Announcements Field trip this Saturday to Cottonwood Canyon area 7:30 AM at loading dock. We will map some really cool stuff! Please review map symbols. We may return after sunset.

Outline for Today 1. More about geometry and kinematics of thrust systems 2. Forced folds 3. Mechanical "paradox" of moving large thrust sheets 4. Thrust belt evolution: Critical Taper theory 5. Foreland basins 6. Two examples 7. Economic applications

the architecture of many fold-thrust belts "thin-skinned" deformation

development of duplexes

Summary Thrust systems: 1. Accommodate significant crustal shortening 2. Basal detachment/decollement; decoupling within the crust 3. Faults have ramp and flat geometries 4. Thrusts place older/higher grade rocks over younger/lower grade rocks 5. Faults cut up-section 6. Faults generally propagate (get younger) toward the foreland 7. Younger and structurally deeper faults rotate older faults to steeper angles

Mt Kidd Fold and thrust belts! Forced folds (D&R )

Free folds: fold profiles are based entirely on physical- mechanical properties of the layers Forced folds: geometry related to movement over fault ramps- "they just go along for the ride, and some of the beds happen to fined themselves in awkward places and are required to stretch or bend" 2-main types of forced folds: fault-bend folds fault-propagation folds

Fault-bend folds

Fault-propagation folds

monoclines as "drape" folds

"thick-skinned" basement-involved shortening

Colorado Plateau monoclines may be related to thick-skinned deformation

Major issues “mechanical paradox” of thrusting - why such thin sheets (e.g. 100 km long/2-3 km thick) can remain intact during faulting? What happened to the missing basement?

“mechanical paradox” of thrusting - why such thin sheets (e.g. 100 km long/2-3 km thick) can remain intact during faulting?

Recall Byerlee's Law Question: How much shear stress is needed to cause movement along a preexisting fracture surface, subjected to a certain normal stress?  c = tan  (  N ), where tan  is the coefficient of sliding friction

 c = tan  (  N ), where tan  is the coefficient of sliding friction

Possible explanation- water pressure plays a big role  c = tan  (*  N ), where tan  is the coefficient of sliding friction and *  N =  N – fluid pressure

What drives a thrust belt?? Old timers thought that decollements beneath thrust belts dipped away from the elevated hinterland- and therefore gravity "sliding" was the main mechanism

But now we know that decollements to thrust belts dip toward the hinterland. Thrust belts move uphill!

Elevated fluid pressure certainly decreases the stress required to move a thrust belt. Gravitational stresses due to elevated topography also aids sliding. BUT, a push from the rear is still necessary

Critical Taper Thrusts belts are wedge shaped- characterized by a topographic slope (  ) and a decollement dip (  ) Only at some critical angle (  +  ), will the thrust belt propagate

The critical taper angle is controlled by the coefficient of friction along the decollement and the frictional sliding strength of the rock EPISODIC propagation

Thrust belts create topographic loads that flex the lithosphere like a person on a diving board- foreland basins!

Important terminology/concepts role of elevated pore fluid pressure in movement of thrust sheets Critical taper theory / wedge theory foreland basin development

Example 1: Tibet Geographic Setting

Regional Geologic Setting

Geometry

Footwall rocks include high-pressure blueschists that formed at depths of >35 km!

Tectonic significance

Example 2: Canadian Cordillera

Roche Ronde

Boulle Range

Roche Ronde Boulle Range

Cross-sections Thrust faults cut up-section only! (or section-parallel) Every flat or ramp in the FW should correspond to an equivalent flat or ramp in the WH Bed thickness is preserved (conservation of volume and mass) Other than that - it’s all interpretation!

Roche Ronde Boulle Range

Relevance to oil exploration

On Thursday: Normal faults