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Luttrell, K, Sandwell D.  2012.  Constraints on 3-D stress in the crust from support of mid-ocean ridge topography. Journal of Geophysical Research-Solid Earth. 117   10.1029/2011jb008765   AbstractWebsite

The direction of crustal stresses acting at mid-ocean ridges is well characterized, but the magnitude of these stresses is poorly constrained. We present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity. The topography is divided into a short-wavelength portion, created by rifting, magmatism, and transform faulting, and a long-wavelength portion associated with the cooling and subsidence of the oceanic lithosphere. The short-wavelength surface and Moho topography are used to calculate the spatially varying 3-D stress tensor in the crust by assuming that in creating this topography, the deviatoric stress reached the elastic-plastic limiting stress; the Moho topography is constrained by short-wavelength gravity variations. Under these assumptions, an incompressible elastic material gives the smallest plastic failure stress associated with this topography. This short-wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults. However, the addition of an in-plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms. By adjusting the magnitude of the regional stress, we determine a lower bound for in situ ridge-perpendicular extension of 25-40 MPa along the slow spreading mid-Atlantic ridge, 40-50 MPa along the ultra-slow spreading ridges in the western Indian Ocean, and 10-30 MPa along the fast spreading ridges of the southeastern Indian and Pacific Oceans. Furthermore, we constrain the magnitude of ridge-parallel extension to be between 4 and 8 MPa in the Atlantic Ocean, between -1 and 7 MPa in the western Indian Ocean, and between -1 and 3 MPa in the southeastern Indian and Pacific Oceans. These observations suggest that a deep transform valley is an essential feature of the ridge-transform spreading center.

Sandwell, DT, Smith WHF.  2009.  Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge segmentation versus spreading rate. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006008   AbstractWebsite

Three approaches are used to reduce the error in the satellite-derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS-1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50-80 km long while it increases dramatically at higher rates (100-1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.