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Results for “Geological Society of America Bulletin, Part I”

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At least 253 records · Page 14Linked to original sources
USGS1953

Beaverhead formation, a Laramide deposit in Beaverhead County, Montana

The name Beaverhead formation is proposed for a thick sequence of conglomerate, sandstone, siltstone, and limestone that crops out over an area of at least 400 square miles in Beaverhead County, Montana, extends southward across the Montana-Idaho boundary, and may extend eastward into Madison County. These rocks are clearly sedimentary by-products of the Laramide orogeny and probably range from late Cretaceous to early Eocene. The Beaverhead formations consists predominantly of conglomerate. In part of the area, an upper and a lower conglomerate member are separated by a middle member of limestone. Where the limestone member is inconspicuous or absent, the upper and lower conglomerate members cannot be differentiated. The most nearly complete and best-exposed known section of the Beaverhead formation, designated the type section, is near the mouth of McKnight Canyon, 6 miles west of Dell, Montana. Here the formation can be divided into four mappable units; elsewhere no more than three units can be recognized. In the McKnight Canyon section, the top and bottom members are dominantly conglomerate, composed of pebbles, cobbles, and subordinate boulders set in a sandy matrix cemented by calcite; breccia beds occur locally. This coarse debris was derived from rocks of Precambrian, Paleozoic, and Mesozoic age and consists largely of limestone and quartzite. The intermediate member consists of two mappable units: a lower thick, massive limestone, locally concretionary, and an upper sequence of interbedded siltstone, sandstone, arkose, limestone, and subordinate conglomerate. At McKnight Canyon, where the base and the top of the formation have been faulted and eroded, the exposed thickness of the section is approximately 9700 feet. The Beaverhead formation rests unconformably on rocks as young as the Colorado group and as old as early Paleozoic, and it probably rests unconformably upon rocks as young as the Montana group and as old as Precambrian. It is unconformably overlain by vertebrate-bearing fluviatile or lacustrine tuffaceous beds of Eocene and Oligocene age. The coarse debris that composes the formation was eroded from nearby mountains that were uplifted in Late Cretaceous, Paleocene, and early Eocene time, and was deposited in basins adjacent to these mountains. The resulting rocks, a product of Laramide orogeny, were later folded and displaced by overthrusting and block faulting. In places, these rocks are overlain by thrust sheets of Paleozoic rocks. © 1953, The Geological Society of America, Inc.

Montana↗
USGS1970

Premetamorphic down-to-basin faulting, folding, and tectonic dewatering, Rangeley area, western Maine

The Rangeley area of western Maine is underlain by a thick sequence of dominantly eugeosynclinal metasedimentary rocks of Ordovician, Silurian, and Devonian age. The dominant structural pattern of these rocks is defined by tight, upright, northeast-trending passive flow folds and by three major normal faults along which younger rocks on the southeast are down-faulted against older rocks on the northwest. Each normal fault, together with a major syncline and a complementary anticline farther southeast, defines a geometrically related fault-fold unit. In best-exposed units, displacement along the faults increases in the direction of plunge of the synclines and of increasing structural relief in the syncline-anticline pairs. A genetic relation between normal faulting and folding is inferred. The dominant fault-fold pattern represents the oldest recognized deformation in the area. Slaty or phyllitic cleavage of this deformation is typically subparallel to the axial surfaces of folds, but locally crosses the faults and the axial surfaces of tight folds at low angles. Metamorphosed clastic dikes along the cleavage suggest that cleavage formation was in part a diagenetic dewatering process. This process probably graded, however, into low-grade metamorphism at depth. It was quickly followed by emplacement of large plutons, local superposed passive slip and flexural slip folding, and by two recognized events of greenschist and amphibolite facies metamorphism. Porphyroblasts of these events have grown across slip cleavages as well as older phyllitic cleavage, and metamorphic zones cross the dominant fault-fold pattern. Deformation, as well as sedimentation, is considered to have been controlled by the ancestral Merrimack synclinorium—a strongly linear two-sided trough that persisted at least from Late Ordovician through Early Devonian time. The fault-fold pattern is inferred to have evolved over a long period of time, as follows: (1) Rapid deposition of 15,000 to 20,000 ft of nearly-impermeable clastic sediments in Late Ordovician and Early Silurian time on the southeast-dipping slope of the sedimentary trough; mass weakened in depth by excess fluid pressure. (2) Continuing sedimentation, down-to-basin creep with associated slump faulting and folding, probably beginning in Middle Silurian time; faults flattened basinward in depth along lower boundary of zone of excess fluid pressure. (3) Horizontal compression developed parallel to slide direction as mass piled against material in the trough; incipient slaty cleavage developed normal to compression, improving vertical permeability. (4) Pore fluids expelled vertically, permitting the slumping mass to compact horizontally, and fold with at least 25 percent shortening. The process culminated in Early Devonian time, during and after deposition of the youngest exposed rocks in the area.

