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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.

Levitt, DA, Sandwell DT.  1996.  Modal depth anomalies from multibeam bathymetry: Is there a south Pacific superswell? Earth and Planetary Science Letters. 139:1-16.   10.1016/0012-821x(95)00247-a   AbstractWebsite

A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called 'South Pacific Superswell' are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close. Depth-age results indicate that there is only a small anomaly (< 200 m) over 15-35 Ma Pacific Plate seafloor with little age-dependent shallowing, suggesting that the lithosphere east of the main hot-spot locations on the 'superswell' is normal. An important implication is that, in sparsely surveyed areas, depths from ETOPO-5 are significantly different from true depths even at large scales (similar to 1000 km) and thus are unsuitable for investigations of anomalies associated with depth-age regressions. We find that seafloor slopes on conjugate profiles of the Pacific and Nazca Plates from 15 to 35 Ma are both slightly lower than normal, but are within the global range. Proximate to the SEPR, seafloor slopes are very low (218 m Myr(-1/2)) on the Pacific Plate (0-22 Ma) and slightly high (similar to 410 m Myr(-1/2)) on the Nazca Plate (0-8 Ma); slopes for older Pacific seafloor (22-37 Ma) are near normal (399 m Myr(-1/2)). Seafloor slopes are even lower north of the Marquesas Fracture Zone but are highly influenced by the Marquesas Swell. We find that the low subsidence rate on young Pacific seafloor cannot be explained by a local hot-spot or a small-scale convective model exclusively and a stretching/thickening model requires implausible crustal thickness variation (similar to 30%).

Schubert, G, Sandwell DT.  1995.  A Global Survey of Possible Subduction Sites on Venus. Icarus. 117:173-196.   10.1006/icar.1995.1150   AbstractWebsite

About 10,000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on Venus. All the sites have narrow deep trenches elongate along strike with arcuate planforms, ridge-trench-outer rise topographic profiles typical of terrestrial subduction zones, large outer rise curvatures >10(-7) m(-1), fractures parallel to the strike of the trench on the outer trench wall and outer rise, and no cross-strike fractures across the trench. Both the northern and southern margins of Latona Corona are possible subduction sites. Identification of a major graben between the two principal outer ridges in southern Latona Corona is evidence of back-are extension in the corona; the amount of extension is estimated to be more than 2-11 km. The moment exerted by the ridges of southern Latona Corona is insufficient to bend the lithosphere into the observed outer rise shape; a negatively buoyant subducted or underthrust slab is needed. Depending on the unknown trench migration rate, lithospheric subduction can make a significant contribution to mantle cooling on Venus. Venusian chasmata could have a dual character. They may be propagating rifts near major volcanic rises, and subduction trenches far from the rises in the lowlands. Subduction and rifting may occur in close proximity on Venus, unlike on Earth. Rifting induced by hotspots on Venus may be necessary to break the lithosphere and allow subduction to occur. Such a process could result in gradual lithospheric subduction or global, episodic overturn of the lithosphere. (C) 1995 Academic Press, Inc.

Schubert, G, Moore WB, Sandwell DT.  1994.  Gravity over Coronae and Chasmata on Venus. Icarus. 112:130-146.   10.1006/icar.1994.1174   AbstractWebsite

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km(-1). The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km(-1). The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (approximate to 100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata. (C) 1994 Academic Press, Inc.

Phipps Morgan, J, Sandwell DT.  1994.  Systematics of Ridge Propagation South of 30-Degrees-S. Earth and Planetary Science Letters. 121:245-258.   10.1016/0012-821X(94)90043-4   AbstractWebsite

New high-resolution Geosat altimetry data south of 30 degrees S reveal numerous propagating ridge wakes along intermediate- and slow-spreading ridges. These new examples provide a large enough database to establish systematics of ridge propagation. Almost all active propagating ridges propagate down a regional along-axis gravity or bathymetry gradient. The sense of the propagating ridge offset (right lateral vs, left lateral) is related to recent changes in spreading direction. We find there is a significant difference between the propagation of ridges with an axial high morphology which propagate at greater than similar to 50% of their full-spreading rate and ridges with a median valley morphology which usually propagate at similar to 25% of their spreading rate. The axial high propagators leave behind an asymmetric wake; the outer pseudofault appears as a continuous linear trough/step while the sheared zone appears as a chain of small gravity bumps. While we clearly see the propagating ridge wakes from offsets greater than similar to 10 km at slow- and intermediate-spreading ridges, at ridges spreading faster than similar to 75 mm/yr the amplitude of the wake topography decreases to the point where we no longer see these wakes in Geosat altimetry data. The systematics seen in this new data set support a fracture mechanics model for the dynamics of ridge propagation.

Royer, J-Y, Gahagan LM, Lawver LA, Mayes CL, Nuernberg D, Sandwell DT, Scotese CR.  1990.  A tectonic chart for the Southern Ocean derived from Geosat altimetry data. AAPG Studies in Geology. 31( St. John B, Ed.).:89-99., Tulsa, OK, United States (USA): American Association of Petroleum Geologists, Tulsa, OK AbstractWebsite

Presented is a new tectonic fabric map of the southern ocean south of 45S, derived from Geosat altimeter profiles and published bathymetric charts and magnetic anomaly picks. The interpretation of the Geosat data is based on an analysis of the first derivative of the geoid profiles (i.e., vertical deflection profiles). To improve the accuracy and resolution of the vertical deflection profiles, 22 repeat cycles from the first year of the Geosat/Exact Repeat Mission (Geosat/ERM) were averaged. At wavelengths less than about 200 km, the vertical deflection is highly correlated with sea-floor topography and thus reveals major features in areas that were previously unsurveyed. The density of the Geosat data is greatest in the high latitudes where lineated bathymetric features such as fracture zones, spreading ridges, trenches, and rifted margins stand out. To construct the tectonic fabric chart, the Geosat data are analyzed in combination with available shipboard bathymetric data and magnetic anomaly identifications. (Auth.)