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 PMSee 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 shorteningStress: 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

forelandlow-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 Plateaumonoclines 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

SummaryThrust systems:1. Accommodate significant crustal shortening2. Basal detachment; decoupling within the crust3. Faults have ramp and flat geometries4. Fault place older/higher grade rocks over younger/lower grade rocks5. Faults cut up-section6. Faults generally propagate (get younger) toward the foreland7. Younger and structurally deeper faults rotate older faults to steeper angles

What is it?1. breached anticline

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?

Mt Kidd

Fold and thrust belts!

Next lecture: forced folds and thrust belt mechanics; D&R (413-423; 336-339)

Important terminology/conceptshinterland 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