Maine↗
USGS2002

The last interglacial period on the Pacific Coast of North America: Timing and paleoclimate

New, high-precision U-series ages of solitary corals ( Balanophyllia elegans ) coupled with molluscan faunal data from marine terraces on the Pacific Coast of North America yield information about the timing and warmth of the last interglacial sea-level highstand. Balanophyllia elegans takes up U in isotopic equilibrium with seawater during growth and shortly after death. Corals from the second terrace on San Clemente Island (offshore southern California), the third terrace on Punta Banda (on the Pacific Coast of northern Baja California), and the Discovery Point Formation on Isla de Guadalupe (in the Pacific Ocean offshore Baja California) date to the peak of the last interglacial period and have U-series ages ranging from ca. 123 to 114 ka. The first terrace on Punta Banda has corals with ages ranging from ca. 83 to 80 ka, which corresponds to a sea-level highstand formed in the late last interglacial period. U-series analyses of corals from the Cayucos terrace (central California) and the Nestor terrace at Point Loma (southern California) show that these fossils have evidence of open-system history, similar to what has been reported by other workers for the same localities. Nevertheless, a model of continuous, secondary U and Th uptake shows that two ages of corals are likely present at these localities, representing the ca. 105 and ca. 120 ka sea-level highstands reported elsewhere. U-series ages of last interglacial corals from the Pacific Coast overlap with, but are on average younger than the ages of corals from Barbados, the Bahamas, and Hawaii. This age difference is explained by the nature of the geomorphic response to sea-level change: fringing or barrier reefs on low-latitude coastlines have an accretionary growth style that keeps pace with rising sea level, whether on a tectonically rising or stable coastline. In contrast, midlatitude, high-energy coastlines are sites of platform cutting during the early part of a sea-level high stand and terrace scouring and concomitant sediment and fossil deposition as sea level starts to recede. The youngest ages of corals from the Pacific Coast suggest that sea level was still relatively high at ca. 116 ka, which is not in agreement with other estimates of relatively large global ice volume at that time. Reliably dated, ca. 120 ka marine-terrace deposits on the Pacific Coast have fossil mollusks that indicate water temperatures as warm or warmer than at present. In contrast, ca. 80 ka marine deposits reported here and elsewhere have fossil mollusks indicating cooler-than-modern water temperatures. The presence of both ca. 105 ka and ca. 120 ka corals on the Nestor and Cayucos terraces explains a previously enigmatic mixture of warm-water and cool-water mollusks. At ca. 105 ka, a relatively high sea level with cool waters may have "captured" the terrace formed during the 120 ka sea-level highstand, in areas of low uplift rate. The inference of cooler-than-modern waters off the Pacific Coast of North America at ca. 80 ka and ca. 105 ka, based on marine-terrace faunas, does not agree with estimates of sea-surface temperatures derived from alkenone studies in the Santa Barbara Basin. However, cooler water temperatures at these times are in agreement with paleo-temperature estimates from planktonic foraminiferal data for the Santa Barbara Basin. All records, from central California to Baja California, whether from marine terraces or offshore cores, indicate at least seasonably warmer-than-modern waters during the peak of the last interglacial period at ca. 120 ka.

Pacific Coast↗
USGS1994

Correlation of basinal carbonate cycles to nearshore parasequences in the Late Cretaceous Greenhorn seaway, Western Interior U.S.A.

Upper Cretaceous limestone-shale couplets developed within the late transgressive stage of the Greenhorn cyclothem may be correlated from carbonate-dominated (basinal) sequences in central Kansas and Colorado westward to clastic cycles in southern Utah. Six such basinal couplets have been traced to corresponding upward-coarsening progradational cycles developed on the western margin of the Western Interior basin. In the central basin in Colorado and Kansas, these sedimentary cycles are represented by limestone-shale and marlstone-shale couplets ∼0.5-1.0 m in thickness. More calcareous parts of these couplets may be correlated westward into condensed, fossiliferous concretion and shell beds in proximal offshore lithofacies of Arizona and Utah. These concretion and shell beds are physically traceable farther landward (westward) into bioturbated, fossil-rich, transgressive lag deposits that bound 10- to 20-m-thick coarsening-upward progradational strand-plain deposits (parasequences) in southwestern Utah. Thus, the progra-dational phase of parasequence deposition correlates with accumulation of clay-rich sediment in the central basin, and the transgressive phase is characterized by reduced terrigenous input and deposition of carbonate-rich sediment. We consider Milankovitch-style orbital forcing of climate and tectonically induced fluctuations in rates of foredeep basin subsidence as possible forcing mechanisms for these basinwide events. Based on the widespread distribution of the limestone-shale couplets, as well as on estimated sedimentation rates and geochronology, it has been widely speculated that these carbonate cycles reflect Milankovitch cycles with periodicities on the order of 20 k.y. to 100 k.y. If so, then stratigraphic data suggest that orbital forcing of climate affected eustasy and/or sediment input and biogenic production in the Western Interior basin. Alternatively, thrusting events in the Sevier orogenic belt may have produced episodic changes in the rates of foredeep basin subsidence and consequent changes in base level, which could have controlled the deposition of the Greenhorn parasequences and carbonate cycles. In either case, correlation of these units demonstrates a consistent basinwide sedimentary response to high-frequency base level or sediment input changes in the Western Interior epicontinental basin.

