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H. Gary Greene

Publications and source records attributed to H. Gary Greene.

At least 55 records · Page 3Linked to original sources
USGS2005

Neotectonics of the offshore Oak Ridge fault near Ventura, southern California

The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200-400 ka ago and that the fault tip is buried by ??? 1 km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reflection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reflection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.

Bulletin of the Seismological Society of America↗
USGS1993

Neogene folding and faulting in southern Monterey Bay, Central California, USA

The goal of this study was to determine the Neogene structural history of southern Monterey Bay by mapping and correlating the shallow tectonic structures with previously identified deeper occurring structures. Side scan sonographs and Uniboom seismic reflection profiles collected in the region suggest that deformation associated with both compressional and transcurrent movement is occurring. Strike-slip movement between the North American and Pacific plates started as subduction ceased 21 Ma, creating the San Andreas fault system. Clockwise rotation of the Pacific plate occurred between 3.4 and 3.9 Ma causing orthogonal convergence between the two plates. This plate rotation is responsible for compressional Neogene structures along the central California coast. Structures exhibit transpressional tectonic characteristics such as thrust faulting, reverse faulting and asymmetrical folding. Folding and faulting are confined to middle Miocene and younger strata. Shallow Mesozoic granitic basement rocks either crop out or lie near the surface in most of the region and form a possible décollement along which the Miocene Monterey Formation has decoupled and been folded. Over 50% of the shallow faults strike normal (NE-SW) to the previously identified faults. Wrench fault tectonics complicated by compression, gradual uplift of the basement rocks, and a change in plate convergence direction are responsible for the observed structures in southern Monterey Bay.

Marine Geology↗
USGS1992

Geology of the d'Entrecasteaux-New Hebrides arc collision zone: Results from a deep submersible survey

During the SUBPSO1 cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-are slope. In this report we use diving observations and samples, as well as dredging results, to analyse the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the mechanisms of scamount subduction. These data indicate that the Bougainville guyot is a middle Eocene island arc volcano capped with reef limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antiform that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the flat top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining.

Tectonophysics↗
USGS1991

Offshore and onshore liquefaction at Moss Landing spit, central California, - result of the October 17, 1989, Loma Prieta earthquake

As a result of the October 17, 1989, Loma Prieta (Santa Cruz Mountains, California) earthquake, liquefaction of the fluvial, estuarine, eolian, and beach sediments under a sand spit destroyed the Moss Landing Marine Laboratories and damaged other structures and utilities. Initial studies suggested that the liquefaction was a local phenomenon. More detailed offshore investigations, however, indicate that it occurred over a large area (max. 8 km 2 ) during or shortly after the earthquake with movement of unconsolidated sediment toward and into the head of Monterey submarine canyon. This conclusion is supported by side-scan sonographs, high-resolution seismic-reflection and bathymetric profiles, onshore and sea-floor photographs, and underwater video tapes. Many distinct lobate features were identified on the shallow shelf. These features almost certainly were the result of the October 17 earthquake; they were subsequently destroyed by winter storms. In addition, fresh slump scars and recently dislodged mud debris were found on the upper, southern wall of Monterey submarine canyon.

