Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.

G r a v i t y a n d p o t e n t i a l f i e l d s 892 The Leading Edge August 2013 SPECIAL SECTION: G r a v i t y a n d p o t e n t i a l f i e l d s Toward 1-mGal accuracy in global marine gravity from CryoSat-2, Envisat, and Jason-1 M ore than 60% of the Earth's land and shallow marine areas are covered by > 2 km of sediments and sedimentary rocks, with the thickest accumulations on rifted continental margins (Figure 1). Free-air marine gravity anomalies derived from Geosat and ERS-1 satellite altimetry (Fairhead et al., 2001; Sandwell and Smith, 2009; Andersen et al., 2009) outline most of these major basins with remarkable precision. Moreover, gravity and bathymetry data derived from altimetry are used to identify current and paleo-submarine canyons, faults, and local recent uplifts. These geomorphic features provide clues to where to look for large deposits of sediments. While current altimeter data delineate large offshore basins and major structures, they do not resolve some of the smaller geomorphic features and basins (Yale et al., 1998; Fairhead et al., 2001). Improved accuracy and resolution is desirable: to facilitate comparisons between continental margins; as an exploration tool and to permit extrapolation of known structures from well-surveyed areas; to follow fracture zones out of the deep-ocean basin into antecedent continental structures, to define and compare segmentation of margins along strike and identify the position of the continent-ocean boundary; and to study mass anomalies (e.g., sediment type and distribution) and isostatic compensation at continental margins. In this article, we assess the accuracy of a new global marine gravity model based on a wealth of new radar altimetry data and demonstrate that these gravity data are superior in quality to the majority of publicly available academic and government ship gravity data. New radar altimeter data Gravity field accuracy depends on four factors: spatial track density, altimeter range precision, diverse track orientation, and the accuracy of the coastal tide models. Recently three new nonrepeat altimeter data sets have become available: 1) CryoSat-2 was successfully launched in February 2010 and has routinely collected altimetry data over ice, land, and ocean since July 2010 (Wingham et al., 2006). The satellite has a long 369-day repeat cycle resulting in an average ground track spacing of 3.5 km at the equator. 2) The Envisat satellite, which has been in continuous operation since 2002, was running low on the fuel needed for maintaining a repeating ground track. In October 2010, Envisat was placed in a new partly drifting-phase orbit (~30 day repeat) to conserve fuel. Although the spacecraft failed in April 2012, it was able to collect 1.5 years of data along this new ground track. These data combined with 97 repeat cycles in the 35-day ground track make a significant

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SUMMARY Flexure and fracturing of the seafloor on the outer trench wall of subduction zones reflect bending of the lithosphere beyond its elastic limit. To investigate these inelastic processes, we have developed a full nonlinear inversion approach for estimating the bending moment, curvature and outer trench wall fracturing using shipboard bathymetry and satellite altimetry-derived gravity data as constraints. Bending moments and downward forces are imposed along curved trench axes and an iterative method is used to calculate the nonlinear response for 26 sites in the circum-Pacific region having seafloor age ranging from 15 to 148 Ma. We use standard thermal and yield strength envelope models to develop the nonlinear moment versus curvature relationship. Two coefficients of friction of 0.6 and 0.3 are considered and we find that the lower value provides a better overall fit to the data. The main result is that the lithosphere is nearly moment saturated at the trench axis. The effective elastic thickness of the plate on the outer trench slope is at least three times smaller than the elastic thickness of the plate before bending at the outer rise in agreement with previous studies. The average seafloor depth of the unbent plate in these 26 sites matches the Parsons & Sclater depth versus age model beyond 120 Ma. We also use the model to predict the offsets of normal faults on the outer trench walls and compare this with the horst and graben structures observed by multibeam surveys. The model with the lower coefficient of friction fits the fault offset data close to the trench axis. However, the model predicts significant fracturing of the lithosphere between 75 and 150 km away from the trench axis where no fracturing is observed. To reconcile these observations, we impose a thermoelastic pre-stress in the lithosphere prior to subduction. This pre-stress delays the onset of fracturing in better agreement with the data.

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