摘要:

Uplifted rift margins are a common feature of continents and oceans. Two variants of rift flank morphologies have been recognized: One in which the topography warps down from an inland high toward the continental margin, and one where the tropographic peak lies close to the continental margin. The Great Escarpment of southern Africa and the Transantarctic Mountains are examples of the first and the second variants of rift flanks, respectively. Both rift flanks are bordered on their landward side by broad continental basins: the Kalahari and the Wilkes hinterland basins. If these basins are interpreted as flexural “outer lows” that deepen in unison with the uplift of the rift flanks, the lithosphere on the uplifted side is very rigid in both cases (elastic thicknessTeof 100 ± 20 km for southern Africa and 110 ± 20 km for East Antarctica). We suggest that the variation in rift flank morphology is caused by the isostatic response to uplift forces of elastic plates sharing different boundary conditions. We model the uplift of the Transantarctic Mountains as an upward deflection of an elastic plate which is broken at the front of the Transantarctic Mountains, and we model the uplift of the Great Escarpment as an upward deflection of a continuous elastic plate that is modified by the downward load of sediments on the continental margin. Although the Transantarctic Mountain uplift is young (60–0 Ma) and the southern African uplift is old (<100 Ma), the different isostatic responses of the two margins are not a function of age, because most loading (sedimentation) and unloading (erosion) took place shortly after rifting. Detailed modeling of topography, gravity, geological markers, and the locations of depocenters suggests that lithospheric rigidity decreases under the Transantarctic Mountains, whereas in southern Africa the decrease occurs not under the Great Escarpment but far seaward under the continental shelf and slope. If the distribution of lithospheric rigidity is indicative of the thermal regime of the lithosphere, then uplifted rift flanks are not always underlain by a thermal anomaly. This and other geological evidence indicate that a single mechanism cannot explain the uplift of both the Antarctic and the African

ISSN:0148-0227    

DOI:10.1029/91JB02231

年代:1992

数据来源: WILEY

摘要:

A damage mechanics model for shear failure under compressive loading is used to calculate the shear strength of a fault. Based on field studies of the structure of natural fault zones, the distribution of starter flaws (initial damage) is assumed to be fractal with dimensionDf= 2.6. For this fractal dimension, the largest flaws dominate the fracture process at low confining stress and the shear strength scales as the inverse square root of the largest flaw size (as in tensile loading). At higher levels of confining stress appropriate to the base of the seismogenic zone, the strength becomes independent of the distribution of flaw sizes and depends only on the density of starter flaws. When the initial damage is sufficiently high, the damage initiation surface coincides with the failure surface, and the fault zone appears to obey the same friction law which controls slip on the individual microfractures. The initial damage corresponding to the fractal distribution of flaws measured in a natural fault zone is large enough for this to occur.

ISSN:0148-0227    

DOI:10.1029/91JB02338

年代:1992

数据来源: WILEY

摘要:

The irregular spatial distribution and velocity independence of basal friction derived from Landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone. Rigid bedrock substrata may contact the base of the ice stream in small (<100 km2) areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in Landsat images.

ISSN:0148-0227    

DOI:10.1029/91JB02454

年代:1992

数据来源: WILEY