Arizona, Colorado, Kansas, Utah↗
USGS2010

Neotectonics and paleoseismology of the Limón and pedro miguel faults in Panamá: earthquake hazard to the Panamá canal

We present new geologic, tectonic geomorphic, and geochronologic data on the slip rate, timing, and size of past surface ruptures for the right-lateral Lim??n and Pedro Miguel faults in central Panam??. These faults are part of a system of conjugate faults that accommodate the internal deformation of Panam?? resulting from the ongoing collision of Central and South America. There have been at least three surface ruptures on the Lim??n fault in the past 950-1400 years, with the most recent during the past 365 years. Displacement in this young event is at least 1.2 m (based on trenching) and may be 1.6-2 m (based on small channel offsets). Awell-preserved 4.2 m offset suggests that the penultimate event also sustained significant displacement. The Holocene slip rate has averaged about 6 mm=yr, based on a 30-m offset terrace riser incised into a 5-ka abandoned channel. The Pedro Miguel fault has sustained three surface ruptures in the past 1600 years, the most recent being the 2 May 1621 earthquake that partially destroyed Panam?? Viejo. At least 2.1 m of slip occurred in this event near the Canal, with geomorphic offsets suggesting 2.5-3 m. The historic Camino de Cruces is offset 2.8 m, indicating multimeter displacement over at least 20 km of fault length. Channel offsets of 100-400 m, together with a climate-induced incision model, suggest a Late Quaternary slip rate of about 5 mm=yr, which is consistent with the paleoseismic results. Comparison of the timing of surface ruptures between the Lim??n and Pedro Miguel faults suggests that large earthquakes may rupture both faults with 2-3 m of displacement for over 40 km, such as is likely in earthquakes in the M 7 range. Altogether, our observations indicate that the Lim??n and Pedro Miguel faults represent a significant seismic hazard to central Panam?? and, specifically, to the Canal and Panam?? City.

Bulletin of the Seismological Society of America↗
USGS2024

Precariously balanced rocks in northern New York and Vermont, U.S.A.: Ground-motion constraints and implications for fault sources

Precariously balanced rocks (PBRs) and other fragile geologic features have the potential to constrain the maximum intensity of earthquake ground shaking over millennia. Such constraints may be particularly useful in the eastern United States (U.S.), where few earthquake‐source faults are reliably identified, and moderate earthquakes can be felt at great distances due to low seismic attenuation. We describe five PBRs in northern New York and Vermont—a region of elevated seismic hazard associated with historical seismicity. These boulders appear to be among the most fragile PBRs in the region, based on reports from hobbyists. The PBRs are glacial erratics, best evidenced by glacial striations on bedrock pedestals. The pedestals themselves are locally high knobs, often situated on regionally high topography; this setting limits soil development and indicates that any outwash deposits were likely ephemeral. As a result, PBR ages can be reliably established by the retreat of the last continental ice sheet, ∼15–13 ka. To quantify the fragility of the PBRs, we surveyed them with ground‐based light detection and ranging and calculated geometric parameters from the point clouds, field observations, and seismic responses. Preliminary validation of the 2023 time‐independent U.S. National Seismic Hazard Model (NSHM) shows that the existence of PBRs is generally consistent with the median site‐specific hazard curves. Only the Blue Ridge Road site suggests a modest reduction in hazard. To visualize the ensemble of data, we mapped the minimum permissible distance to potential source faults around each PBR site as a function of source magnitude by using the ground‐motion models from the 2023 NSHM. Viewed in this manner, our data are consistent with potential M ∼6.5 earthquake‐source faults in many parts of the Lake Champlain Valley and northern Adirondack Mountains. Our work illustrates a potential pathway for better constraining earthquake‐source faults in regions of cryptic faults.

Bulletin of the Seismological Society of America↗
USGS2025

Re-evaluating the tectonic affinity of Proterozoic crustal provinces in the Southwest USA: Detrital zircon evidence for a Laurentian source for the Yavapai and Mojave Provinces

