Transcript Slide 1
Announcements Talk This Thurs. 4 pm, Rm. Haury Bldg. Rm 216, "Tertiary structural and stratigraphic evolution of the Greater Tucson area", by Jon Spencer. Write 1 paragraph summary (+1% extra credit) Field trip to Silverbell Mine this Saturday: 7:45 AM to 4 PM See me after class for information. (+1% extra credit) midterm exam stats Thrust systems: geometry and kinematics (D&R: 319-336) the architecture of many fold-thrust belts "thin-skinned" deformation Himalayas Strain: can accommodate MAJOR shortening Stress: s1 is horizontal Principal stress directions? s1 is horizontal, s3 is vertical thrust systems generally propagate toward the foreland Canadian Rockies Thrusts root into a basal decollement, below which shortening is accommodated by a different mechanism; decoupling between upper and lower crust may be highergrade rocks and ductile shear zones in hinterland hinterland foreland low-grade rocks and brittle faults in foreland faults cut up-section thrusts take advantage of preexisting planes of weakness In almost all cases, thrusts place older and/or higher grade rocks on younger and/or lower grade rocks Example from the Argentinian Cordillera Exshaw thrust-hangingwall flat, footwall ramp imbricate fan development of duplexes duplex terminology also- horses! duplex: outcrop-scale map pattern of a duplex lateral ramps compartmental faulting and tear faults footwall rocks are commonly deformed into synclines Footwall syncline in the Canadian Cordillera Footwall syncline in the Canadian Cordillera also, triangle zones "thick-skinned" basement-involved shortening Colorado Plateau monoclines may be related to thick-skinned deformation The observation that faults do not continue around the entire Earth suggests that they must terminate Generally, a gradual decrease in slip toward fault termination Younger and structurally deeper faults lead to rotation of older faults Summary Thrust systems: 1. Accommodate significant crustal shortening 2. Basal detachment; decoupling within the crust 3. Faults have ramp and flat geometries 4. Fault 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 1. breached anticline What is it? 2. tip lines 4. klippe 5. window 6. lateral ramp 7. blind thrust 8. branch lines 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? • Why are almost all faults dipping one way when rock mechanics predict equal chance for both thetas around sigma 1? Fold and thrust belts! Mt Kidd Next lecture: forced folds and thrust belt mechanics; D&R (413-423; 336-339) Important terminology/concepts hinterland vs. foreland foreland propagation of thrusting thin-skinned vs. thick-skinned deformation blind thrust duplexes, their development, and map pattern roof thrust, floor thrust, and horses basal decollement hinterland vs. foreland deformation ramp-flat thrust geometries and terminology klippe vs. window allochthonous vs. autochthonous imbricate fan lateral ramp tear faults footwall synclines pop-up structures and backthrusts fault tips rotation of old faults during slip along younger faults