti, Zr) that is characteristic of subduction-related magmatic rocks. On chondrite-normalized rare earth element diagrams, these samples display light rare earth element enrichment (LREE) and a flat pattern for the heavy rare earth elements (HREE). In spite of the contrasting degree of differentiation between the coastal plutons and inland volcanic rocks, there is a relatively small variation in the isotopic composition of these two suites. Initial 87Sr/86Sr ratios obtained and reported previously for Tertiary plutonic rocks of western Oaxaca range from 0.7042 to 0.7054 and ??Nd values, from -3.0 to +2.4, and for the volcanic rocks, from 0.7042 to 0.7046 and 0 +2.6. The range of these isotope ratios and those reported for the basement rocks in this region suggest a relatively low degree of old crustal involvement for most of the studied rocks. The Pb isotopic compositions of the Tertiary magmatic rocks also show a narrow range [(206Pb/204Pb) = 18.67-18.75; (207Pb/204Pb) = 15.59-15.62; (208Pb/204Pb) = 38.44-38.59], suggesting a similar source region for the volcanic and plutonic rocks. Trace elements and isotopic compositions suggest a mantle source in the subcontinental lithosphere that has been enriched by a subduction component. General tectonic features in this region indicate a more active rate of transtensional deformation for the inland volcanic region than along the coastal margin during the main events of Oligocene magmatism. The lower degree of differentiation of the inland volcanic sequences, particularly the upper unit of the northern sector, compared to the plutons of the coastal margin, suggests that the differentiation of the Tertiary magmas in southern Mexico was controlled to a great extent by the characteristics of the different strain domains. (C) 2000 Elsevier Science B.V. All rights reserved.

Conference Paper↗

USGS1998

Tectonic setting of synorogenic gold deposits of the Pacific Rim

More than 420 million oz of gold were concentrated in circum-Pacific synorogenic quartz loades mainly during two periods of continental growth, one along the Gondwanan margin in the Palaeozoic and the other in the northern Pacific basin between 170 and 50 Ma. These ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary-rock dominant terranes. The auriferous veins contain only a few percent sulphide minerals, have gold:silver ratios typically greater than 1:1, show a distinct association with medium grade metamorphic rocks, and may be associated with large-scale fault zone. Ore fluids are consistently of low salinity and are CO2-rich. In the early and middle Palaeozoic in the southern Pacific basin, a single immense turbidite sequence was added to the eastern margin of Gondwanaland. Deformation of these rocks in southeastern Australia was accompanied by deposition of at least 80 million oz of gold in the Victorian sector of the Lachlan fold belt mainly during the Middle and Late Devonian. Lesser Devonian gold accumulations characterized the more northerly parts of the Gondwanan margin within the Hodgkinson-Broken River and Thomson fold belts. Additional lodes were emplaced in this flyschoid sequence in Devonian or earlier Palaeozoic times in what is now the Buller Terrane, Westland, New Zealand. Minor post-Devonian growth of Gondwanaland included terrane collision and formation of gold-bearing veins in the Permian in Australia's New England fold belt and in the Jurassic-Early Cretaceous in New Zealand's Otago schists. Collision and accretion of dozens of terranes for a 100-m.y.-long period against the western margin of North America and eastern margin of Eurasia led to widespread, lattest Jurassic to Eocene gold veining in the northern Pacific basin. In the former location, Late Jurassic and Early Cretaceous veins and related placer deposits along the western margin of the Sierra Nevada batholith have yielded more than 100 million oz of gold. Additional significant ore-forming events during the development of North America's Cordilleran orogen included those in the Klamath Mountains region, California in the Late Jurassic and Early Cretaceous; the Klondike district, Yukon by the Early Cretaceous; the Nome and Fairbanks districts, Alaska, and the Bridge River district, British Columbia in the middle Cretaceous; and the Juneau gold belt, Alaska in the Eocene. Gold-bearing veins deposited during the Late Jurassic and Early Cretaceous terrane collision that formed the present-day Russian Far East have been the source for more than 130 million oz of placer gold. The abundance of gold-bearing quartz-carbonate veins throughout the Gondwanan, North American and Eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth. Such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata. The common temporal association between gold veining and magmatism around the Pacific Rim reflects these thermal episodes. Melting of the lower thickened crust during arc formation, slab rollback and extensional tectonism, and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initation of the ore-forming processes.

Ore Geology Reviews↗

USGS2003

Channel response to tectonic forcing: field analysis of stream morphology and hydrology in the Mendocino triple junction region, northern California

