Transcript Slide 1
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 Fold and thrust belts! Mt Kidd Forced folds (D&R 413-423) Free folds: fold profiles are based entirely on physicalmechanical 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? sc = tanf(sN), where tanf is the coefficient of sliding friction sc = tanf(sN), where tanf is the coefficient of sliding friction Possible explanation- water pressure plays a big role sc = tanf(*sN), where tanf is the coefficient of sliding friction and *sN = sN – 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 shapedcharacterized by a topographic slope (a) and a decollement dip (b) Only at some critical angle (a+b), 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