Outline Terminology Ramp and Flat thrusts Animations Class exercise in cutoffs General rules for thrusting??

See if they have one without erosion Animations of thrust belts from kontastan See if they have one without erosion

Thrust Belt Anatomy – the Alps Thrust Fault Terminology foreland, hinterland thin-skinned, thick-skinned

Thrust Sheet Anatomy Thrust ramps How does hangingwall deform? frontal ramp lateral ramp

Original layer cake template Thrust Fault Terminology

Thrust Fault Terminology Are strata parallel to the thrust…

Thrust Fault Terminology Transport Direction Foreland Hinterland Ramp - flat thrusts propagate very quickly. Subsequent sliding on the thrust produces distinct types of folds with angular hinges (called fault-bend folds).

Kinematics of Ramp-Flat Thrusts Open file in 10. thrust folder (fault bend fold model)

Kinematics of Ramp-Flat Thrusts Open file in 10. thrust folder (and then duplex model) Duplex, horses Note these are all in sequence

Orogenic wedges Concept of critical taper Many large thrust belts are critically tapered or are in a steady state (between erosion and uplift from shortening. Discuss how changes in taper effect thrust belt evolution (Uneven erosion like Puli, intersect salt basin like Ebro

Kinematics of a thin-skinned thrust belt

Class exercise in cutoffs Class exercise in cutoffs This exercise is designed to help you recognize concordant and discordant contacts in thrust belts (e.g. stratigraphic cutoffs) and use these to infer thrust kinematics. For the handout: Identify and draw a line along the thrust in the outcrop Identify and draw in lines that trace stratigraphic contacts Assuming the rocks are not overturned, determine whether you are looking at a footwall flat, footwall ramp, hangingwall flat or hangingwall ramp. Mark them on the fig Then determine the direction the hangingwall is moving and add an arrow denoting that direction

Class exercise in cutoffs Class exercise in cutoffs This exercise is designed to help you recognize concordant and discordant contacts in thrust belts (e.g. stratigraphic cutoffs) and use these to infer thrust kinematics. For the handout: Identify and draw a line along the thrust in the outcrop Identify and draw in lines that trace stratigraphic contacts Assuming the rocks are not overturned, determine whether you are looking at a footwall flat, footwall ramp, hangingwall flat or hangingwall ramp. Mark them on the fig Then determine the direction the hangingwall is moving and add an arrow denoting that direction

Interpretation Find the thrust fault…

Print 25 copies of this image for class exercise

Interpretation Find the thrust fault…

Identify the flats and ramps…

Interpretation Has the fault on the right been subsequently rotated or otherwise folded? How must was the thrust on the left rotated to create the geometry seen?

Throw, Heave, and Slip Q Heave Q Throw Q Slip Note that in reality, the over-hanging lip on the hanging-wall collapses onto the top of the footwall (Throw)2 + (Heave)2 = (Slip)2 SinQ = (Throw/Slip) CosQ = (Heave/Slip)

General Rules of thrust faulting: Rule 1 Older deeper rocks are emplaced above younger shallower rocks, thrusts repeat strata in boreholes and bury rocks in map view

General Rules of thrust faulting: Rule 2 Assuming layer-cake stratigraphy, thrusts cut up-section in the direction of transport Original layer cake template

General Rules of thrust faulting: Rule 3 Thrusts propagate in the direction of transport By that we mean new thrusts break out in front on old ones Show duplex movie Transport Direction

General Rules of thrust faulting: Rule 4 Hanging wall and footwall cutoffs and cutoff angles should match up and be equal (except where fault slip is consumed by shear in sediments)

An astounding duplex . Slip on each slice is about half of its length, making a hindward-dipping sequence. Duplexes form in response to the strength of the layer being shortened and the layer upon which sliding occurs (the base of the stack. A delicate interplay must thus exist between rock strength and frictional sliding

General Rules of thrust faulting: Rule 5 Older thrusts are carried on younger thrusts

General Rules of thrust faulting: Rule 6 Higher older thrusts become folded as younger thrusts climb ramps

Influence of pore fluid pressures A paradox in thrust belts is that the thrust sheets themselves are often very broad, yet relatively thin. The Lewis thrust in Montana is a classic example of such a fault, many times wider than it is thick yet with at least 10 km of slip. There is almost no strain at the thrust contact, with < 1m of relatively undeformed shale. The best explanation is that the thrust contact was under very high pore fluid pressure, effectively, the lithostatic load. A great analogy is a plastic puck on an air-hockey table, it has almost no friction due to the air lifting it off the table.

Drawing a balanced cross section In the following exercise, please complete the cross sections of a fault bend fold (an anticline) formed above a simple thrust ramp. The section should Maintain bedding thickness The dip of the back limb should be equal to the dip of the thrust Use interlimb bisectors as axial surfaces (shown as dashed lines) Pin bends in the fold to bends in the thrust The thrust steps up from left to right The thrust is marked by the thicker line on both sides of the section Be careful drawing in the strata as there are exact solutions (the key is locating where the bends in the faults are located). A Buff One card makes a good straight edge.