Abstract:
Plate motion is a remarkable Earth process and is widely ascribed to two primary driving forces: slab pull and ridge push. With the release of the first- and second-order stress fields since 1989, a few features of tectonic stresses provide strong constrain on these forces. The observed stresses are mainly distributed on the uppermost brittle part of the lithosphere. A modeling analysis, however, reveals that the stress produced by ridge push is dominantly distributed in the lower part of the lithosphere; Doglioni and Panza recently made an in-depth investigation on slab pull and found this force cannot be in accordance with observations. These findings of ridge push and slab pull suggest that there needs other force to be responsible for plate motion and tectonic stress. Here, we propose that the pressure of deep ocean water against the wall of continent yields enormous force (i.e., ocean-generated force) on the continent. The continent is fixed on the top of the lithosphere, this attachment allows ocean-generated force to be laterally transferred to the lithospheric plate. We show that this force may combine other forces to form force balances for the lithospheric plate, consequently, the African, Indian, South American, Australian, and Pacific plates obtain a movement of 4.52, 6.09, 2.11, 3.52, and 6.62 cm/yr, respectively. A torque balance modelling shows that the error between the movements calculated for 121 sample locations and the movements extracted from GSRM v.2.1 is less than 0.8 mm/yr in speed and 0.3o in azimuth.Keywords:
Ridge push
Slab
Seafloor Spreading
Classification of discontinuities
Ridge push
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This paper develops new absolute models of plate motion relative to ocean ridges: PRF-2000VEL model with respect to the fixed Pacific Ridge, SRF-2000VEL model with respect to Southern mid-Atlantic ridge and NRF-I2000VEL model with respect to Northern mid-Atlantic ridge, analyze and compare the absolute motions of lithospheres relative to Pacific Ridge, Southern mid\|Atlantic and Northern mid-Atlantic Ridge, respectively, which shows the Eastern Pacific mid-ridge and the mid-Atlantic Ridge is extending at the even 10.9mm/a rate.
Mid-Atlantic Ridge
Ridge push
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Linear analytical solutions for bottom-trapped subinertial oscillatory flow over simple ridge topographies in a stratified (two-layer) rotating fluid are presented. Results are compared to moored current meter observations of bottom-intensified motions over the Endeavour Segment of Juan de Fuca Ridge in the northeast Pacific. The solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest. For a given internal deformation radius, the degree of current amplification increases with increased bottom slope, ridge height, and oscillation frequency. Amplification decreases with increased width of the ridge relative to the deformation radius.
Crest
Clockwise
Ridge push
Oscillation (cell signaling)
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Global observations of mid‐ocean ridge (MOR) bathymetry demonstrate an asymmetry in axial depth across ridge offsets that is correlated with the direction of ridge migration. Motivated by these observations, we have developed two‐dimensional numerical models of asthenospheric flow and melting beneath a migrating MOR. The modification of the flow pattern produced by ridge migration leads to an asymmetry in melt production rates on either side of the ridge. By coupling a simple parametric model of three dimensional melt focusing to our simulations, we generate predictions of axial depth differences across offsets in the MOR. These predictions are quantitatively consistent with the observed asymmetry.
Ridge push
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When seawater, that penetrates the lithosphere through faults and fractures at mid-ocean ridges, gets in contact with mantle rocks serpentine may form. Serpentine bearing rocks are considerably weaker than their source rock thereby causing a drastic change in the rheological strength of the affected lithosphere. Serpentinization is limited by temperature and the availability of active fluid bearing faults. Its maximum depth was previously considered not to exceed 4 km beneath the sea floor.
Yield strength envelopes (YSE) represent vertical profiles that predict the maximum stress supported by the lithosphere as a function of depth. We calculated YSEs for the axial lithosphere at an amagmatic Southwest Indian Ridge segment for different geotherms, serpentinization depths and mineralogical compositions in the ductile regime. Assuming the earthquake distribution is somehow linked to the rheological strength profile we then interpreted those YSEs that best correlate with the depth frequency distribution of local earthquakes. By doing so we could constrain the thermals structure, the mineralogical compositions and the deformation mode in the lithosphere. The YSEs show a thick mechanical lithosphere (30–35 km) at the ridge axis that is weakened in its uppermost 8-13 km due to serpentinization. Incorporating the axial morphology we propose a distinct mode of deformation that may also be applicable to other magma starved ultraslow spreading mid ocean ridge segments. Here, deformation and lithospheric accretion are essentially governed by deep reaching boundary faults that are well lubricated and hence aseismic due to extensive, deep-reaching serpentinization.
Ridge push
Lithospheric flexure
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Pacific Plate
Overpressure
Ridge push
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Abstract Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0–80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, φ, with SKS δt that are much longer (1.7–3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.
Seafloor Spreading
Ridge push
Mid-Atlantic Ridge
Neutral buoyancy
Seismometer
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Ultra-slow spreading ridges such as the South West Indian ridge or the Arctic ridge system are oddities amongst oceanic ridges. Conversely to faster oceanic ridges, petrographic and seafloor studies have shown that they are characterized by low melt supply and present low crustal thicknesses and heat flow; these features are interpreted as an evidence for a cooler sublithospheric mantle. In cartoonish sketches of plate tectonics, ridges open above upwellings, subduction zones occur over downwellings, and plates are riding over the mantle convection cells. In this study, we designed a simple yet dynamically consistent thermal convection model to test the impact of farfield forces on spreading ridges and show that this pattern is disrupted by plate tectonics. In particular, continental collisions modulate the spreading rates because resisting forces build up at plate boundaries. As a consequence, this modifies the surface boundary conditions and therefore the underlying mantle flow. We show that the ideal convection cell pattern quickly breaks down when plate motion is impeded by continental collisions in the far field. Not only the decreasing spreading rates are diagnostic, but in the same time, (i) the heat flow is decreasing at the ridge, (ii) the thermal structure of the cooling lithosphere no longer matches the cooling half-space model, and (iii) the mantle temperature beneath the ridge drops by more than 100 degrees. We compare our model predictions to available observables and show that this simple mechanism explains the atypical thermo-mechanical evolution of the South West Indian ridge and Arctic ridge system. Last, the recent S wave seismic tomography model of Debayle and Ricard (2012) reveals that only away from those two ridges does lithospheric thickening departs from the half-space cooling model, in accord with our model predictions.
Hotspot (geology)
Seafloor Spreading
Ridge push
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[1] Global observations of mid-ocean ridge (MOR) bathymetry demonstrate an asymmetry in axial depth across ridge offsets that is correlated with the direction of ridge migration. Motivated by these observations, we have developed two-dimensional numerical models of asthenospheric flow and melting beneath a migrating MOR. The modification of the flow pattern produced by ridge migration leads to an asymmetry in melt production rates on either side of the ridge. By coupling a simple parametric model of three dimensional melt focusing to our simulations, we generate predictions of axial depth differences across offsets in the MOR. These predictions are quantitatively consistent with the observed asymmetry.
Ridge push
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