Out-of-Plane Strains Associated with Transverse Ramps: a Field and Theoretical Study

Theodore G. Apotria, John H. Spang, David V. Wiltschko

Transverse ramps, occurring at all scales in both compressional and extensional terranes, are faults that cut across the stratigraphy, oblique to the transport direction. These are regions where the common assumption of plane-strain is violated, and may be responsible for folds oblique to the regional trend, strike-parallel fold closure, normal faults, high-angle reverse faults, and zones of fracture and cleavage overprint.

The South Fork thrust is a transverse thrust fault in the hanging wall of the Absaroka thrust about 25 mi north of Kemmerer, Wyoming. Second-order folds and faults documented in the vicinity of the transverse fault indicate a significant component of strain out of the transport plane that varies from compressive to extensional along the length of the fault. The range in trend of fold axes in the hanging wall near the subsurface ramp is 180°-242°, indicating non-plane material transport up the ramp. The orientation of pressure solution surfaces and fractures also varies systematically in the vicinity of the ramp.

Three-dimensional kinematic models indicate that the finite strain tensor can be decomposed into components of in-plane and out-of-plane deformation. The out-of-plane components include shearing within the hinges of fault-bend folds over the transverse ramp, and lateral shortening or extension due to deflection of the hanging wall into the plane of the transverse ramp. The sense and magnitude of the strain depends on the frontal ramp dip, transverse ramp dip (ß), the angle between the transport direction and the transverse ramp (^agr), and position along the ramp. For certain geometries where ß is high and ^agr is low, the model predicts the existence of transverse faults in the hanging wall (tear faults).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.