Models for crustal growth commonly involve the accretion of dominantly juvenile crust to continental margins. However, tracking the provenance and tectonic affinity of dominantly juvenile crustal provinces is challenging. This difficulty is highlighted by uncertainty over whether the Yavapai and Mojave Provinces, part of the >1300-km-wide system of Proterozoic orogens in southwestern Laurentia, (1) have similar crustal and tectonic histories and (2) if they formed on or near Laurentian, Australian, or Antarctic cratons. Here, we contribute new large-n detrital zircon U-Pb geochronology and Sm-Nd whole-rock isotope geochemistry to help constrain the provenance of the Yavapai Province and address these broader questions. Yavapai Province metasedimentary rocks from central Colorado in the southwestern USA have abundant pre-1.80 Ga detrital zircon grains, with ca. 1.85 Ga, 2.30 Ga, and 2.50−2.70 Ga peaks, and variable amounts of 1.79−1.78 Ga grains. Evolved whole-rock Sm-Nd isotopic compositions from these rocks, including 2.36−2.08 Ga model ages, also suggest mixing between 1.79 Ga and 1.78 Ga Yavapai Province arcs and early Proterozoic to Archean sources. Nearly identical pre-1.8 Ga detrital and inherited zircon age distributions suggest that the Yavapai and Mojave Provinces formed on and/or incorporated similar material. The Trans-Hudson orogen, and to a slightly lesser extent the Penokean orogen, provide the closest matches to the pre-1.80 Ga material in the Yavapai and Mojave Provinces. This similarity, coupled with a weaker resemblance to Australian and Antarctic sources, support a Laurentian affinity for the Yavapai and Mojave Provinces. We envision Paleoproterozoic arc formation on both oceanic crust and material of Laurentian affinity and multiple phases of arc-back-arc genesis, closure, and accretionary tectonism along the long-lived margin of the supercontinent Columbia (Nuna).

Arizona, Colorado, Idaho, New Mexico, Utah, Wyomin↗
USGS2010

Ground-motion modeling of Hayward fault scenario earthquakes, part II: Simulation of long-period and broadband ground motions

We simulate long-period ( T> 1.0–2.0 s) and broadband ( T> 0.1 s) ground motions for 39 scenario earthquakes ( M w 6.7–7.2) involving the Hayward, Calaveras, and Rodgers Creek faults. For rupture on the Hayward fault, we consider the effects of creep on coseismic slip using two different approaches, both of which reduce the ground motions, compared with neglecting the influence of creep. Nevertheless, the scenario earthquakes generate strong shaking throughout the San Francisco Bay area, with about 50% of the urban area experiencing modified Mercalli intensity VII or greater for the magnitude 7.0 scenario events. Long-period simulations of the 2007 M w 4.18 Oakland earthquake and the 2007 M w 5.45 Alum Rock earthquake show that the U.S. Geological Survey’s Bay Area Velocity Model version 08.3.0 permits simulation of the amplitude and duration of shaking throughout the San Francisco Bay area for Hayward fault earthquakes, with the greatest accuracy in the Santa Clara Valley (San Jose area). The ground motions for the suite of scenarios exhibit a strong sensitivity to the rupture length (or magnitude), hypocenter (or rupture directivity), and slip distribution. The ground motions display a much weaker sensitivity to the rise time and rupture speed. Peak velocities, peak accelerations, and spectral accelerations from the synthetic broadband ground motions are, on average, slightly higher than the Next Generation Attenuation (NGA) ground-motion prediction equations. We attribute much of this difference to the seismic velocity structure in the San Francisco Bay area and how the NGA models account for basin amplification; the NGA relations may underpredict amplification in shallow sedimentary basins. The simulations also suggest that the Spudich and Chiou (2008) directivity corrections to the NGA relations could be improved by increasing the areal extent of rupture directivity with period.

California↗
USGS2003

Holocene evolution of the western Orinoco Delta, Venezuela

The pristine nature of the Orinoco Delta of eastern Venezuela provides unique opportunities to study the geologic processes and environments of a major tropical delta. Remote-sensing images, shallow cores, and radiocarbon-dating of organic remains form the basis for describing deltaic environments and interpreting the Holocene history of the delta. The Orinoco Delta can be subdivided into two major sectors. The southeast sector is dominated by the Rio Grande-the principal distributary-and complex networks of anastomosing fluvial and tidal channels. The abundance of siliciclastic deposits suggests that fluvial processes such as over-bank flooding strongly influence this part of the delta. In contrast, the northwest sector is represented by few major distributaries, and overbank sedimentation is less widespread relative to the southeast sector. Peat is abundant and occurs in herbaceous and forested swamps that are individually up to 200 km2 in area. Northwest-directed littoral currents transport large volumes of suspended sediment and produce prominent mudcapes along the northwest coast. Mapping of surface sediments, vegetation, and major landforms identified four principal geomorphic systems within the western delta plain: (1) distributary channels, (2) interdistributary flood basins, (3) fluvial-marine transitional environments, and (4) marine-influenced coastal environments. Coring and radiocarbon dating of deltaic deposits show that the northern delta shoreline has prograded 20-30 km during the late Holocene sea-level highstand. Progradation has been accomplished by a combination of distributary avulsion and mudcape progradation. This style of deltaic progradation differs markedly from other deltas such as the Mississippi where distributary avulsion leads to coastal land loss, rather than shoreline progradation. The key difference is that the Orinoco Delta coastal zone receives prodigious amounts of sediment from northwest-moving littoral currents that transport sediment from as far away as the Amazon system (???1600 km). Late Holocene progradation of the delta has decreased delta-plain gradients, increased water levels, and minimized over-bank flooding and siliciclastic sedimentation in the northwest sector. These conditions, coupled with large amounts of direct precipitation, have led to widespread peat accumulation in interdistributary basins. Because peat-forming environments cover up to 5000 km2 of the delta plain, the Orinoco may be an excellent analogue for interpreting ancient deltaic peat deposits.

