Mechanical model for single thrust sheets Part I: Taxonomy of crystalline thrust sheets and their relationships to the mechanical behavior of erogenic belts

Abstract

Crystalline thrust sheets (CTS) are those partially or wholly composed of metamorphic and/or igneous rocks. They have been known for more than a century. We have recognized several types: (1) thin-skinned, basement-cover sheets; (2) ophiolites; (3) tectonic slides; (4) basement uplifts; and (5) composite thrust sheets. The overthrust index ratio , B, may be obtained by dividing the total estimated displacement on CTS by the distance from the 0-mgal contour on the prominent gravity gradient in an orogen to the last thrust in the foreland. A fundamental property of CTS is that the largest crystalline thrust sheet is always larger than the largest foreland thrust sheet in the same orogen ; this property is doubtlessly related to then- relative strengths. Because they generally contain no pre-existing, subhorizontal weak zones, CTS are probably generated within the ductile-brittle transition zone. Thicknesses of CTS are thus determined by the geo-thermal gradient and crustal lithology. A mechanical model primarily for types 1,2, and 5 CTS relates cross-strike width (x) of thrust sheets to the compressive stress (σ c ), friction (τ B ), and dip (α) of the basal thrust; thickness of the thrust sheet at the hinter end (t); and average thickness (t). The model, based upon an energy conservation argument, predicts x = σc t /(τ B + g¯ρ;α¯t) where g is gravity (980 gal) and ρ is average density of the allochthonous mass. This equation can be used to relate data from ancient, deeply eroded orogens to processes that were active during emplacement of CTS. For observed widths and thickness, the model requires emplacement of composite (type 5) thrust sheets by tectonic compression (σ c ∼50 MPa) and low-friction thrusts (τ B B ≃ 50 MPa).