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At least 163 records · Page 9Linked to original sources

USGS2007

Geologic map of the Craters of the Moon 30' x 60' Quadrangle, Idaho

The Craters of the Moon 30 x 60 minute quadrangle shows the geology of the northern two-thirds of the Craters of the Moon (COM) lava field and volcanic structures of the northern and central parts of the Great Rift volcanic rift zone. The COM lava field is the largest, predominantly Holocene lava field in the conterminous United States. The northwest corner of the map shows older sedimentary, intrusive, and volcanic rocks that range in age from Ordovician to Miocene. These rocks provide evidence of compressional fold and thrust events of the Antler and Sevier orogenies. Compression was followed by voluminous volcanism represented by the Challis Volcanic Group. Basin-and-Range faulting followed in Neogene time. The COM lava field covers about 1,600 square kilometers and contains about 30 cubic kilometers of lava flows and associated vent deposits. Stratigraphic relationships, paleomagnetic studies, and radiocarbon ages indicate that the field formed during eight eruptive periods designated as H, the oldest, to A, the youngest. Each eruptive period was several hundred years or less in duration and separated from other eruptive periods by non-eruptive recurrence intervals of several hundred to about 3,000 years. The first eruptive period began about 15,000 carbon-14 years ago and the latest one ended about 2,100 carbon-14 years ago. All available field, paleomagnetic, radiocarbon, and argon-40/argon-39 data are incorporated in this map and they quantitatively refine the volcanic and paleomagnetic history of the pre-Holocene lava fields and the COM lava field. In a sense, these data determine the 'pulse rate' for Pleistocene and Holocene basaltic volcanism in the area of this map. Twenty-three new argon-40/argon-39 geochronologic data reveal a fairly complete and continuous record of basaltic volcanism in the Craters of the Moon 30 x 60 minute quadrangle for the last 500 ka. The ages cluster into age groupings at ~30 ka, 50-70 ka, 100-125 ka, 260-290 ka, 320-340 ka, and 475 ka. There are apparent periods of ~30 to 60 ka duration when little or no volcanic activity took place between groups. Magnetic polarity and remanent inclination and declination directions for most lava flows in the quadrangle have normal magnetic polarity; they were emplaced during the Brunhes Normal Polarity Chron and are younger than 780,000 years. Directions of remanent magnetization and the new argon-40/argon-39 ages were used to correlate and approximately date lava flows and lava fields for this map.

Scientific Investigations Map↗

USGS2002

Magnetobiochronologic synthesis of ODP Leg 178 rise sediments from the Pacific sector of the Southern Ocean: Sites 1095, 1096, and 11011

During Ocean Drilling Program (ODP) Leg 178, eight holes were drilled at three sites (1095, 1096, and 1101) on the continental rise along the western Antarctic Peninsula. The rise sediments proved to be good paleomagnetic recorders and provided continuous magnetostratigraphic records at all three sites. Biosiliceous microfossils, particularly diatoms and radiolarians, were present in the upper Miocene through lower Pliocene sections. In the upper Pliocene to Pleistocene sections, biosiliceous microfossils were rare but calcareous nannofossils and foraminifers were present. This paper summarizes the biostratigraphy and magnetostratigraphy of Leg 178 continental rise sites and is the first attempt at direct calibration of Antarctic biostratigraphic events to the geomagnetic polarity timescale in the Pacific sector of the Southern Ocean.

Proceedings of the Ocean Drilling Program: Scienti↗

USGS2015

Glaciological and marine geological controls on terminus dynamics of Hubbard Glacier, southeast Alaska

Hubbard Glacier, located in southeast Alaska, is the world's largest non-polar tidewater glacier. It has been steadily advancing since it was first mapped in 1895; occasionally, the advance creates an ice or sediment dam that blocks a tributary fjord (Russell Fiord). The sustained advance raises the probability of long-term closure in the near-future, which will strongly impact the ecosystem of Russell Fiord and the nearby community of Yakutat. Here, we examine a 43-year record of flow speeds and terminus position to understand the large-scale dynamics of Hubbard Glacier. Our long-term record shows that the rate of terminus advance has increased slightly since 1895, with the exception of a slowed advance between approximately 1972 and 1984. The short-lived closure events in 1986 and 2002 were not initiated by perturbations in ice velocity or environmental forcings, but were likely due to fluctuations in sedimentation patterns at the terminus. This study points to the significance of a coupled system where short-term velocity fluctuations and morainal shoal development control tidewater glacier terminus position.