Geological Society of America Bulletin↗
USGS2008

Rock magnetic characterization of faulted sediments with associated magnetic anomalies in the Albuquerque Basin, Rio Grande rift, New Mexico

Variations in rock magnetic properties are responsible for the many linear, short-wavelength, low-amplitude magnetic anomalies that are spatially associated with faults that cut Neogene basin sediments in the Rio Grande rift, including the San Ysidro normal fault, which is well exposed in the northern part of the Albuquerque Basin. Magnetic-susceptibility measurements from 310 sites distributed through a 1200-m-thick composite section of rift-filling sediments of the Santa Fe Group and prerift Eocene and Cretaceous sedimentary rocks document large variations of magnetic properties juxtaposed by the San Ysidro fault. Mean volume magnetic susceptibilities generally increase upsection through eight map units: from 1.7 to 2.2E-4 in the prerift Eocene and Cretaceous rocks to 9.9E-4-1.2E-3 in three members of the Miocene Zia Formation of the Santa Fe Group to 1.5E-3-3.5E-3 in three members of the Miocene-Pleistocene Arroyo Ojito Formation of the Santa Fe Group. Rock magnetic measurements and petrography indicate that the amount of detrital magnetite and its variable oxidation to maghemite and hematite within the Santa Fe Group sediments are the predominant controls of their magnetic property variations. Magnetic susceptibility increases progressively with sediment grain size within the members of the Arroyo Ojito Formation (deposited in fluvial environments) but within members of the Zia Formation (deposited in mostly eolian environments) reaches highest values in fine to medium sands. Partial oxidation of detrital magnetite is spatially associated with calcite cementation in the Santa Fe Group. Both oxidation and cementation probably reflect past flow of groundwater through permeable zones. Magnetic models for geologic cross sections that incorporate mean magnetic susceptibilities for the different stratigraphic units mimic the aeromagnetic profiles across the San Ysidro fault and demonstrate that the stratigraphic level of dominant magnetic contrast changes with different exposure levels into the fault. These data indicate that tectonic juxtaposition of primary variations of magnetic properties of strata across the fault is the source of the associated magnetic anomaly. This study indicates that magnetic anomalies over faults and folds can be generated by sediments (1) deposited within tectonic basins having volcanic or basement source areas rich in magnetite, (2) having depositional environments with sufficient but varying energy to transport dense magnetic minerals and cause stratigraphic changes of magnetic properties, and (3) having magnetic minerals preserved owing to their youth or nonreactive geochemical environments. ?? 2007 Geological Society of America.

Geological Society of America Bulletin↗
USGS2024

SCEC/USGS Community Stress Drop Validation Study: How spectral fitting approaches influence measured source parameters

Spectral source parameters used to estimate an earthquake’s stress drop (Δσ) can vary significantly across measurement approaches. The Statewide California Earthquake Center/U.S. Geological Survey Community Stress-Drop Validation Study was initiated to compare source parameter estimates, focusing initially on a dataset from the 2019 Ridgecrest earthquake sequence. As part of that validation effort, here we focus on one potential source of uncertainty: whether spectral fitting approaches alone, applied to a common set of spectra from the 2019 Ridgecrest sequence result in different source parameter estimates. By using a common set of benchmark spectra analyzed across a consistent frequency band of 1–40 Hz, we eliminate many sources of variability. A subgroup of validation study participants volunteered to estimate the low-frequency displacement (Ω0) and corner frequency ( f c ) by fitting a smooth function to benchmark displacement spectra. Participants used linear- or log-sampled spectra, assumed a Brune or Boatwright spectral model, and applied different misfit criteria. We compare 17 approaches used to estimate Ω0, f c , and Δσ for 54 earthquake spectra. Our results reveal that 35% of events have Δσ estimates within a factor of two, whereas others exhibit variations exceeding an order of magnitude. The variability in Ω0 and f c can largely be attributed to whether a spectrum is consistent with the smooth function of an idealized simple crack model. The trade-off between Ω0 and f c may be more pronounced when using linearly sampled spectra, as higher frequency spectral bumps control the fits. As expected, methods that assumed a Boatwright model tended to have lower Ω0 and somewhat higher f c compared to those assuming a Brune model, although resulting Δσ estimates are similar. When compared to the overall validation study results, the fitting approach alone may account for between 5% and 90% (25% on average) of the total variability in spectral Δσ.

