LOW-TAPER INTERCUTANEOUS WEDGES - A MODEL FOR OPPOSING SHEAR SENSES ON COEVAL, SUBPARALLEL SHEARS

Recent interpretations of the eastern part of the triangle zone, at th e front of the Canadian Rocky Mountain thrust belt, as a low-taper int ercutaneous wedge imply that the upper and basal decollements of the w edge formed as subparallel and coeval structures with opposite senses of displacement. Previous theoretical models of frontal orogenic struc tures are difficult to apply to the low-taper intercutaneous wedge, be cause these models do not fully consider the effects of high and fluct uating pore-fluid pressures, anisotropic rock properties, and the dyna mics of fault propagation. Concepts derived from the theoretical model s as well as from recent studies of modern accretionary wedges, are in tegrated into a model which allows and explains opposing shear senses on coeval, yet subparallel shears. A blind foreland-directed thrust fa ult, the basal decollement, propagates from the deformation belt into the foreland. The decollement propagates into the ''stable'' region, w here mean and differential stresses are too low for new faults to init iate. A zone of high pore-fluid pressure surrounds and precedes the mi grating decollement tip. At the low differential stresses envisioned f or this ''stable'' domain, the vertical orientation of the minimum eff ective stress, subhorizontal bedding anisotropies, and high pore-fluid pressure together promote horizontal extensional failure above the ti pline of the decollement. Since this new failure plane is above the or iginal decollement, it starts to accommodate the movement on the forel and-directed decollement by hinterland-directed displacement, thereby becoming the upper decollement. Once it has formed, this extensional f racture or zone behaves as an essentially frictionless surface, across which shear Stresses cannot be transmitted. As foreland-directed move ment on the basal decollement continues, this free surface starts to g lide, accommodating the tectonically driven movement along the basal d ecollement, and becoming the upper decollement. The upper decollement propagates towards the foreland, and back towards the hinterland, wher e it may cut up-section to break to surface as a backthrust. Little ne t slip can be accomplished until the basal and upper decollements have been linked. Once a linking ramp is formed, insertion of the wedge co mmences and strata in the foreland are delaminated.