Alaska↗

USGS2017

Systems for measuring thickness of temperate and polar ice from the ground or from the air.

Equipment has been designed and tested for ground-based and airborne sounding of temperate glaciers. The transmitter is a free-running pulse generator that uses avalanche-mode transistor breakdown to create high-voltage pulses. The transmit and receive antennas are resistively loaded dipoles; for the airborne system, a twin-lead transmit element and a three-layer coaxial receive element are used on the inboard end of the respective antennas. The sounders are broadband systems; oscilloscopes are used for receivers. The oscilloscope trace is recorded photographically in the ground-based systems. A sampling oscilloscope is used in the airborne system—the sampling process strobes the waveform to audio frequencies so that it can be recorded on magnetic tape. Echoes have been obtained from ice depths of 550 m using the airborne system and about 1 000 m using the ground-based system.

Journal of Glaciology↗

USGS2006

Analyses of unusual long-period earthquakes with extended coda recorded at Katmai National Park, Alaska, USA

A swarm of six long-period (LP) events with slowly decaying coda wave amplitudes and durations up to 120 s, was recorded by seismic stations located in the proximity of Mt. Griggs, a fumarolically active volcano in the Katmai National Park, Alaska, during December 8–21, 2004. Spectral analyses reveal the quasi-monochromatic character of the waveforms, dominated by a 2.5 Hz mode frequently accompanied by a weaker high-frequency onset (6.0–9.0 Hz). Particle motion azimuths and inclination angles show a dominant WNW-ESE direction of polarization for all the signals, and suggest that seismic energy is radiated by a stable source at shallow depth. Damping coefficients between 0.0014 and 0.0063 are estimated by fitting an exponential decay model to the signal's coda; corresponding quality factors range from 78 to 351. The source of the waveforms is modelled as a resonant cavity filled with a fluid/gas mixture.

Alaska↗

USGS1997

Temporal gravity and height changes of the Yellowstone caldera, 1977 - 1994

This paper describes the longest record of gravity measurements in the area of the Yellowstone caldera, Wyoming. The temporal gravity changes, at the ±12 µGal (10 −8 ms −2 ) precision level, are compared with changes in heights from leveling and GPS. The gravity field decreased across the caldera from 1977 to 1983 during the uplift and attained a maximum decrease of up to −60± 2 µGal along the Caldera axis. The gravity field then reversed polarity to increasing values, of up to 60±12 µGal between 1986 and 1993. The ratio between height and gravity changes varied during the entire time, but converged over the latter period following the free-air gravity gradient. General ground deformation deduced from leveling showed caldera-wide uplift of ∼15 mm/a during the period of gravity decrease, then from leveling and GPS, subsidence of ∼25 mm/a during the gravity increase.

Geophysical Research Letters↗

USGS2017

Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40–58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region’s unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

Alaska↗

USGS1996

40Ar/39Ar geochronology and paleomagnetism of Independence volcano, Absaroka volcanic supergroup, Beartooth mountains, Montana

