A crack model of creep processes on deep sections of faults

A crack model in antiplane shear configuration is shown representing creep processes interpreted in terms of 'viscous' deformation of a narrow plastic layer, characterized by inhomogeneous rheological properties, embedded within a homogeneous elastic medium. The evolution in time of slip and stress over the crack plane is studied through a truncated expansion in Chebyshev polynomials, and convergence is proved to be fast in the simple examples considered. Finite-stress solutions are found which are compatible with constitutive relations of elasto-plastic materials and furthermore these allow us to simulate creep propagation and stress transfer between locked and unlocked fault segments. This model provides a simple interpretation of the shallow depth of the seismogenic layer observed in several areas of the world and lends itself to modelling creep processes during either post-seismic rebound or pre-seismic stress buildup. Stress transfer is accomplished mostly by the slow extension of the creeping section. During a seismic cycle it is envisaged that different regimes dominate over deep, intermediate and shallow sections of faults: (i) slow pre-seismic stress build-up accompanied by creep and stress migration toward intermediate depths; (ii) brittle fracture over shallow and intermediate sections of faults; (iii) post-seismic rebound over intermediate and deep sections of faults. The present crack model, while providing finite-stress solutions, allows a better understanding of how stress may accommodate at different depths over a fault plane during a seismic cycle.     

Stratigraphic signatures of translation of thrust-sheet top basins over low-angle detachment faults

Abstract Low‐angle detachment faults and thrust‐sheet top basins are common features in foreland basins. However, in stratigraphic analysis their influence on sequence architecture is commonly neglected. Usually, only eustatic sea level and changing flexural subsidence are accounted for, and when deformation is considered, the emphasis is on the generation of local thrust‐flank unconformities. This study analyses the effects of detachment angle and repetitive detachment activation on stratigraphic stacking patterns in a large thrust‐sheet top basin by applying a three‐dimensional numerical model. Model experiments show that displacement over low‐angle faults (2–6°) at moderate rates (～5.0 m kyr^?1) results in a vertical uplift component sufficient to counteract the background flexural subsidence rate. Consequently, the basin‐wide accommodation space is reduced, fluvio‐deltaic systems carried by the thrust‐sheet prograde and part of the sediment supply is spilled over towards adjacent basins. The intensity of the forced regression and the interconnectedness of fluvial sheet sandstones increases with the dip angle of the detachment fault or rate of displacement. In addition, the delta plain is susceptible to the formation of incised valleys during eustatic falls because these events are less compensated by regional flexural subsidence, than they would be in the absence of fault displacement.     

Dynamic stress variations due to shear faults in a plane-layered medium

A complete set of expressions is presented for the computation of elastic dynamic stress in plane-layered media. We use a discrete-wavenumber reflectivity method to compute the stress field radiated by arbitrary moment-tensor sources. The expressions derived here represent an interesting tool for both-the observational and theoretical analysis of dynamic stress changes associated with earthquake phenomena. Dynamic stress changes associated with a strike-slip fault having unilateral rupture are shown. This modelling, which is similar to the 1992 Landers California earthquake, illustrates the effects of distance, directivity and depth on transient stress changes.     

Three-dimensional-seismic coherency signature of Niger Delta growth faults: integrating sedimentology and tectonics

This case study of growth faults and associated deltaic sedimentation in the shallow‐offshore Niger Delta uses an integrated analysis of three‐dimensional (3D)‐seismic coherence facies and wireline data that supports an evaluation of the sedimentary response to delta tectonics. The study area comprises four fault blocks bounded by a set of kilometre‐scale, basinward‐dipping, synsedimentary normal faults. Correlation of highly variable growth stratigraphy across faults was achieved by a systematic visualization and interpretation of series of coherence horizon‐slices: the detection and matching of erosive and depositional patterns (e.g. channels) across faults allowed the establishment of sedimentology‐controlled links between diverse footwall and hanging‐wall growth successions. At the same time, this interpretation approach helped to visualize seismic‐sedimentological and seismic‐geomorphological features survey‐wide at all depth levels. The integration of this extensive 3D database with lithology information from wireline logs provides a powerful tool for subsurface sedimentology interpretation. Synoptic analysis of the 3D‐seismic sedimentology interpretation with stratigraphy based fault‐kinematic analysis using throw vs. depth plots (Th–Z plots) enabled a discussion of the relation between delta tectonics and sedimentary‐system development, and the evaluation of the Th–Z method for subsurface‐lithology prediction. The interpretation results document that both motion analysis of synsedimentary deltaic faults and Th–Z‐based lithology prediction are only feasible when supported by detailed 3D information on palaeoenvironment and palaeotopography at and around studied fault systems. We therefore recommend the use of fast‐track fault‐kinematic and subsurface‐lithology predictions based on Th–Z plots only when supported by comprehensive 3D seismic‐sedimentological interpretations.     

Errors in extension measurements from planar faults observed on seismic reflection lines

Abstract The preferred model for the extension of brittle crust involves the rotation of planar fault blocks. We show that in general the distortion at the ends of the blocks does not affect the measurement of extension. The horizontal displacement on a normal fault, the heave, is observed with little distortion on a seismic reflection time-section. It can be used to estimate the amount of extension. We demonstrate that the sum of the heaves is not equal to the actual elongation if the blocks have rotated. However, the error in the extension factor, β, introduced by equating elongation with the sum of the heaves is small. It increases with the amount of rotation from 0 for no rotation to 13% for the maximum observed angle of rotation of 30^o. We compare this value with the practical error introduced by uncertainties in seismic velocities when the elongation is measured from a depth-converted seismic section. This latter error is significantly smaller being approximately 5% for an error in velocity of 20% when the rotation angle is less than 30^o.     

The field of crustal velocity in Asia calculated from Quaternary rates of slip on faults

Rates of Quaternary faulting, when treated as measures of strain rates within regions several times larger than the depths of brittle faulting, yield a velocity field for Asia that matches the average relative velocity between the Indian and Eurasian plates ove the past 2 Myr (NUVEL-1A), and the velocity of Shanghai with respect to Eurasia, measured with Very Long Baseline Interferometry (VLBI). By using rates of slip on discrete surfaces (faults) to define average strain rates, we explicitly assume tha the large-scale deformation can be treated as that of a continuum. Depending on the scales over which strain rates are averaged, 87 to 93 per cent of the assumed strain rates are fit within assigned 1-sigma uncertainties, and 97 to 99 per cent are fit withi 2-sigma. The internal consistency of these data and the agreement with independen rates (NUVEL-1A and VLBI) test the validity of treating large-scale deformation of continents as that of a continuum. In the calculated velocity field, a large fraction, ˜ 85 per cent, of India's convergence with Eurasia is absorbed by crustal, or lithospheric, thickening. Among consistently misfit assumed strain rates are those dominated by high rates of strike-slip faulting. Strain associated with 20-30 mm a^-1 of slip on the Altyn Tagh, Karakax, and Karakorum faults is inconsistent with the distributions of strain in surrounding regions. We conclude that these rates, all of which are based on correlations of ages of offset landforms with changes in global climate, and not on radiometric dating, have been systematically overestimated. Lateral transport out of India's northward path is relatively minor; South China is calculated to move east-southeast at less than 10 mm a^-1.