91 research outputs found
THE ROLE OF FRICTIONAL STRENGTH
[1] At a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc. In this paper, we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins. In numerical models, we vary thickness of a serpentinized layer in the mantle wedge (15 to 25 km) and the frictional strength of both the sediments and serpentinized mantle (friction angle 1° to 15°, or static friction coefficient 0.017 to 0.27) to control the amount of frictional coupling between the plates. With plastic strain weakening in the lithosphere, our numerical models can attain stable subduction geometry over millions of years. We find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths. In the case of low sediment and serpentinite friction, the subduction interface has a shallow dip, while the subduction zone develops an accretionary prism, a broad forearc high, a deep forearc basin, and a shallow trench. In the high friction case, the subduction interface is steep, the trench is deeper, and the accretionary prism, forearc high and basin are all absent. The resultant free-air gravity and topographic signature of these subduction zone models are consistent with observations. We believe that the low-friction model produces a geometry and forearc structure similar to that of accretionary margins. Conversely, models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin. We find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones
Genetic Relations Between the Aves Ridge and the Grenada Back-Arc Basin, East Caribbean Sea
The Grenada Basin separates the active Lesser Antilles Arc from the Aves Ridge, described as a CretaceousâPaleocene remnant of the âGreat Arc of the Caribbean.â Although various tectonic models have been proposed for the opening of the Grenada Basin, the data on which they rely are insufficient to reach definitive conclusions. This study presents, a large set of deepâpenetrating multichannel seismic reflection data and dredge samples acquired during the GARANTI cruise in 2017. By combining them with published data including seismic reflection data, wideâangle seismic data, well data and dredges, we refine the understanding of the basement structure, depositional history, tectonic deformation and vertical motions of the Grenada Basin and its margins as follows: (1) rifting occurred during the late Paleoceneâearly Eocene in a NWâSE direction and led to seafloor spreading during the middle Eocene; (2) this newly formed oceanic crust now extends across the eastern Grenada Basin between the latitude of Grenada and Martinique; (3) asymmetrical preâMiocene depocenters support the hypothesis that the southern Grenada Basin originally extended beneath the presentâday southern Lesser Antilles Arc and probably partly into the presentâday forearc before the late OligoceneâMiocene rise of the Lesser Antilles Arc; and (4) the Aves Ridge has subsided along with the Grenada Basin since at least the middle Eocene, with a general subsidence slowdown or even an uplift during the late Oligocene, and a sharp acceleration on its southeastern flank during the late Miocene. Until this acceleration of subsidence, several bathymetric highs remained shallow enough to develop carbonate platforms
IODP Expedition 334: An Investigation of the Sedimentary Record, Fluid Flow and State of Stress on Top of the Seismogenic Zone of an Erosive Subduction Margin
The Costa Rica Seismogenesis Project (CRISP) is an
experiment to understand the processes that control nucleation
and seismic rupture of large earthquakes at erosional
subduction zones. Integrated Ocean Drililng Program
(IODP) Expedition 334 by R/V JOIDES Resolution is the first
step toward deep drilling through the aseismic and seismic
plate boundary at the Costa Rica subduction zone offshore
the Osa Peninsula where the Cocos Ridge is subducting
beneath the Caribbean plate. Drilling operations included
logging while drilling (LWD) at two slope sites (Sites U1378
and U1379) and coring at three slope sites (Sites U1378â1380)
and at one site on the Cocos plate (Site U1381). For the first
time the lithology, stratigraphy, and age of the slope and
incoming sediments as well as the petrology of the subducting
Cocos Ridge have been characterized at this margin.
The slope sites recorded a high sediment accumulation rate
of 160â1035m m.y.-1 possibly caused by on-land uplift triggered
by the subduction of the Cocos Ridge. The geochemical
data as well as the in situ temperature data obtained at
the slope sites suggest that fluids are transported from
greater depths. The geochemical profiles at Site U1381 reflect
diffusional communication of a fluid with seawater-like
chemistry and the igneous basement of the Cocos plate
(Solomon et al., 2011; Vannucchi et al., 2012a). The
present-day in situ stress orientation determined by borehole
breakouts at Site U1378 in the middle slope and Site
U1379 in the upper slope shows a marked change in stress
state within ~12 km along the CRISP transect; that may
correspond to a change from compression (middle slope) to
extension (upper slope)
Locked and loading megathrust linked to active subduction beneath the Indo-Burman Ranges
The Indo-Burman mountain rangesmarkthe boundary between the Indian and Eurasian plates, north of the SumatraâAndaman subduction zone. Whether subduction still occurs along this subaerial section of the plate boundary, with 46mm/yr of highly oblique motion, is contentious. About 21mm/yr of shear motion is taken up along the Sagaing Fault, on the eastern margin of the deformation zone. It has been suggested that the remainder of the relative motion is taken up largely or entirely by horizontal strike-slip faulting and that subduction has stopped. Here we present GPS measurements of plate motions in Bangladesh, combined with measurements from Myanmar and northeast India, taking advantage of a more than 300 km subaerial accretionary prism spanning the Indo-Burman Ranges to the GangesâBrahmaputra Delta. They reveal 13â17mm/yr of plate convergence on an active, shallowly dipping and locked megathrust fault. Most of the strike-slip motion occurs on a few steep faults, consistent with patterns of strain partitioning in subduction zones. Our results strongly suggest that subduction in this region is active, despite the highly oblique plate motion and thick sediments. We suggest that the presence of a locked megathrust plate boundary represents an underappreciated hazard in one of the most densely populated regions of the world
Mantle wedge hydration in Nicaragua from Local Earthquake Tomography
The continental margin of Nicaragua and Costa Rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition, a joint on- and offshore local earthquake tomography was performed. LowâP-wave velocities and highâVp/Vsâratios, indicative for hydration, were found in the upper-mantle and lowermost crust beneath the Sandino Basin. The mantle wedge hydration can be estimated to 2.5 wt. per cent beneath south Nicaragua. In contrast, the mantle wedge beneath north Costa Rica is weakly or not hydrated. The hydration leads to a local gap in the seismicity in Nicaragua. The lateral transition between the hydrated and non-hydrated areas occurs within a distance of about 10 km. This transition coincides with a change in the crustal thickness in the order of 5â10 km, thickening to the south, and in the tectonic regimes. The change in the tectonic regimes towards a stronger extension along the margin of Nicaragua could be the key for understanding the observations: the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of lowâVp, highâVp/Vsâand low seismicity
IODP expedition 334: An investigation of the sedimentary record, fluid flow and state of stress on top of the seismogenic zone of an erosive subduction margin
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