Independence volcano is a major volcanic complex in the lower part of the Absaroka Volcanic Supergroup (AVS) of Montana and Wyoming. Recently reported Rb–Sr mineral dates from the complex give apparent ages of 91 and 84 Ma, whereas field relationships and the physical and compositional similarity of the rocks with other dated parts of the AVS indicate an Early to Middle Eocene age for eruption and deposition. To resolve the conflict between age assignments based on stratigraphic correlations and Rb–Sr dates, we report new paleomagnetic data and 40 Ar/ 39 Ar dates for Independence volcano. Paleomagnetic data for the stock and an andesite plug that cuts the stock are well grouped, of reverse polarity, and yield a virtual geomagnetic pole that is essentially identical to Late Cretaceous and Tertiary reference poles. The reverse polarity indicates that the magnetization of these rocks is probably younger than the Cretaceous normal superchron, or less than about 83.5 Ma. Hornblende from a volcanic breccia near the base of the volcanic pile gives a 40 Ar/ 39 Ar age of 51.57 Ma, whereas biotites from a dacite sill and a granodiorite stock that forms the core of the volcano give dates that range from 49.96 to 48.50 Ma. These dates record the age of eruption and intrusion of these rocks and clearly show that the age of Independence volcano is Early to Middle Eocene, consistent with stratigraphic relations. We suggest that the Rb–Sr mineral dates from the Independence stock and related intrusions are unreliable.

Montana, Wyoming↗

USGS2023

Paleomagnetism of the Harrat Rahat volcanic field, Kingdom of Saudi Arabia—Geologic unit correlations and geomagnetic cryptochron identifications

Paleomagnetic rock samples were collected from 173 drill sites in the Quaternary alkali basaltic volcanic field of northern Harrat Rahat, Kingdom of Saudi Arabia. Laboratory measurements on these samples established that lava flows and vent complexes—identified and mapped from field characteristics, rock types, and compositions as products of single or temporally close eruptions—typically record single, or very similar, directions of remanent magnetization. Correlations defined through geologic mapping, spatial association, geochemistry, geochronology, and identical mean remanent directions indicate at least 16 brief episodes of temporally clustered eruptions. These episodes had durations of a few centuries or less. Anomalous remanent magnetic directions were found for at least 13 mapped lavas of northern Harrat Rahat, which demonstrate that they were acquired during brief geomagnetic cryptochrons during the Brunhes Normal Polarity Chron. These uncharacteristic directions enhance the opportunity to identify common eruptive episodes, and to better understand and evaluate assessments of eruption ages based on 40 Ar/ 39 Ar geochronology. Combining paleomagnetic and regional archaeomagnetic results for the youngest eruptions allows us to evaluate their historical age assignments and, in one case, refute a previously assigned provisional age.

Harrat Rahat volcanic field↗

USGS2023

The Far-Field imprint of the late Paleozoic Ice Age, its demise, and the onset of a dust-house climate across the Eastern Shelf of the Midland Basin, Texas

The late Paleozoic is a period of pronounced climatic and tectonic change, characterized by the onset and disappearance of continental-scale glaciers across polar Gondwana, the formation of Pangea, and widespread large igneous province volcanism. The low-latitude equatorial tropics are assumed to be places of persistent warm and wet climatic conditions throughout the Phanerozoic, which through intense silicate weathering, exert a major influence on Earth’s climate via the consumption of atmospheric carbon through carbonic hydrolytic weathering, formation of clay minerals and deliverability of alkalinity to ocean basins. Here we investigate the late Paleozoic sedimentary record of the Eastern Shelf of the Midland Basin in order to refine the climatic and provenance record of this region. The Eastern Shelf of the Midland Basin was situated within the equatorial tropics throughout the late Paleozoic and was connected to the open ocean through a network of fluvial systems that drained into the marine Midland Basin. We present new U-Pb zircon geochronology (19 samples, 2591 analyses) and sedimentary petrography (11 samples, 5800 grain counts), which we integrate with previously published paleobotany, paleosol chemistry and clay mineralogy to provide a holistic climate and tectonic record from this region. We observe major changes in sedimentary processes that we attribute to the formation of Pangea, eustatic changes linked to a dynamic high-latitude glaciation and teleconnections with low latitude hydrology, and a long-term shift in the Earth climate system all of which result in a dynamic sediment provenance history. Late Pennsylvanian and earliest Permian deposits are enriched in zircons with local affinity and interpreted to reflect local uplift and repeat incision across the basin margin, the latter a result of glacioeustatic forcing during an “everwet” climate. A major paleoenvironmental shift occurs in the late early Permian, which is reflected by the transition from fluvial to mixed fluvial-aeolian and ultimately aeolian dominant sedimentation by the late Permian. The transition from fluvial to aeolian dominant sedimentation is accompanied by a change in clay chemistry, sedimentary rock textual maturity, paleosol morphology and a threefold increase in Paleozoic zircons in the mid to late Permian strata. Widespread loess deposits across equatorial Pangea during the Permian have been used to argue for the possibility of equatorial glaciers situated in highland settings during the early Permian. Conversely, our data suggest initiation of a substantial component of aeolian deposition across the field areas, which is coincident with widespread ice loss across high latitude Gondwana, and ultimately highlights the teleconnections between high latitude glaciation and the low latitude hydrologic cycle.

