Geology 3120 Horses and Wedges 1 Outline Horses

Geology 3120 Horses and Wedges 1

Outline • Horses and Wedges? • Duplexes • Fenster and Klippe • Critical Taper Wedges in Thrust Faulting 2

Imbricate Fan Thrust sheets splay off a sole thrust 3

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Duplex • Sole thrust and roof thrust • Blocks are surrounded by faults - horses 5

Duplex structures in quartzite 6

In-Sequence Thrusting 7

Out-of-Sequence Thrusting 8

Duplex Categories • Dip of foreland hinterland duplexes • Displacement verses length of horses 9

Hinterland-Dipping Duplex • Displacement < length of horse • Most common 10

Antiformal Duplex • Displacement = length of horse • Rare 11

Forward-Dipping Duplex • Displacement > length of horse • Almost never 12

Upheaval Dome Duplex • Map the horses • What kind of duplex is this? 13

Fenster (Window) and Klippe Fenster 14

Chief Mountain, Glacier National Park 15

Chief Mountain • Compression • Thrust initiation • Thrust transport • Erosion 16

Critical Tapered Wedge (Coulomb Wedge) • Shoveling snow • Bulldozing soil • Thrust systems in mountain belts 17

Controlling Factors a b m i, l mb If mi > mb, then (a + b) = f(internal friction mi, basal friction mb, fluid pressure l) 18 (a + b) does not depend on length, width, or any size parameter

Wedge States a b • Subcritical - no slip on the detachment; sedimentation and thrusting build the wedge, erosion degrades the wedge • Critical - metastable state of a and b ”Steady State” • Supercritical - slip on the detachment, no internal deformation of the wedge 19

Taiwan West East 20

Sean Willett’s movie of particle paths through a critical wedge Need url for his website to run movie… 21

Phaedra Upton’s movies of critical wedge through Puli, Taiwan Vertical strain = red; erosion included Vertical strain = red; erosion not included 22

References Slide 1 http: //www. mitani. com. au/pws. htm Slides 3, 4, 6, 7, 15, 17, 18 Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2 nd ed. , John Wiley & Sons, New York, 776 p. , 1996. Slide 5 http: //earth. leeds. ac. uk/faults/thrust/belts/duplex. htm Slides 8, 9, 10, 11, 12 Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co. , New York, 532 p. , 1992 23