An empirical calibration of the shear stress model for bedrock incision is presented, using field and hydrologic data from a series of small, coastal drainage basins near the Mendocino triple junction in northern California. Previous work comparing basins from the high uplift zone (HUZ, uplift rates around 4 mm/year) to ones in the low uplift zone (LUZ, ∼0.5 mm/year) indicates that the HUZ channels are about twice as steep for a given drainage area. This observation suggests that incision processes are more effective in the HUZ. It motivates a detailed field study of channel morphology in the differing tectonic settings to test whether various factors that are hypothesized to influence incision rates (discharge, channel width, lithology, sediment load) change in response to uplift or otherwise differ between the HUZ and LUZ. Analysis of regional stream gaging data for mean annual discharge and individual floods yields a linear relationship between discharge and drainage area. Increased orographic precipitation in the HUZ accounts for about a twofold increase in discharge in this area, corresponding to an assumed increase in the erosional efficiency of the streams. Field measurements of channel width indicate a power-law relationship between width and drainage area with an exponent of ∼0.4 and no significant change in width between the uplift rate zones, although interpretation is hampered by a difference in land use between the zones. The HUZ channel width dataset reveals a scaling break interpreted to be the transition between colluvial- and fluvial-dominated incision processes. Assessments of lithologic resistance using a Schmidt hammer and joint surveys show that the rocks of the study area should be fairly similar in their susceptibility to erosion. The HUZ channels generally have more exposed bedrock than those in the LUZ, which is consistent with protection by sediment cover inhibiting incision in the LUZ. However, this difference is likely the result of a recent pulse of sediment due to land use in the LUZ. Therefore, the role of sediment flux in setting incision rates cannot be constrained with any certainty. To summarize, of the four response mechanisms analyzed, the only factor that demonstrably varies between uplift rate zones is discharge, although this change is likely insufficient to explain the relationship between channel slope and uplift rate. The calibrated model allows us to make a prediction of channel concavity that is consistent with a previous estimate from slope–drainage area data. We show that the inclusion of nonzero values of critical shear stress in the model has important implications for the theoretical relationship between steady-state slope and uplift rate and might provide an explanation for the observations. This analysis underscores the importance of further work to constrain quantitatively threshold shear stress for bedrock incision.

California↗

USGS2000

Recent tectonics of the Blanco Ridge, eastern blanco transform fault zone

Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the ∼150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector estimates of 43 earthquakes ( M w ≥3.8) that occurred along the eastern BTFZ over the last 5 years reveal that the Blanco Ridge is a high-angle right-lateral strike-slip fault, with a small component of dip-slip motion, where the Juan de Fuca plate is the hanging wall relative to the Pacific plate. Furthermore, the Cascadia and Gorda basins are undergoing normal faulting with extension predominantly oblique to the transform trend. Seafloor submersible observations agree with previous hypotheses that the active transform fault trace is the elongate basin that runs the length of the BR summit. Brecciated and undeformed basalt, diabase, and gabbro samples were collected at the four submersible survey sites along the Blanco Ridge. These petrologic samples indicate the Blanco Ridge is composed of an ocean crustal sequence that has been uplifted and highly fractured. The petrologic samples also appear to show an increase in elevation of the crustal section from east to west along the Blanco Ridge, with gabbros exposed at a shallower depth farther west along the southern (Pacific plate side) BR ridge flank. Further supporting evidence for BR uplift exists in the seismic reflection profiles across the BR showing uplift of turbidite sequences along the north and south ridge base, and gravity and magnetics profiles that indicate possible basement uplift and a low-density zone centered on the ridge's Pacific plate side. The BR formation mechanism preferred here is first, uplift achieved partially through strike-slip motion (with a small dip-slip component). Second, seawater penetration along the fault into the lower crust upper mantle, which then enhanced formation and intrusion of a mantle-derived serpentinized-peridotite diapir into the shallow ocean crust, causing further uplift along the fault.

Marine Geophysical Research↗

USGS1999

Paleomagnetism of an east-west transect across the Cascade arc in southern Washington: Implications for regional tectonism

Paleomagnetic data from a transect across the Cascade arc in southern Washington were collected to test models of vertical axis rotations for this part of the Pacific Northwest and to provide information on the tectonic history of the St. Helens seismic zone. The 75 site-mean directions are divided into three groups based on isotopic, fission track, and stratigraphic ages. Group 1 consists of samples from rocks deposited between 39 and 30 Ma and shows a mean clockwise vertical axis rotation of 34° ± 13°, group 2 consists of samples from rocks deposited between 30 and 24 Ma and shows a mean clockwise rotation of 17° ± 11°, and group 3 consists of samples from rocks between 24 and 16 Ma and shows a mean clockwise rotation of 20° ± 12°. Although these three values of rotation are statistically indistinguishable at the 95% confidence level, we interpret them in combination with other data to indicate differential rotation across the St. Helens seismic zone (SHZ). The available paleomagnetic data for Eocene-Oligocene rocks west of the seismic zone show clockwise vertical axis rotations of 30° ± 8° and 35° ± 9° consistent with the value for group 1. The rotational values for groups 2 and 3 and the intrusive suite of Kidd Creek (13 Ma), despite their different ages, have similar values east of the SHZ. Comparing these groups of data east and west of the seismic zone indicates a differential rotation of 10° ± 3° across it. In addition, the rates of rotation in southern Washington are similar to those for rocks in the western Cascades of Oregon and indicate that rotational deformation of Miocene age extends northward into southern Washington and eastward into the axis of the arc.

Journal of Geophysical Research B: Solid Earth↗

USGS2001

Density structure of the lithosphere in the southwestern United States and its tectonic significance

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ?? 3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher-density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m~3 higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.

Journal of Geophysical Research B: Solid Earth↗