Texas↗

USGS2018

Late Neogene–Quaternary tephrochronology, stratigraphy, and paleoclimate of Death Valley, California, USA

Sedimentary deposits in midlatitude continental basins often preserve a paleoclimate record complementary to marine-based records. However, deriving that paleoclimate record depends on having well-exposed deposits and establishing a sufficiently robust geochronology. After decades of research, we have been able to correlate 77 tephra beds exposed in multiple stratigraphic sections in the Death Valley area, California, United States. These correlations identify 25 different tephra beds that erupted from at least five different volcanic centers from older than 3.58 Ma to ca. 32 ka. We have informally named and determined the ages for seven previously unrecognized beds: ca. 3.54 Ma tuff of Curry canyon, ca. 3.45 Ma tuff of Furnace Creek, ca. 3.1 Ma tuff of Kit Fox Hills, ca. 3.1 Ma tuff of Mesquite Flat, ca. 3.15 Ma tuff of Texas Spring, 3.117 ± 0.011 Ma tuff of Echo Canyon, and the ca. 1.3 Ma Amargosa ash bed. Several of these tephra beds are found as far northeast as central Utah and could be important marker beds in western North America. Our tephrochronologic data, combined with magnetic polarity data and 40 Ar/ 39 Ar age determinations, redefine Neogene sedimentary deposits exposed across 175 km 2 of the Death Valley area. The alluvial/lacustrine Furnace Creek Formation is a time-transgressive sedimentary sequence ranging from ca. 6.0 to 2.5 Ma in age. The ca. 2.5–1.7 Ma Funeral Formation is typically exposed as a proximal alluvial-fan facies overlying the Furnace Creek Formation. We have correlated deposits in the Kit Fox Hills, Salt Creek, Nova Basin, and southern Death Valley with the informally named ca. 1.3–0.5 Ma Mormon Point formation. In addition, our correlation of the late Pleistocene Wilson Creek ash bed 15 in the Lake Rogers deposits represents the first unambiguous sequences deposited during the Last Glacial Maximum (marine isotope stage [MIS] 2) in Death Valley. Based on this new stratigraphic framework, we show that the Pliocene and Pleistocene climate in Death Valley is consistent with the well-established marine tropical/subtropical record. Pluvial lakes in Death Valley and Searles Valley began to form ca. 3.5–3.4 Ma in the late Pliocene during MIS MG5. Initiation of lakes in these two hydrologically separated valleys at the same time at the beginning of a cooling trend in the marine climate record suggests a link to a cooler, wetter (glacial) regional climate in North America. The Death Valley lake persisted until ca. 3.30 Ma, at the peak of the M2 glaciation, after which there is no evidence of Pliocene lacustrine deposition, even at the peak of the Northern Hemisphere Glaciation (ca. 2.75 Ma). If pluvial lakes in the Pliocene are an indirect record of glacial climate conditions, as they are for the Pleistocene, then a glacial climate was present in western North America for ∼200,000 yr during the Pliocene, encompassing MIS MG5–M2. Pleistocene pluvial lakes in Death Valley that formed ca. 1.98–1.78 Ma, 1.3–1.0 Ma, and ca. 0.6 Ma (MIS 16) are consistent with other regional climate records that indicate a regional glacial climate; however, Death Valley was relatively dry at ca. 0.77 Ma (MIS 19), when large lakes existed in other basins. The limited extent of the MIS 2 marsh/shallow lake in the Lake Rogers basin of northern Death Valley reflects the well-known regional glacial climate at that time; however, Death Valley received relatively lower inflow and rainfall in comparison.