California↗
USGS1973

Submarine chert-argillite slide-breccia of Paleozoic age in the southern Klamath Mountains, California

A unique chert-argillite breccia—a breccia with an argillite matrix, in which nearly all the fragments are chert—underlies an area of at least 60 sq mi in the southern Klamath Mountains of California. Rocks of this composition have not been reported previously, in the Klamath Mountains or elsewhere, but in northwestern Trinity County they make up a large part of the western Paleozoic and Triassic belt. The remainder of the belt consists of northerly trending sinuous zones of two other principal lithologies: radiolarian chert, rhythmically interbedded with slate or phyllite, crops out in three zones in the eastern, central, and western parts of the area; and massive metabasalt and metadiabase intruded by gabbro and serpentinite occur in a zone about 3 mi wide in the eastern part of the area. Although the predominance of chert fragments makes the chert-argillite breccia apparently unique, rocks that are generally similar, consisting of angular to subangular clasts in a pelitic matrix, have been studied in many localities. Field relations and petrographic features of the chert-argillite breccia—specifically the angularity of clasts, absence of a sandy matrix, preservation of Radiolaria in both chert clasts and pelitic matrix, presence of contorted flow laminae in the matrix, large size of some of the exotic blocks, close association with chert, pillow lava, and limestone, absence of stretching or slickensiding of clasts, and overall dimensions of the unit measurable in miles—variously preclude origin of the rock as a normal sedimentary conglomerate, a tectonic breccia, a tillite, or a subaerial mudflow, but include features common to numerous submarine-slide deposits. We interpret the chert-argillite breccia as the result of uplift and subsequent sliding of a thick deposit of inter-layered chert and pelitic ooze. Brittle fracturing of the chert and mobilization of water-saturated ooze produced the observed features. Fossiliferous limestone blocks within the slide-breccia indicate that it is no older than Silurian or Devonian, and it predates Jurassic metamorphism and plutonism. The relation of the breccia to a major northwest-trending fault zone, and to serpentinite within this zone, suggests that the formation of the breccia was generally synchronous with the faulting and ultramafic intrusion. Rocks of the central metamorphic belt, east of the mapped area, were metamorphosed during a Devonian orogeny, and we suggest that the submarine slide-breccia was formed as a frontal effect of the same orogeny.

California↗
USGS1991

Sedimentation and tectonics of the Sylhet trough, Bangladesh

The Sylhet trough, a sub-basin of the Bengal Basin in northeastern Bangladesh, contains a thick fill (12 to 16 km) of late Mesozoic and Cenozoic strata that record its tectonic evolution. Stratigraphic, sedimentologic, and petrographic data collected from outcrops, cores, well logs, and seismic lines are here used to reconstruct the history of this trough. The Sylhet trough occupied a slope/basinal setting on a passive continental margin from late Mesozoic through Eocene time. Subsidence may have increased slightly in Oligocene time when the trough was located in the distal part of a foreland basin paired to the Indo-Burman ranges. Oligocene fluvial-deltaic strata (Barail Formation) were derived from incipient uplifts in the eastern Himalayas. Subsidence increased markedly in the Miocene epoch in response to western encroachment of the Indo-Burman ranges. Miocene to earliest Pliocene sediments of the Surma Group were deposited in a large, mud-rich delta system that may have drained a significant proportion of the eastern Himalayas. Subsidence rates in the Sylhet trough increased dramatically (3-8 times) from Miocene to Pliocene-Pleistocene time when the fluvial Tipam Sandstone and Dupi Tila Formation were deposited. This dramatic subsidence change is attributed to south-directed overthrusting of the Shillong Plateau on the Dauki fault for the following reasons. (1) Pliocene and Pleistocene strata thin markedly away from the Shillong Plateau, consistent with a crustal load emplaced on the northern basin margin. (2) The Shillong Plateau is draped by Mesozoic to Miocene rocks, but Pliocene and younger strata are not represented, suggesting that the massif was an uplifted block at this time. (3) South-directed overthrusting of the Shillong Plateau is consistent with gravity data and with recent seismotectonic observations. Sandstone in the Tioam has a marked increase in sedimentary lithic fragments compared to older rocks, reflecting uplift and erosion of the sedimentary cover of the Shillong Plateau. If the Dauki fault has a dip similar to that of other Himalayan overthrusts, then a few tens of kilometers of horizontal tectonic transport would be required to carry the Shillong Plateau to its present elevation. Uplift of the Shillong Plateau probably generated a major (∼300 km) westward shift in the course of the Brahmaputra River.

