Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.t12g..01o&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #T12G-01
Physics
5104 Fracture And Flow, 5475 Tectonics (8149), 8005 Folds And Folding, 8010 Fractures And Faults, 8045 Role Of Fluids
Scientific paper
We demonstrate the effects of mechanical stratigraphy and pore fluid pressure on the fault structure and topography of thrust-related anticlines. We show that nucleation of secondary backthrusts is evidence of mechanically well-stratified, layered crust. Previously, Roering et al. [1997] found that interlayer slip above an active thrust fault results in positive Coulomb failure stress changes (ΔCFS), which favor secondary backthrust formation over updip propagation when the upper tip of the primary thrust is near an interface with low shear strength. Niño et al. [1998] subsequently showed that increasing displacement along a non-propagating basement-confined thrust results in backthrust strain within overlying layered rock. In this study, we use the Hoek-Brown (H-B) failure criterion to evaluate the tendency for backthrusting above propagating blind thrust faults. Half-space boundary element models loaded under increasing remote principal stress ratios are used to create predictive maps of H-B stability. The primary thrust is propagated and secondary faults are nucleated by adding fault segments in regions of predicted H-B failure from the previous loading step. Mechanical stratification is modeled as horizontal planes of low shear strength within a rock mass of higher uniform shear strength. We investigate patterns of thrust faulting within mechanically layered and mechanically homogeneous crust. Models of H-B failure around blind thrust faults in homogeneous crust show that these thrusts primarily propagate updip and that backthrusts are not predicted to form. This is consistent with predictions from ΔCFS models. In layered crust, the tendency for backthrust nucleation is dependent on the strength of the rock layer that the primary thrust is propagating through. When the upper tip propagates through a weak layer or interface, backthrusts are not predicted to form by H-B failure because of the low shear strength, or correlatively high pore fluid pressure, of the material. Although the rock within the backthrust region experiences positive ΔCFS, the absolute magnitudes of the H-B failure stresses are insufficient to nucleate faulting. When the upper tip propagates through a strong layer or interface, H-B and ΔCFS models predict an equal tendency for backthrust nucleation and updip propagation of the primary thrust. The resulting anticline of layered crust is composed of the larger primary thrust-related fold with superimposed smaller anticlines above each backthrust. Backthrusts nucleating at shallower depths result in the smaller anticlines being increasingly closer to the axis of the primary fold. For folds in either homogeneous or layered crust, the flanks of the resulting anticlines are steepest on the side of the fold directly above the upper thrust tip. This provides a means of determining the dip direction of each thrust from topography. Mechanical stratification within a thrust-related anticline is reflected in backthrust-related topography. The presence of backthrusts, inferred from slope analysis of smaller anticlines superimposed on the larger primary thrust-related fold, indicates a layered mechanical stratification within the anticline. Where subsurface data are unavailable, such as on the moon or Mars, topographic observations of thrust-related anticlines can be used to infer mechanical stratigraphy. In terrestrial studies, the lack of backthrusts within anticlines of well-defined mechanical stratigraphy is evidence for high pore fluid pressure (low effective shear strength) during seismogenesis.
Okubo Chris H.
Schultz Richard A.
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