Deformation and Basin Formation along Strike-Slip Faults
Significant advances during the decade 1975 to 1985 in understanding the geology of basins along strike-slip faults include the following: (1) paleomagnetic and other evidence for very large magnitude strike slip in some orogenic belts; (2) abundant paleo-magnetic evidence for the pervasive rotation of blocks about vertical axes within broad intracontinental transform boundaries; (3) greater appreciation for the wide range of structural styles along strike-slip faults; (4) new models for the evolution of strike-slip basins; and (5) a body of new geophysical and geological data for specific basins. In the light of this work, and as an introduction to the remainder of the volume, the purpose of this paper is to summarize the major characteristics of and controls on structural patterns along strike-slip faults, the processes and tectonic settings of basin formation, and distinctive stratigraphic characteristics of strike-slip basins.
Strike-slip faults are characterized by a linear or curvilinear principal displacement zone in map view, and in profile, by a subvertical fault zone that ranges from braided to upward-diverging within the sedimentary cover. Many strike-slip faults, even those involving crystalline basement rocks, may be detached within the middle to upper crust. Two prominent characteristics are the occurrence of en echelon faults and folds, within or adjacent to the principal displacement zone, and the co-existence of faults with normal and reverse separation. The main controls on the development of structural patterns along strike-slip faults are (1) the degree to which adjacent blocks either converge or diverge during strike slip; (2) the magnitude of displacement; (3) the material properties of the sediments and rocks being deformed; and (4) the configuration of pre-existing structures. Each of these tends to vary spatially, and, except for the last, to change through time. It is therefore not surprising that structural patterns along strike-slip faults differ in detail from simple predictions based on the instantaneous deformation of homogeneous materials. In the analysis of structural style, it is important to attempt to separate structures of different ages, and especially to distinguish structures due to strike-slip deformation from those predating or post-dating that deformation. Distinctive aspects of structural style for strike-slip deformation on a regional scale include evidence for simultaneous shortening and extension, and for random directions of vergence in associated thrusts and nappes.
Sedimentary basins form along strike-slip faults as a result of localized crustal extension, and, especially in zones of continental convergence, of localized crustal shortening and flexural loading. A given basin may alternately experience both extension and shortening through variations in the motion of adjacent crustal blocks, or extension in one direction (or in one part of the basin) may be accompanied by shortening in another direction (or in another part of the basin). The directions of extension and shortening also tend to vary within a given basin, and to change through time; and the magnitude of extension may be depth-dependent. Theoretical studies and observations from basins where strike-slip deformation has ceased suggest that many strike-slip basins experience very little thermally driven post-rift subsidence. Strike-slip basins are typically narrow (less than about 50 km wide), and they rapidly lose anomalous heat by accentuated lateral as well as vertical conduction. Detached or thin-skinned basins also tend to be cooler after rifting has ended than those resulting from the same amount of extension of the entire lithosphere. In some cases, subsidence may be arrested or its record destroyed as a result of subsequent deformation. Subsidence due to extension, thermal contraction, or crustal loads is amplified by sediment loading.
The location of depositional sites is determined by (1) crustal type and the configuration of pre-existing crustal structures; (2) variations in the motion of lithospheric plates; and (3) the kinematic behavior of crustal blocks. The manner in which overall plate motion is accommodated by discrete slip on major faults, and by the rotation and internal deformation of blocks between those faults is especially important. Subsidence history cannot be determined with confidence from present fault geometry, which therefore provides a poor basis for basin classification. Every basin is unique, and palinspastic reconstructions are useful even if difficult to undertake.
Distinctive aspects of the stratigraphic record along strike-slip faults include (1) geological mismatches within and at the boundaries of basins; (2) a tendency for longitudinal as well as lateral basin asymmetry, owing to the migration of depocenters with time; (3) evidence for episodic rapid subsidence, recorded by thick stratigraphic sections, and in some marine basins by rapid deepening; (4) the occurrence of abrupt lateral facies changes and local unconformities; and (5) marked differences in stratigraphic thickness, facies geometry, and occurrences of unconformities from one basin to another in the same region.
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