Geological Society of America Bulletin↗

USGS2016

A large refined catalog of earthquake relocations and focal mechanisms for the Island of Hawai'i and its seismotectonic implications

We present high-quality focal mechanisms based on a refined earthquake location catalog for the Island of Hawai'i, focusing on Mauna Loa and Kīlauea volcanoes. The relocation catalog is based on first-arrival times and waveform data of both compressional and shear waves for about 180,000 events on and near the Island of Hawai'i between 1986 and 2009 recorded by the seismic stations at the Hawaiian Volcano Observatory. We relocate all the earthquakes by applying ray tracing through an existing three-dimensional velocity model, similar event cluster analysis, and a differential-time relocation method. The resulting location catalog represents an expansion of previous relocation studies, covering a longer time period and consisting of more events with well-constrained absolute locations. The focal mechanisms are obtained based on the compressional-wave first-motion polarities and compressional-to-shear wave amplitude ratios by applying the HASH program to the waveform cross correlation relocated earthquakes. Overall, the good-quality (defined by the HASH parameters) focal solutions are dominated by normal faulting in our study area, especially in the active Ka'ōiki and Hīlea seismic zones. Kīlauea caldera is characterized by a mixture of approximately equal numbers of normal, strike-slip, and reverse faults, whereas its south flank has slightly fewer strike-slip events. Our relocation and focal mechanism results will be useful for mapping the seismic stress and strain fields and for understanding the seismic-volcanic-tectonic relationships within the magmatic systems.

Hawai'i↗

USGS2007

Upper mantle anisotropy from teleseismic SKS splitting beneath Lützow-Holm Bay Region, East Antarctica

Investigations of SKS wave splitting of teleseismic events from digital seismographs recorded at eight stations around the Lützow-Holm Bay Region have lead to understanding the evolution of the Antarctic Plate. The observed delay times of SKS splitting are up to 1.3 s, which are generally equal to the global average. A two-layer model reveals that the lower layer anisotropy is caused by the recent asthenospheric flow, as compared with the Absolute Plate Motion by the HS3-NUVEL1 model. The upper layer anisotropy corresponds well to polarization of NE–SW convergence direction between East and West Gondwana in Pan-African age. We suggest that the upper layer anisotropy was formed during Pan-African orogeny and was possibly influenced by the preexisting structure during Gondwana break-up. The origin of anisotropy is the Lattice Preferred Orientation of olivine which was caused by both paleo-tectonic events and the recent asthenospheric flow.

Open-File Report↗

USGS1984

Computer analysis of digital well logs

A comprehensive system of computer programs has been developed by the U.S. Geological Survey for analyzing digital well logs. The programs are operational on a minicomputer in a research well-logging truck, making it possible to analyze and replot the logs while at the field site. The minicomputer also serves as a controller of digitizers, counters, and recorders during acquisition of well logs. The analytical programs are coordinated with the data acquisition programs in a flexible system that allows the operator to make changes quickly and easily in program variables such as calibration coefficients, measurement units, and plotting scales. The programs are designed to analyze the following well-logging measurements: natural gamma-ray, neutron-neutron, dual-detector density with caliper, magnetic susceptibility, single-point resistance, self potential, resistivity (normal and Wenner configurations), induced polarization, temperature, sonic delta-t, and sonic amplitude. The computer programs are designed to make basic corrections for depth displacements, tool response characteristics, hole diameter, and borehole fluid effects (when applicable). Corrected well-log measurements are output to magnetic tape or plotter with measurement units transformed to petrophysical and chemical units of interest, such as grade of uranium mineralization in percent eU3O8, neutron porosity index in percent, and sonic velocity in kilometers per second.

