The hotspot-generated Louisville Ridge is a 4000-km chain of seamounts (typically 2-2.5 km high and 10-40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26øS) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zoneshow a west dipping subsurface platform 2-3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1-2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestdchtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount. The principal effects of hotspot-ridge collision with a sediment-starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell,(3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km 3 of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc-derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the -37,000 km 3 of material which fills subducting grabens each million years is arc-derived volcanic and pelagic sediment. Dupont, J., Morphologie et structures superficielles de l'arc insulaire des Tonga-Kermadec, in Contribution a l'EtudeGeodynamique du Sud-Ouest Pacifique, vol. 147, pp. 263-282, Office de la Recherche Scientifique et Technique Outre-Mer, Paris, 1982. Dupont, J., and R.H. Herzer, Effect of subduction of the Louisville Ridge on the structure and morphology of the Tonga Origins of nonvolcanic seamounts in a forearc environment, in Seamounts, Islands and 93, 3078-3104, 1988. McCann, W.R., and R.E. Haberman, Morphologic and geologic effects of the subduction of bathymetric highs, Pure Appl. Geophys., in press, 1989. McCarthy, J., and D.W. Scholl, Mechanisms of subduction accretion along the central Aleutian Trench, Geol. Soc. Leg 91 Scientific Party, Tectonic evolution of the southwestern tropical Pacific basin (abstract), Eos Trans AGU, 64, 315, 1983. of Leg 91 basalts and...
Advances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea-floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high-resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon-channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent-shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea-floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long-wave, upstream-migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude-supercritical over the lee side of the bedform and Froude-subcritical over the stoss side. Numerical modelling and seismic-reflection imagery support an interpretation of weakly asymmetrical to near-symmetrical aggradation of predominantly fine-grained net-depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, protoSan Mateo channel comprises a series of relatively coarse-grained net-erosional cyclic steps, which nucleated out of sea-floor perturbations across the tectonically active lower slope. Thus, the interaction between turbiditycurrent processes and sea-floor perturbations appears to be fundamentally important to channel initiation, particularly in high-gradient systems. Offshore of southern California, and in analogous deep-water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.
Using older and in part fl awed data, Ruff (1989) suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700-1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thicksediment trenches, they account for ~53% of instrumental era IPTs ≥Mw8.0, ~75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts >Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0-9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean fl oor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of highmagnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.
Mobile Bay is a wide (25–50 km), shallow (3 m), highly stratified estuary on the Gulf coast of the United States. In May 1991 a series of instruments that measure near‐surface and near‐bed current, temperature, salinity, and middepth pressure were deployed for a year‐long study of the bay. A full set of measurements were obtained at one site in the lower bay; all but current measurements were obtained at a midbay site. These observations show that the subtidal currents in the lower bay are highly sheared, despite the shallow depth of the estuary. The sheared flow patterns are partly caused by differential forcing from wind stress and river discharge. Two wind‐driven flow patterns actually exist in lower Mobile Bay. A barotropic response develops when the difference between near‐surface and near‐bottom salinity is less than 5 parts per thousand. For stronger salinity gradients the wind‐driven currents are larger and the response resembles a baroclinic flow pattern. Currents driven by river flows are sheared and also have a nonlinear response pattern. Only near‐surface currents are driven seaward by discharges below 3000 m3/s. At higher discharge rates, surface current variability uncouples from the river flow and the increased discharge rates drive near‐bed current seaward. This change in the river‐forced flow pattern may be associated with a hydraulic jump in the mouth of the estuary.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.