announcements field trip this saturday to collosal cave area 7:30 am at loading dock. we will map...
Post on 21-Dec-2015
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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!
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
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