72 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
Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data
Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as ‘opportunistic’ since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts
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 U1378–1380)
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 160–1035m 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)
Dynamical effects of subducting ridges: Insights from 3-D laboratory models
We model the subduction of buoyant ridges and plateaus to study their effect
on slab dynamics. Oceanic ridges parallel to the trench have a stronger effect
on the process of subduction because they simultaneously affect a longer trench
segment. Large buoyant slab segments sink more slowly into the asthenosphere,
and their subduction result in a diminution of the velocity of subduction of
the plate. We observe a steeping of the slab below those buoyant anomalies,
resulting in smaller radius of curvature of the slab, that augments the energy
dissipated in folding the plate and further diminishes the velocity of
subduction. When the 3D geometry of a buoyant plateau is modelled, the dip of
the slab above the plateau decreases, as a result of the larger velocity of
subduction of the dense "normal" oceanic plate on both sides of the plateau.
Such a perturbation of the dip of the slab maintains long time after the
plateau has been entirely incorporated into the subduction zone. We compare
experiments with the present-day subduction zone below South America.
Experiments suggest that a modest ridge perpendicular to the trench such as the
present-day Juan Fernandez ridge is not buoyant enough to modify the slab
geometry. Already subducted buoyant anomalies within the oceanic plate, in
contrast, may be responsible for some aspects of the present-day geometry of
the Nazca slab at depth
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 Sumatra–Andaman 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 Ganges–Brahmaputra Delta. They reveal 13–17mm/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
Synchronization of growth branching and flowering processes in the south american tropical tree Cecropia obtusa (Cecropiaceae)
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