Geological Society of America Bulletin↗
USGS1999

Site response for Seattle and source parameters of earthquakes in the Puget Sound Region

We analyzed seismograms from 21 earthquakes ( M L 2.0-4.9) recorded by digital seismographs we deployed in urban Seattle to determine site response and earthquake stress drops. The seismometers were situated on a wide variety of geologic units, including artificial fill (e.g., Kingdome, Harbor Island), Pleistocene age soils (glacial till and outwash deposits of Seattle's hills), modified land (downtown Seattle, Space Needle), and Tertiary sedimentary rock. Two mainshock-aftershock sequences were recorded: the June 1997 Bremerton sequence (mainshock M L 4.9) and the February 1997 South Seattle sequence (mainshock M L 3.5), along with other events in the Puget Sound region. We developed a new inversion procedure to estimate site response, source corner frequencies, and seismic moments from the S -wave spectra. This inversion uses corner frequencies determined from spectral ratios of mainshock-aftershock pairs as constraints. The site responses found from the inversion are not relative to the rock site but are relative to an idealized site with a flat frequency response. The response of the rock site is also found from the inversion. The inversion results show high response for the sites on artificial fill, more moderate amplification for most sites on stiff Pleistocene soils or modified land, and low response for the rock site. Some sites display resonances, such as a strong 2-Hz resonance at our site near the Kingdome, which is caused by the surficial layers of fill and younger alluvium. The sites in West Seattle exhibit high amplification, even though they are on relatively stiff soils of glacial outwash. This may be partly caused by basin surface waves produced by conversion of incident S waves. This high response in West Seattle is consistent with damage reports from the 1949 ( m b 7.1) and 1965 ( m b 6.5) earthquakes. Stress-drop estimates for the events we recorded were generally low, between 0.4 and 25 bars, although some of the events may have had higher stress drops that could not be resolved because of the limited passband. We calculated a stress drop of 24 bars for the Bremerton mainshock and 10 bars for the South Seattle mainshock.

Washington↗
USGS1984

Mineralogy and chemistry of massive sulfide deposits from the Juan de Fuca Ridge

Six hydrothermal vent sites and associated benthic communities were located in the axial valley of the southern Juan de Fuca Ridge using transponder-navigated bottom photography. The hydrothermal deposits form ledges and shallow mounds within a central zone characterized by a linear bathymetric depression and numerous collapse features. The flat valley floor adjacent to the central zone consists of ferrobasalt lobate flows and sheet flows; sediment cover is minimal. Vent sites are characterized by concentrations of tube worms, clams, benthic siphonophores, and several unidentified fauna. Two types of massive sulfide were dredged from one of the vent sites. Type A samples are angular slabs of dark gray Zn-rich sulfide with interlayers and a thin, partly oxidized crust of Fe sulfide. These layered sulfide aggregates appear to be fragments of a sulfide wall enclosing an active hydrothermal vent. The outer sulfide wall is composed mainly of colloform Fe sulfide and Fe-poor sphalerite deposited under lower-temperature conditions whereby sea water and hydrothermal fluid mix above the discharge point. With continued sulfide deposition, the wall inhibits mixing of sea water and hydrothermal fluid. Inside the wall, the intensifying hydrothermal system deposits a higher-temperature assemblage of granular Fe-rich sphalerite, wurtzite, pyrite, and minor Cu-Fe sulfide. The zonation in wurtzite from Fe-rich cores to Fe-poor rims may result from a late-stage cooling of the hydrothermal fluid and(or) a change in fluid chemistry. The sulfide wall grows outward where a rupture in it permits the escape of high-temperature fluid and then deposition of a secondary shell over the breakthrough point. As temperature increases, earlier-formed minerals dissolve, and Zn, Fe, and Pb migrate toward the outer sulfide wall. Tube worms flourished in the outer wall of type A samples, and abandoned tube structures served as conduits for the late-stage hydrothermal fluids. Type B sulfide samples are subrounded, spongy-textured fragments composed almost entirely of dendritic aggregates of pale, Fe-poor colloform sphalerite and opaline silica. This type of sulfide is deposited in open space by moderate- to low-temperature fluid discharging at a slow but variable rate; the fluid becomes increasingly oxidizing, resulting in late-stage deposition of hematite, barite, and sulfur. Type B samples show little evidence of burrowing animals; this type of sulfide may be deposited in settings peripheral to sites of focused discharge.

Geological Society of America Bulletin↗
USGS2020

ShakeAlert Earthquake Early Warning System Performance During the 2019 Ridgecrest Earthquake Sequence

