Thermo-Mechanical Properties and Evolution of Pull-Apart Basins: ABSTRACT

Abstract

Pull-apart basins are generally characterized by two component subsidence; an initial essentially instantaneous isostatic subsidence (S1) dependent on the ratio of crustal to lithospheric thickness (Cz/1z) and the stretching factor s, followed by a slower exponentially decaying thermal subsidence (St) controlled by the thermo-elastic properties of the continental lithosphere which, in turn, can be characterized by a thermal time constant ^tgr. Rapid short-lived subsidence (Ridge basin) is indicative of either (1) inhomogeneous crustal stretching without major sublithospheric involvement, or (2) extremely small lithospheric diffusivities. The former implies a thin-skinned origin for pull-apart basins and suggests that the s atial and temporal distribution of bounding faults and splays typical of pull-apart basins result from inhomogeneous brittle failure of the upper crust. Crustal, extensional or shear-strength profiles for various geothermal gradients and degrees of wetness adequately explain two-layer extension with intra-crustal decollement. However, the effects of lateral heat flow decrease the thermal time constant by allowing a basin to subside more quickly because of both lateral and vertical cooling. The size of this effect is dependent on the width of the stretched lithosphere. The effective ^tgr of a 100 km (60 mi) wide rift is 36 m.y. and for a 25 km (15 mi) rift is 6 m.y., whereas the actual thermal time constant in both cases is 62.8 m.y. Lateral heat flow amplified rift subsidence while produ ing complementary uplift in adjacent unstretched regions. However, the flexural rigidity of the lithosphere severely attenuates the deformation caused by the lateral flow of heat. Although the deformation is highly dependent on the mechanical properties of the lithosphere, ^tgr is independent. Diachronous rift shoulders or peripheral uplifts may produce important hydrocarbon gradients and result from various combinations of lateral heat flow, flexural arching, and normal-fault decoupling. Continental lithospheric rigidities appear to increase with age following an orogenic or thermal event, suggesting that the long-term mechanical behavior of the continental lithosphere is similar to that of the oceanic lithosphere. However, high rigidities (1032 dyne-cm) associated with Archean or Proterozoic terranes and modeling of plate deformation suggest that the long-term thermal behavior of continental lithosphere is governed by a cooling plate model with a 200-250 km (124-155 mi) lithospheric thickness, nearly twice the 125 km (78 mi) estimated for the oldest oceanic lithosphere. This has important implications for the evolution of sedimentary basins. A doubling of the lithospheric thickness implies a quadrupling of ^tgr, yet basin subsidence models have assumed tha ^tgr for the oceanic and continental lithospheres are similar. A large ^tgr allows basin subsidence to continue over significantly longer times, but lateral heat flow, in addition to vertical, must be included in basin models to obtain accurate subsidence and temperature estimates. In particular, S1 is highly dependent on the age of the underlying basement. End_of_Article - Last_Page 1201------------