Displacement and strain patterns of extensional orogens

Abstract

Rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust. Strain compatibility arguments indicate that a finite extension can be accommodated by displacements across (1) planar, nonrotating faults or ductile shear zones, (2) shear zones which rotate above a horizontal detachment (the domino model), or (3) shear zones which rotate as a result of a horizontally oriented, pure shear stretching component (the plastic model). Listric, normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component (in the plastic model) or the area loss component (in the planar shear zone model). Within the geometric framework of these various models, the effects of superposed simple shear, pure shear extension, and area change on the rock fabric are investigated. These displacement components, which may be superposed sequentially or simultaneously, determine the state of finite strain associated with a given magnitude of tectonic extension. The relationships between displacement and strain are expressed as graphs of foliation dip (or strain ratio) versus shear strain and as graphs of foliation dip (or strain ratio) versus tectonic extension. The shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone, whereas the tectonic extension graphs better illustrate these effects on the regional scale. The shear strain and tectonic extension graphs can be used by field geologists (1) to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation, (2) to distinguish between possible extension models from field data, and (3) to evaluate the influence of shear zone attitude and thickness upon the amount of displacement. Applications of these methods are illustrated for field areas in metamorphic core complexes of Arizona and in the Basin and Range province of the North America Cordillera.

The Devonian basins of western Norway: tectonics and kinematics of an extending crust

“…In contrast, all the data support a model with a dipping detachment (Kligfield et al 1984). In a first hypothesis (Fig.…”

“…A model associated with a horizontal detachment Kligfield et al 1984) is not realistic due to; (i) the uniform sense of shear along the detachment and (ii) the huge amount of extension that would be required (stretching factor/3 = 4.4, McKenzie 1978) without intense and generalized plutonism and magmatism.…”

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

“…Various mechanisms have been proposed to explain how crustal extension is accommodated during orogenic collapse: horst and graben faulting with blocks potentially extending to the base of the crust (Stewart 1980), block faulting that detaches into an underlying shear zone or anastomosing network of shear zones (Miller et al 1983;Smith and Bruhn 1984;Kligfield et al 1984;Hamilton 1987), and block faulting in the hanging wall of a major shear zone that penetrates the entire crust, soling out at the crust-mantle boundary or even deeper (Wernicke 1985). Due to its high spatial resolution, deep seismic reflection surveying is the best method to discriminate between different models of crustal extension at depth, and in the well-studied extensional Basin and Range province of the western United States, models in which block faulting in the upper crust is separated from ductile extension in the lower crust appear most consistent with the available seismic data (Allmendinger et al 1987).…”

Thermal and mechanical development of the East African Rift System

“…Typically, continental rift basins are bordered along one side by one or more normal faults showing large vertical displacements, which we define as a border fault segment. Structural relations in rifts indicate that the high-angle border faults become low-angle detachment faults or shear zones at depth (e.g., Wernicke and Burchfiel, 1982), and rheological considerations of extended continental crustal materials support these observations (e.g., Kligfield et al, 1984). It is generally agreed that extension occurs along these normalslip detachments, hence the original location and orientation of border fault segments influences later stages of rifting and passive margin development.…”