Geology↗
USGS1991

Tectonic evolution of submarine canyons along the California continental margin

The development of submarine canyons along active-plate margins commonly is influenced by tectonic processes. Recent studies of submarine canyons along the transform margin of western North America show that the origin and subsequent evolution of many canyons are correlatable with plate motion and plate-margin deformation. Elements of canyon morphology such as bends and meanders commonly are controlled by faults and folds that are relatable to the structural fabric of the continental shelf and slope. Some canyon heads that appear to be displaced from their lower reaches are explainable as the result of movement along strike-slip faults associated with the plate margin. Many submarine canyons along the California margin are not associated with large rivers and thus may owe their origins either to pre-Holocene fluvial or structural processes. Some modern canyons appear to be associated with pre-Pleistocene ancestral canyons. Because of both vertical and horizontal tectonic movements during the past 20 Ma, some California submarine canyons have been repeatedly filled and exhumed; the most recent exhumation began during the latest lowstand of sea level and continues today. Canyons that today have their upper reaches on the continental slope or outermost shelf, distant from large rivers or other sources of sediment supply, commonly appear to have been laterally displaced along offshore faults. Palinspastic reconstructions along these faults commonly reveal a genetic relation between such canyons and canyons heading nearshore, from which they were offset. For example, detailed studies of the Ascension-Monterey Submarine Canyon system in Monterey Bay suggest that several smaller canyons on the outer shelf and upper slope have been displaced northwestward from the headward part of Monterey Canyon by right slip along offshore faults of the Palo Colorado-San Gregorio, Ascension, and Monterey Bay fault zones. Many other canyons on the California margin have developed along, or had their courses abruptly altered by, structural zones, owing either to canyon cutting along a zone of weakness or to fault displacement. Mass wasting associated with zones of faulting and slumping, which may have been seismically induced, also may affect canyon form. Clearly, submarine canyons along the California margin commonly owe their origin and morphologic development to influences other than fluvial erosion during sea-level lowstands. A chief influence has been the San Andreas fault system.

California↗
USGS1988

Acoustic stratigraphy and hydrothermal activity within Epi Submarine Caldera, Vanuatu, New Hebrides Arc

Geological and geophysical surveys of active submarine volcanoes offshore and southeast of Epi Island, Vanuatu, New Hebrides Arc, have delineated details of the structure and acoustic stratigraphy of three volcanic cones. These submarine cones, named Epia, Epib, and Epic, are aligned east-west and spaced 3.5 km apart on the rim of a submerged caldera. At least three acoustic sequences, of presumed Quaternary age, can be identified on single-channel seismic-reflection profiles. Rocks dredged from these cones include basalt, dacite, and cognate gabbroic inclusions with magmatic affinities similar to those of the Karua (an active submarine volcano off the southeastern tip of Epi) lavas. ?? 1988 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1986

Ascension Submarine Canyon, California - Evolution of a multi-head canyon system along a strike-slip continental margin

Ascension Submarine Canyon, which lies along the strike-slip (transform) dominated continental margin of central California, consists of two discrete northwestern heads and six less well defined southeastern heads. These eight heads coalesce to form a single submarine canyon near the 2700 m isobath. Detailed seismic stratigraphic data correlated with 19 rock dredge hauls from the walls of the canyon system, suggest that at least one of the two northwestern heads was initially eroded during a Pliocene lowstand of sea level ∼3.8 m.y. B.P. Paleogeographic reconstructions indicate that at this time, northwestern Ascension Canyon formed the distal channel of nearby Monterey Canyon and has subsequently been offset by right-lateral, strike-slip faulting along the San Gregorio fault zone. Some of the six southwestern heads of Ascension Canyon may also have been initially eroded as the distal portions of Monterey Canyon during late Pliocene-early Pleistocene sea-level lowstands (∼2.8 and 1.75 m.y. B.P.) and subsequently truncated and offset to the northwest. There have also been a minimum of two canyon-cutting episodes within the past 750,000 years, after the entire Ascension Canyon system migrated to the northwest past Monterey Canyon. We attribute these late Pleistocene erosional events to relative lowstands of sea level 750,000 and 18,000 yrs B.P. The late Pleistocene and Holocene evolution of the six southeastern heads also appears to have been controlled by structural uplift of the Ascension-Monterey basement high at the southeastern terminus of the Outer Santa Cruz Basin. We believe that uplift of this basement high sufficiently oversteepened submarine slopes to induce gravitational instability and generate mass movements that resulted in the erosion of the canyon heads. Most significantly, though, our results and interpretations support previous proposals that submarine canyons along strike-slip continental margins can originate by tectonic trunction and lateral offset.

California↗