Circular↗

USGS1978

Atlantic walrus (Odobenus rosmarus rosmarus): A literature survey and status report

It is generally agreed that the genus Odobenus includes only one species, O. rosmarus . At least two subspecies are widely recognized: O. r. rosmarus , the Atlantic walrus, and O. r. divergens , the Pacific walrus. A third nominal subspecies, O. r. laptevi , the Laptev walrus, is designated by some Soviet researchers; and the taxonomic status of the Kara Sea walrus is undetermined. The range and abundance of nearly all walrus stocks have been seriously reduced by intensive human exploitation. The Atlantic walrus, the principal subject of this study, remains plentiful in only three known areas of concentration: northern Hudson Bay and northern Foxe Basin in the eastern Canadian Arctic, and the Thule district of northwest Greenland. This animal is no longer the object of large-scale commercial hunting, but is still subject to heavy subsistence hunting by native groups in some areas. The status of the walrus population in Canada was investigated in the 1950's and is believed to have changed little since that time. There are probably no more than about 10,000 walruses in Canadian waters, virtually all of them in the eastern arctic. The biology, ecology, and exploitation of walruses in the Thule district were studied during the 1940's, and some information is available concerning recent catch levels and hunting practices. The Polar Eskimos of north Greenland continue to organize much of their cultural and economic life around the hunting of walruses, and the species remains abundant in their area, numbering at least a few thousand. The walrus stocks off west Greenland and in the Greenland, Barents, and Kara seas are the most critically depleted. No systematic field investigations of walruses east of Greenland have been made since the Kara Sea and Franz Josef Land populations were studied in the mid-1930's. Opportunistic sighting records and compilations of historical catch information indicate that the herds of many tens of thousands that once inhabited Svalbard, Bear Island, Franz Josef Land, Novaya Zemlya, and other parts of the Eurasian Arctic have been very nearly extirpated. Measures to curb the international trade in walrus ivory and skins may benefit the Atlantic subspecies in Canada and north Greenland. Enforcement of existing regulations pertaining to hunting by aborigines, and perhaps the imposition of further restrictions on such activities, are necessary if the subspecies is to approach full recovery. Field studies in the northeast Atlantic and off west Greenland would help determine the current status of walruses and provide a better understanding of the requirements for their recovery in these areas. The Laptev walrus seems to have been reduced by overexploitation, but available information, most of it translated from Russian, is inconclusive. This subspecies was thought to number around 4,000 to 5,000 in 1975, and Soviet scientists have remarked on the need for a reduction in hunting pressure and protection of its habitat.

Wildlife Research Report↗

USGS2014

Quantifying fall migration of Ross's gulls (Rhodostethia rosea) past Point Barrow, Alaska

The Ross’s gull ( Rhodostethia rosea ) is a poorly known seabird of the circumpolar Arctic. The only place in the world where Ross’s gulls are known to congregate is in the near-shore waters around Point Barrow, Alaska, where they undertake an annual passage in late fall. Ross’s gulls seen at Point Barrow are presumed to originate from nesting colonies in Siberia, but neither their origin nor their destination has been confirmed. Current estimates of the global population of Ross’s gulls are based largely on expert opinion, and the only reliable population estimate is derived from extrapolations from previous counts conducted at Point Barrow, but these data are now over 25 years old. In order to update and clarify the status of this species in Alaska, our study quantified the timing, number, and flight direction of Ross’s gulls passing Point Barrow in 2011. We recorded up to two-thirds of the estimated global population of Ross’s gulls (≥27,000 individuals) over 39 days with numbers peaking on 16 October when we observed over 7,000 birds during a 3-h period.

Alaska↗

USGS2009

Microseismicity at the North Anatolian Fault in the Sea of Marmara offshore Istanbul, NW Turkey