During July 2019, a sequence of earthquakes including a Mw6.4 foreshock and a Mw7.1 mainshock occurred near Ridgecrest, California. ShakeAlert, the U.S. Geological Survey (USGS) ShakeAlert public Earthquake Early Warning (EEW) system being developed for the U.S. West Coast, was operational during this time, though public alerting was only available within LA County. ShakeAlert created alert messages for many of the earthquakes, including the two largest events, and for many of the larger aftershocks. In this study, we dissect log files and replay data through the system to reconstruct the sequence of events and analyze the performance of the system during that time period. While the system performed reasonably well overall, the sequence also revealed various issues and short comings that will be addressed in impending and future system upgrades, with most parts of the system working as they should. ShakeAlert correctly detected and rapidly characterized both the Mw6.4 and Mw7.1 earthquakes within 6.9 s of their origin times and created alert messages that were available to ShakeAlert’s pilot users. No public alerts were sent out by the ShakeAlertLA cellphone app (the only publicly available alerting method at the time) because the predicted shaking for LA County was below the app’s alerting threshold of MMI 4.0. For the Mw6.4 event this was accurate. For the Mw7.1 event, public alerts for LA County were warranted, but ShakeAlert underpredicted the shaking levels because both the point-source and the finite-fault algorithms underestimated the magnitude of the earthquake by 0.8 units. A number of software and hardware issues that were responsible for the magnitude underestimation of the mainshock have been identified and will be addressed in future ShakeAlert releases. We also analyze the hypothetical alerting performance of ShakeAlert had public alerting been available throughout southern California with a lower alerting threshold of 〖MMI〗_alert=2.5MMI 2.5. We find that, despite the magnitude underestimation, ShakeAlert could have provided timely warnings to a large fraction of affected sites, including some of the near-epicentral sites with high ground motion intensities.

California↗
USGS1952

Magmatic differentiation in tertiary and quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska

Samples of 17 volcanic rocks of Tertiary and Quaternary age from Adak and Kanaga islands have been chemically analyzed and studied microscopically. Spectrograms have been made of 10 of them. The rocks from Adak represent one center of possibly older Tertiary age and two centers of younger Tertiary or Quaternary age. The rocks from Kanaga Island represent both a shield volcano of possibly Tertiary age, partly destroyed by the formation of a caldera, and a young cone of Quaternary age that has grown within the caldera. All the rocks are basalt or andesite. Modally, all are characterized by relatively large crystals of plagioclase more calcic than andesine, and by one or more of the following ferromagnesian minerals: olivine, hypersthene, augite, and hornblende. Apatite and iron ores are common, and late silica minerals and orthoclase occur interstitially in the groundmasses of some rocks . As analyses of no more than four samples are available for each center, the small differences between sets of analyses representing different centers are of doubtful significance. Consequently, the analyses representing all the centers have been plotted on each of the several diagrams used. The several types of variation diagrams show that the province is a calc-alkaline one. The alkali-lime index is in the neighborhood of 63. This very high value is comparable with that for Katmai and is only slightly less than the maximum for the Japanese volcanic rock series. The quantities of minor constituents present are not exceptional for the rock types analyzed; the rocks from Adak are apparently more strontium-rich than those from Kanaga . The chemical analyses of the more basic rock types, as compared with the average analysis of plateau basalt, suggest that the Aleutian parental magma could have been derived from a plateau basalt magma by the addition of plagioclase and the subtraction of pyroxene, iron ore, and some quartz. The distribution of the minor elements can be explained more easily by postulating that, at least on Kanaga Island , some sediments have been assimilated. The derivation of the analyzed rocks from the Aleutian parental magma is most easily explained by the hypothesis that the plagioclase remained in suspension while the ferromagnesian minerals were settling out.

Alaska↗
USGS1983

Lead-isotopic compositions of diverse igneous rocks and ore deposits from southwestern New Mexico and their implications for early Proterozoic crustal evolution in the western United States

U-Pb zircon measurements from some of the basement rocks in southwestern New Mexico have ages that range from 1,650 to 1,450 m.y. The largest ore deposits occur near Silver City in the southwest part of the region studied. They are associated with Laramide alkali to calc-alkaline plutons, and their lead-isotopic compositions are the least radiogenic in the region. This lead exhibits lower-crustal-upper-mantle characteristics, and for the major producing porphyry copper deposits, the 206 Pb/ 204 Pb ratios are less than 18.0. Lead from more silicic mid-Tertiary volcanic rocks and associated ore deposits has somewhat higher 208 Pb/ 204 Pb ratios that reflect a greater crustal involvement in their origin. In a regional trend to the northeast, ore lead becomes more radiogenic, and at Hansonburg, 200 km from Silver City, lead in the Mississippi-type deposits is clearly derived from upper-crustal sources. On the 207 Pb/ 204 Pb- 206 Pb/ 204 Pb plot, data from all of the rocks and ores that we have analyzed form an array that lies below the average orogene curve of Doe and Zartman (1979). A common source is implied for the lead, the isotopic composition of which was similar to that found in 1,750-m.y.-old stratiform deposits as far apart as Pecos, New Mexico, and Jerome, Arizona. Such a composition indicates that over a large region of the southwestern United States, continental crust developed between 1,750 and 1,450 m.y. ago, possibly in an island-arc environment. Basement rocks that are ∼ 1,750 m.y. old extend northward through Colorado to Utah. Galena data obtained in previous studies show that the fraction of older sialic lead in those rocks increases toward the Archean craton in Wyoming. The crust apparently developed southward from Wyoming in stages at 2,400 m.y. ago or before, 2,100 m.y. ago, and 1,750 m.y. ago, with incorporation of older sialic material in each stage as far south as Milford, Utah.

Geological Society of America Bulletin↗