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At least 991 records · Page 55Linked to original sources
USGS1990

Evolution of the Tertiary La Honda basin, central California

Tertiary strata of the La Honda basin are exposed in the Santa Cruz Mountains along the central California coast south of San Francisco. The basin fill has a composite thickness of more than 14,500 m and consists of sedimentary and volcanic rocks that in places rest on granitic basement rocks of the Salinia terrane. Paleogene strata are mainly turbidite sandstone and hemipelagic mudstone that accumulated in deep-sea fan and basin plain environments at lower bathyal to abyssal depths. Neogene rocks are mainly shallow-marine shelf sandstone and upper to middle bathyal siliceous mudstone. Both Paleogene and Neogene strata exhibit rapid lateral variations in thickness and facies, several local and regional unconformities, numerous folds, and ubiquitous faults. The complicated geology and geologic history of the La Honda basin reflect the fact that, throughout its history, the basin has been located at or near the tectonically active plate boundary between the North American continent and various oceanic plates of the Pacific basin. The La Honda basin originated during the Paleocene, perhaps during an episode of wrench tectonism associated with oblique subduction and arrival of the Salinia terrane. Major restructuring of the basin during the Oligocene-including uplift and erosion of the basin margins, movement along the Zayante-Vergeles fault, and deposition of two sand-rich deep-sea fans–apparently resulted from the approach of the Farallon-Pacific spreading ridge and its collision with the California continental margin. During the late Oligocene and early Miocene, widespread volcanism and marine transgression accompanied an episode of regional transtension along the San Andreas fault system. Deposition of shallow-marine sandstones and deeper-water siliceous mudstones occurred during much of the Miocene and Pliocene but was interrupted at least three times by brief episodes of uplift and erosion associated with transpressional wrench tectonism along the San Andreas fault. Marine deposition ended and uplift of the modern Santa Cruz Mountains began during the late Pliocene in response to the most-recent episode of regional transpression. Five small oil fields in the La Honda basin have produced a total of 1.7 million barrels of oil and 300 million cubic feet of gas, mostly from reservoirs in Eocene turbidite sandstone and Miocene limestone.

Conference Paper↗
USGS2019

Channel modification and evolution alter hydraulic connectivity in the Atchafalaya River basin increasing vulnerability to sea-level rise

Channel dredging and erosion in the Atchafalaya River basin have resulted in changes to the hydraulic connectivity of this floodplain swamp that have not been previously quantified. In this study, analyses were conducted to determine hydraulic and geomorphic factors that have changed since channel closure in 1962. Results indicated changes occurred in the Atchafalaya main channel cross-section between 1962 and 2010, and hydraulic and geomorphic changes were detected in portions of the interior eastern basin floodplain. Analyses of hydrographs in relation to floodplain elevations indicated that there was a lack of mineral sediment deposition sufficient to offset subsidence and rising sea level. This deficit has resulted in extended hydroperiods over the floodplain which could prevent tree regeneration and promote hypoxia.

Louisiana↗
USGS2013

Tectono-magmatic evolution and distribution of porphyry Cu systems in the Central Tethys Region of Turkey, the Caucasus, Iran, and southern Pakistan

Recent compilation of geodynamic, geochemical, geochronologic, and ore deposits data provided an opportunity to review the continental margin, intra-oceanic, and post-collisional tectonic settings in the Central Tethys Region. These settings formed during sequential rifting of microcontinents from the passive margin of Gondwana, their northward transport across the Neo-Tethys Ocean, and their collision with the active margin of Eurasia.

Tethys region↗
USGS2022

Evolution of design ground motions in California: NEHRP 2009 to 2020

The U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) is used in construction codes, such as the National Earthquake Hazard Reduction Program (NEHRP) Provisions, to develop ground motions for structural and geotechnical design. When the NSHM is updated (e.g. changes to its earthquake rupture forecast or ground motion models), or the manner in which it is implemented in construction codes changes, design ground motion values can increase or decrease at a given location. This study presents an analysis of 21 sites in California to characterize how design ground motion values (SS, S1, SMS, SM1) have changed from the 2009 to 2015 to 2020 NEHRP Provisions. Four sites are presented in greater detail: two in Southern California and two in Northern California. The results show that significant changes in design ground motion values between NEHRP releases are explained by major changes in the NSHM science (e.g., inclusion of basin effects) and its use (e.g., for a broader range of site classes), but in ways that are difficult to ascertain beforehand. It is therefore likely that design ground motion values may continue to evolve between NSHM and construction code updates as the earthquake science and engineering field's understanding continues to advance.

California↗