The North Anatolian Fault Zone (NAFZ) below the Sea of Marmara forms a “seismic gap” where a major earthquake is expected to occur in the near future. This segment of the fault lies between the 1912 Ganos and 1999 İzmit ruptures and is the only NAFZ segment that has not ruptured since 1766. To monitor the microseismic activity at the main fault branch offshore of Istanbul below the Çınarcık Basin, a permanent seismic array (PIRES) was installed on the two outermost Prince Islands, Yassiada and Sivriada, at a few kilometers distance to the fault. In addition, a temporary network of ocean bottom seismometers was deployed throughout the Çınarcık Basin. Slowness vectors are determined combining waveform cross correlation and P wave polarization. We jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision. We observe seismicity rates of 20 events per month for M < 2.5 along the basin. The spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the Çınarcık Basin merge to one single master fault below ∼17 km depth. On the basis of a cross-correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations. Fault plane solutions have a predominant right-lateral strike-slip mechanism, indicating that normal faulting along this part of the NAFZ plays a minor role. Toward the west we observe increasing components of thrust faulting. This supports the model of NW trending, dextral strike-slip motion along the northern and main branch of the NAFZ below the eastern Sea of Marmara.

Istanbul↗

USGS2016

Paleomagnetic record determined in cores from deep research wells in the Quaternary Santa Clara basin, California

Paleomagnetic study of cores from six deep wells provides an independent temporal framework for much of the alluvial stratigraphy of the Quaternary basin beneath the Santa Clara Valley. This stratigraphy consists of 8 upward-fining cycles in the upper 300 m of section and an underlying 150 m or more of largely fine-grained sediment. The eight cycles have been correlated with the marine oxygen isotope record, thus providing one means of dating the section. The section has also proved to contain a rich paleomagnetic record despite the intermittent sedimentation characteristic of alluvial environments. Each well was designed to reach a depth of ~300 m, although 2 were terminated at shallower depth where bedrock was encountered and one (GUAD) was deepened to bedrock at 407.2 m. Cores were taken at intermittent intervals in most of the wells, composing ~20%–25% of their depths. In GUAD an attempt was made to core the entire upper 300 m, with core recovery of 201.8 m (67%). The paleomagnetic framework ranges from the 32 ka Mono Lake excursion near the top of the second sedimentary cycle to below the 780 ka Brunhes-Matuyama geomagnetic reversal beneath the eighth cycle. These ages nicely fit those assigned to the section based on correlation with the marine oxygen isotope record. Several episodes of anomalous magnetic inclinations were also found within the cyclic section in some of the wells. Some of the episodes of anomalous magnetic inclinations are only separated by short normal intervals in a pattern similar to that described for some welldocumented excursions. We consider that a geomagnetic excursion was likely only if the anomalous inclinations were found at approximately the same stratigraphic position in more than one drill hole. A deeper time constraint is provided by the upper boundary (990 ka) of the Jaramillo Normal Polarity Subchron recognized at a depth of 302 m in one deeply penetrating well (GUAD). Approximately 100 m of normal Jaramillo section is evident below that in wells GUAD and EVGR. The reversal that we identify as the 780 ka Brunhes-Matuyama boundary, found at depths of 291–303 m in three wells, indicates an average rate of deposition in this upper section of ~37 cm/k.y. In GUAD, the top of the underlying normally polarized section, which we assign to the upper part of the Jaramillo Normal Polarity Subchron, was found between 301.8 and 304.5 m. The resultant 10 m of reversed polarity section above the Jaramillo seems anomalously short for this 210 k.y. part of the Matuyama Chron, during which several times that thickness of section probably should have accumulated. This observation indicates that a significant unconformity should be present in that short section between the Jaramillo Subchron and the Brunhes-Matuyama boundary. Deeper cores in two wells (GUAD and EVGR) all have normal polarity and seem to represent much of the Jaramillo Subchron, although no base for that subchron was found. The resultant minimum rate of sedimentation for this lower section beneath the unconformity is 170 cm/k.y. The Mono Lake (ca. 32 ka), Pringle Falls (ca. 210 ka), and Big Lost (ca. 565 ka) geomagnetic excursions all seem to be represented in the Santa Clara Valley wells. Possible correlations to the Laschamp (ca. 40 ka) and Blake (ca. 110 ka) excursions are also noted. Three additional excursions that have apparently not been previously reported from western North America occur within cycle 6 (between 536 and 433 ka), near the base of cycle 5 (after 433 ka), and near the middle of cycle 2 (before ca. 75 ka).

California↗