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: )

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

Himalayas Strain: can accommodate MAJOR shortening Stress:  1 is horizontal Principal stress directions?  1 is horizontal,  3 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 higher- grade 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

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

What is it? 1. breached anticline 2. tip lines 4. klippe 5. window6. 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?

Mt Kidd Fold and thrust belts! Next lecture: forced folds and thrust belt mechanics; D&R ( ; )

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