Critical Wedge Taper: Kinematic Response to Surface Processes and Relation to Sevier Orogeny Tarka Wilcox Image: Google Earth, Taiwan

Contents Coulomb Failure - review of Mohr’s circle Critical Wedges - review of wedge mechanics Kinematic Evolution - structural deformation Complications - sub / super-critical wedges Response to Sub/Super-Critical States - deformation Complicated Kinematic Evolution - exhumation Relation to the Sevier Orogeny Current-day Analogs… Acknowledgements

Rock Failure Mechanics  SS NN Failure Envelope Coulomb Failure Material Strength Pore Fluid Pressure 

 critical taper angle Critical Wedge Mechanics   Basal Decollement Backstop Direction of wedge propegation  critical taper angle

Critical Wedge Mechanics Continued….. Controlling Factors: -rock strength -material properties:  -pore fluid pressure: -thickness of incoming material

Kinematic Evolution     Self-similar wedge Thrust faults propegate forward off the basal decollement. Old thrusts are rotated towards the back of the wedge

Complications Complications in the idealized critical wedge model: -Externally influenced changes to the taper angle -Erosion -Lack of erosion -Change in thickness of incoming material -Internally influenced changes to the taper angle -Change in material properties -Resulting change in kinematics Complications

Sub / Super Critical Wedges  critical taper angle decrease in  subcritical increase in  supercritical

Subcritical: Internal deformation Supercritical: Forward deformation Response to Unstable States

Complicated Kinematic Evolution Basal Decollement Backstop Duplexes Back-thrusting Subcritical wedges will deform internally to achieve critical taper. Types of internal deformation include: -Backthrusting -Duplexes

Effect of Erosion Basal Decollement Backstop Duplexes Wedges that are forced into a persistant subcritical state by removal of material from their upper surface may undergo an extended period of internal deformation (e.g., DeCelles and Mitra, 1995). Continued formation of new duplex structures can lead to relatively high rates of material exhumation from the interior of the wedge (e.g., Konstantinovskaia and Malaveille, 2005). Material Flux Exhumation Erosion

Relation to Sevier Orogeny The Sevier fold and thrust belt displays evidence of extended periods of internal (vs. prograded) deformation. (DeCelles and Mitra, 1995) Provenance studies of the sediments deposited in the Sevier foredeep suggest long-term supply of material from a basement cored structural culmination. (DeCelles et al., 1995) Depositional and thrusting events appear to be cyclical. The relationship between these cycles can be explained through the response of wedge taper to cycles in erosion and the corresponding sedimentary deposits. (DeCelles and Mitra, 1995) Taken from Figure 2, in DeCelles and Mitra, 1995

Sevier Orogeny continued… Cycles of wedge deformation in the Sevier: DeCelles and Mitra (1995) propose a 3-stage cycle: -Wedge grows in critical (stable) state, imbricating internally and propegating forward. -Wedge becomes supercritical. This promotes significant forward deformation (>10km). -Wedge is subjected to regional erosion. These erosional events are recorded by major unconformities in the foreland. The corresponding depositional events “catch up” with the tectonic thickening of the wedge and stall the forward propegation of deformation. The stalling is related to the load at the toe of the wedge increasing the friction felt along the decollement. This load can be accomodated via flexural foredeeps or ‘piggy-back’ basins.

Sevier Orogeny continued… Three major episodes of this cycle are seen: -Crawford Thrust event: ca Ma -Ham’s Fork Conglomerate, synorogenic deposit. -Absaroka (lower) Thrust event: ca Ma -Evanston Fm., synorogenic deposit. -Hogsback Thrust event: ca Ma -Wasatch Fm., synorogenic deposit. (DeCelles and Mitra, 1995)

Current-day Analogs… A possibly analagous situation can be seen in modern Taiwan. -Critcally tapered wedge -Extremely high erosion rates -Apparent back-stepping of active deformation in response to erosion-induced subcritical taper -Formation of basement duplex structures, resulting in exhumation of deep rocks

Acknowledgements Thanks to Caffeine

References Cited DeCelles, P.G., Lawton, T.F., Mitra, G. (1995) Thrust timing, growth of structural culminations, and synorogenic sedimentation in the type Sevier orogenic belt, western United States, Geology, 23, 8, DeCelles, P.G., and Mitra, G. (1995) History of the Sevier orogenic wedgein terms of critical taper models, northeast Utah and southwest Wyoming, GSA Bulletin, 107, 4, Konstantinovskaia, E. and Malavieille, J. (2005) Erosion and exhumation in accretionary orogens: Experimental and geological approaches, Geochem. Geophys. Geosyst., 6, Q02006, doi: /2004GC000794