Announcements

Field trip this Saturday to Collosal Cave area7:30 AM at loading dock. We will map some really cool stuff! Please review map symbols and bring

stereonets. We may return after sunset.

Next week: Mihai Ducea will lecture on normal fault systems. Please read D&R pp.340-357

Nov. 13 - Draft #1 of fault paper due!

What is it? (Quiz)

(1)

(2)

(4)

(3)

Colorado Plateaumonoclines may be related to thick-skinned deformation

Today: Mechanics of Thrust Systems

(1) Mechanical "paradox" of moving large thrust sheets

(2) Thrust belt evolution: Critical Taper theory

(3) Foreland basins

(4) Fault project

• “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??

Oldtimers thought that decollements beneath thrust belts dipped away from the elevated hinterland- and therefore gravity "sliding" was the main mechanism

Once armed with the knowledge of fluid pressure, oldtimers really thought they had it figured out.

The can slides at low angles, not because of lower friction, but due to elevated fluid (air) pressure during thermal expansion that counteracts weight of can

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!

Fault Project

Information to research:

- Geographic setting- Regional geologic setting

- Geometry: orientation and spatial dimensions- Kinematics: Sense and magnitude of slip, type of

fault- Timing: slip history, active or inactive?

- Mechanics: relate fault movement to stress- Regional tectonic significance

- Practical significance

Paper Guidelines

6-8 pages text (double spaced) + few illustrations

Need to have the following sections:Abstract

IntroductionGeometry and Kinematics of Fault

SignificanceConclusions

References: At least five cited (only two may be websites); Format: follow that of the journal Geology

Next lecture: Strike-slip fault systems(D&R: 357-371)

Important terminology/concepts

role of elevated pore fluid pressure in movement of thrust sheets

Critical taper theory / wedge theory

foreland basin development