GEOL372: Week 5 Thrust fault systems. Contractional regimes

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1 4/3/15 GEOL372: Week 5 Thrust fault systems Contractional regimes Thrust faulting accommodates shortening σ3 σ1 1

2 Thrust geometry Thrust geometry RAMP segment of fault with different strike and/or dip conneccng flat segments 2

Thrust fault structures Cut up-section in the direction

bedding in competent Cut up-section in both HW and FW

deform as they go over a ramp Geometry of ramp controls

3 Thrust fault structures Cut up-section in the direction of transport Parallel bedding in incompetent Cut across bedding in competent Cut up-section in both HW and FW ramps Ramps and fault bend folds Hangingwall layers deform as they go over a ramp Geometry of ramp controls geometry of fold HW strata deform twice as they translate over ramp 3

4 Thrust structures Thrust structures 4

5 Thrust structures Thrust structures 5

Thrust systems: Imbricates Branch points

(hinterland dipping) Roof thrust Sole/Floor

6 Thrust systems: Imbricates Branch points Imbricate fan Floor (sole thrust) Blind thrusts Shortened and thickened sequence Sole thrust Thrust systems: Duplexes Duplex (hinterland dipping) Roof thrust Sole/Floor thrust Anticline-syncline pairs Bedding fault-parallel at inflection 6

7 In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one In-sequence thrusting Next slip surface Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one 7

In-sequence thrusting displaces Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old

8 In-sequence thrusting displaces Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one 8

9 In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one 9

In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one In-sequence

10 In-sequence thrusting Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one In-sequence thrusting and so it continues... Sole thrust propagates forward towards foreland, then cuts up-section at a ramp Newer horse forms in front of old one 10

11 Thrust geometry Thrust surfaces intersect at branch lines Erosion creates branch and tip points Out of sequence thrusting 11

12 Ramps and back thrusts Ramps are stress guides i.e. rigid obstacles Antiformal stacks Slip on splay faults is large compared to their length (and/or not equal) 12

13 Antiformal stacks Slip on splay faults is large compared to their length (and/or not equal) Antiformal stacks Slip on splay faults is large compared to their length (and/or not equal) 13

Eventually fault breaks through fold limb Fault propagation folds

14 Fault propagation folds As fault tip propagates, units ahead start to fold to accommodate strain Fold moves as tip moves relative to FW Eventually fault breaks through fold limb Fault propagation folds Characteristics of fault propagation folds that are different to fault bend folds 14

15 Fault propagation folds 2 Photo by Gerhard Wiesmayr, University of Vienna 15

16 Photo from Detachment folds Folds above slip along a single detachment (no ramp) HW shortens more than FW Common above very weak detachments Occur at all scales 16

17 4/3/15 Thrust terminology From Twiss and Moores (2006) Fenestre Allochthonous rocks Autochthonous rocks Thrust system geometry Branchline maps 17

18 Thrust system geometry Thrust system geometry 18

deformed Extensive shortening, faulting and folding in

19 Thrust system geometry Shows how thrust surfaces relate to each other Thin skinned thrusting Basement not deformed Extensive shortening, faulting and folding in cover above a décollement (aka detachment, aka sole thrust) 19

20 4/3/15 Fold-thrust belts Photo from Casey Moore 20

21 Thick skinned thrusting Basement is shortened in hinterland Thick thrust sheets, igneous and metamorphic rocks Entire crust is imbricated Where do they form?? Oceanic- ConCnental ConCnental- ConCnental van der Pluijm and Marshak,

22 4/3/15 Video from KonstanCnovskaia and Malavieille, 2005; G- cubed Look for: decollement, horses, duplex structure, backthrusting Folds and thrusts in accretionary prisms A modern example, using seismic imagery from Moore et al. (2007), Science 22

23 The Franciscan Complex: an ancient accretionary wedge Elise Kni[le, UCSC Series of terranes with different rock types stacked up Complicated now by the San Andreas 23

24 24

25 Where do they form?? Oceanic- ConCnental ConCnental- ConCnental Thin skinned thrusting: the Moine Thrust system 25

26 26

27 Dundonnel structure: Peach et al Ben Peach notebook sketches

28 Orogenic wedges Contractional systems are wedge-shape in crosssection Deformation and wedges The wedge shape is like a pile of snow ahead of a plough? Deformation internally and erosion maintain roughly constant shape 28

29 Critical wedges Parameters that control wedge shape: Basal friction Internal friction Erosion rate at surface Geometry of base Everything is on the verge of failure The influence of gravity Gravitational potential can drive collapse of a wedge Some observations of orogens can be explained by gravity alone But overall strain patterns are testable 29

30 Gravitational collapse: continental shelf Weak detachment dips oceanwards Whole sequence of sediments slip down hill Both extension and contraction 100km From Butler and Paton (2010) 30

Lecture 15. Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System

Lecture 15. Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System Lecture 15 Fold-Thrust Belts, and the NJ Ridge and Valley Thrust System Earth Structure (2 nd Edition), 2004 W.W. Norton & Co, New York Slide show by Ben van der Pluijm WW Norton; unless noted otherwise