Mechanics of vein, fault and solution surface formation in the Appalachian valley and ridge, northeastern tennessee, U.S.A.: implications for fault friction, state of stress and fluid pressure

Abstract

Complex patterns of veins, solution surfaces and faults in exposures of the Sevier Shale in the Appalachian fold and thrust belt, Tennessee, U.S.A., result from the interplay between the remote state of stress and stress perturbations caused by slip across thrust faults. In order to gain a better understanding of this interplay and its consequences, a two-dimensional displacement discontinuity boundary-element model (DDBEM) is utilized to interpret structural assemblages from the Sevier Shale. A DDBEM of isolated faults and fault arrays shows that the local state of stress is controlled by the frictional resistance along the faults and the angle between the remote (applied) greatest compressive stress direction and the fault planes. Nowhere is this more evident than within the domains bounded by bed-parallel echelon thrust fault arrays. Here, contrasting structural patterns involving geologically contemporaneous cleavages and bedding-normal veins are recognized. The cleavages are oblique to the trend of the bed-parallel thrust fault arrays implying that the fault arrays were not frictionless during the dissolution process. The bedding-normal veins, however, indicate that the same fault arrays were principal stress planes and nearly frictionless during opening fracturing. Low frictional resistance is interpreted to be due to nearly lithostatic fluid pressures. These contrasting styles are interpreted to result from slip and inter-slip periods of deformation. When the fluid pressure was high, fault friction was low, and the faults slipped. Fracturing dominated the domains between the thrust fault arrays causing the veins to open and a decrease in the fluid pressure. During the inter-slip period the fluid pressure was low; the faults had significant frictional resistance; and pressure solution was the dominant deformation mechanism. The cycling between slip and inter-slip periods and respective states of stress may explain the non-orthogonal relationship between the co-existing veins and cleavages.