Reading Displacement chapter, in library (not available online). Read 18.0-18.4 plus 18.12. Concept map due Wednesday. Davis and Reynolds “ Structural Geology of Rocks and Regions” p. 319-370, also on reserve in library Concept maps not required for D&R chapter

Why Study Reverse Faults? Host the largest, and potentially most destructive earthquakes (subduction zone thrusts). Low dip requires that faults have large surface area in brittle "seismogenic zone" and that this surface area is close to ground surface where we live. Associated with mountain building and collisional tectonics. Low-angle faults with big displacements have been known since 1800's (Lapworth). Influence positions of ore deposits (high-angle reverse faults) and hydrocarbons (thrusts).

Shaw and Shearer: Blind thrust fault beneath Los Angeles metropolitan area, interpreted from seismic reflection profiles and precise earthquake locations

Pittman and Ryan

Initiating subduction Generally will reactivate pre-existing structures, such as rift-related features preserved on passive margins (producing continental arc) or transform faults in mid-ocean ridges (producing island arc)

Structural components of convergent margins

Other elements of thrust belts Thrusting may be thin-skinned (involving only sedimentary cover) or thick-skinned (involving basement) Large overturned folds, or nappes (French word for napkin) may form

Accretionary tectonics First recognized in North American Cordillera Far-traveled terranes may have docked to continent along either reverse or strike-slip faults

X-section through Cordilleran Terranes Sutures bound accreted terranes Exotic terranes include arcs, microcontinents, and slivers of colliding continental material

Much of what we know about thrust terranes is a gift of erosion Klippe (pl. klippen), from German word meaning ‘slice’, is the erosional remnant of a hanging wall Window is an erosional hole through hanging wall, allowing footwall to be viewed Autochthonous material is in place Allocthonous material has been transported along faults from location of origin

Elements of a fold and thrust belt Elements of a fold and thrust belt. Note the asymmetry of both horse geometry and folds that records transport direction (also known as vergence)

Break-thrust fold

Development and 3D form of a fault-bend fold

Fault propagation fold

Trishear zones exhibit tighter folds with increased proximity to fault tip (blind thrust)

Patterns of detachment folds are less regular than other fault-related folds

Extension may also occur locally in association with subduction zones

…Or in association with continent-continent collisions, where ‘orogenic collapse’ is one way that mountain systems achieve isostatic equilibrium

Mechanical paradox Thrust movement on low-angle surface with ‘typical’ coefficient of friction of rock requires stresses high enough to break the rock To understand how this motion takes place, we need to think about concepts of displacement, stress, and fault mechanics

Three descriptions of mechanical interactions Displacement describes the movement of particles with respect to an external reference frame Deformation can be described by a displacement field Translation Rotation Shape change (distortion) = strain Stress is force per unit area

Components of Deformation

Deformation

Three categories of displacement Relative particle motion can be described even if part of the system is fixed in space