Structural lithic units in external orogenic zones

Abstract

Changes in types of thrust fault structures are typical of external orogenic zones, and are inferred here to be related to thickness and facies changes in stratigraphie sequences. Obvious deformation style changes occur from external to intermediate thrust sheets. Similar changes in style are observed, and most easily interpreted, along-strike within individual thrust sheets. To correlate stratigraphic and structural changes, stratigraphic sections are divided into structural lithic units. The subdivisions are based on the dominant deformation styles vertically in individual cross-sections. The major variables in determining deformation style are controlling stratigraphic members within individual stratigraphic/structural units, and the stacking sequence of lithologie types into different structural units. In many areas, the critical taper of one major dominant member regionally controls where structural style changes will occur. Areal change in either internal composition or in stacking sequence causes lateral change in deformation style. Lithological changes in structural units are coincident with changes in frontal thrust ramp positions (connected by major lateral ramps), and with changes in thrust zone geometries (imbricate thrusts in one area versus a duplex zone along strike). They are also associated with changes in thrust shapes, from listric to stepped, and changes from fault-bend-fold to fault-propagation-fold structures. Controlling members may be either strong or weak. Fold dominated regions and thrust faulted regions without major shales or evaporites are usually controlled by strong controlling members. Weak controlling members are major evaporite horizons, or thick shales. A thorough understanding of stratigraphic facies realms therefore may allow predictions of types and changes in structural styles. It can also be very useful in the interpretation of less well-exposed thrust terranes.