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Contractional Regimes
Ch. 16, p. 312-‐
1. Contractional Deformation: Rocks are contracted in regions undergoing horizontal shortening, often at collisional/convergent plate boundaries. The shortening may be taken up in a number of manners:
(a) Volume loss (pressure solution) (b) Thickening (pure shear) (c) Buckling (folds) (d) Faulting (thrust and reverse faults)
[Fig. 16.1. Mechanisms of accommodating horizontal shortening]
2. Contraction by Faulting: Faulting and folding are readily observable forms of contraction. Thrust faults tend to have dips of ~30°; reverse faults (>45°) are steeper but accommodate less shortening. Thrust faults and folds are commonly interrelated and may create important structural traps (e.g., oil and gas).
[Fig. 16.2. Thrust fault and related fold highlighted by sedimentary beds in Chile. The HW is translated over the FW in a thrust fault]
3. Thrusts: Thrust faults commonly have a HW that is thin relative to its length and is called a thrust sheet or nappe. Multiple thrust sheets may be stacked on top of each other, forming a nappe complex.
[Box 16.1. (a) Precambrian gneiss thrust over Cambrian quartzite along the Moine thrust, Scotland. (b) Thrust nappe of Precambrian basement overlying Cambrian quartzites, Scotland]
4. Thrust Nappes: The basement rocks below the thrust underneath a thrust nappe are essentially “in place” (not transported) and are called autochthonous. If the nappe rocks were transported a great distance (10s-‐100s of km), they are allochthonous and the nappe can be called an allochthon.
[Box 16.1. Allochthonous and autochthonous rocks related to the Moine thrust in Scotland]
5. Thrust Nappes: Thrust faults may be listric, curving into a very shallow dip (~a few degrees) below a thrust sheet, forming a décollement. The low angle intersection with the Earth’s surface creates an irregular fault trace. Erosion remnants of nappes are called klippe. Holes through nappes that expose the underlying autochthonous rocks are called fensters.
[Figure. Terminologies related to thrust nappes (Twiss & Moores, 2007)]
6. Thrust Imbrication: Thrust sheets may be internally dissected by numerous thrusts that imbricate the thrust sheet. This creates a number of slices of the thrust sheet called horses that break away from the basal décollement or sole thrust. Numerous back-‐to-‐back horses define a duplex structure (S-‐shaped horses).
[Fig. 16.5. Imbrication zone in a thrust sheet, defined by horses that thrust away from a sole thrust or floor thrust] [Fig. 16.6. Thrust duplex in sandstones in Svalbard]
7. Duplex Development: Imbrication zones form when thrusts initiate across more competent layers (sandstone, limestone) and connect a sole (floor) thrust with a roof thrust.
If the duplex advances in the direction of thrusting (the vergence direction), it is in-‐sequence thrusting (from the hinterland towards the foreland).
[Fig. 16.9. In-‐sequence thrusting produces a duplex that migrates towards the foreland]
8. Duplex Development: The floor thrust and roof thrust bound the horses that define the duplex structure.
[Figure. Duplex development (Twiss & Moores, 2007]
9. Flat-‐Ramp-‐Flat Geometry: The floor thrusts and roof thrusts, connected by thrust ramps, define a flat-‐ramp-‐flat geometry. As thrust sheets get thrust up the ramps and onto the roof thrusts, the stratigraphic section is repeated.
[Figure. Flat-‐ramp-‐flat geometry along a thrust fault (Twiss & Moores, 2007)]
10. Duplex Development: Complex duplex structures may ultimately develop that allow significant horizontal shortening across a thrust nappe. The geometric configuration varies depending on whether the duplexing evolves toward the foreland or the hinterland.
[Fig. 16.7. Cross section through the Caledonian foreland in Norway, showing an imbricated duplex system in the thrust sheet]
11. Duplex Development: As duplexes evolve, the roof thrust is progressively abandoned as new horses get added to the duplex with the development of new thrust ramps.
[Figure. Foreland-‐migrating duplexes (in-‐sequence thrusting) create horses that dip back towards the hinterland] [Figure. Hinterland-‐migrating duplexes (out-‐of-‐sequence thrusting) create a different duplex internal geometry, with horses dipping towards the foreland (Twiss & Moores, 2007)]
12. Duplex Development: Depending on the relative amounts of motion on each thrust ramp, duplexes may become very convoluted and may start to stack horses on top of each other to form an antiformal stack (“piggyback” duplexing).
[Fig. 16.15. Piggyback style of duplexing interpreted from seismic data, Taiwan]
13. Backthrusts: High strain zones within a thrust sheet may result in the development of backthrusts, which dip in the opposite direction to the main thrust fault.
[Fig. 16.10. Development of backthrusts within thrust sheets. Example on the right is from the Caledonian foreland, Norway]
21. Folds in Seismic Data:
[Box 16.2. Fault-‐propagation fold interpreted from seismic reflection data, Colombia. This fold forms an important oil trap and formed by faulting through an initial detachment fold. Deformation here was coincident with sedimentation]
22. Fold-‐and-‐Thrust Belts: These fold types illustrate that thrust faults are commonly associated with folds and often form a tectonic environment called a fold-‐and-‐thrust belt. They form in plate convergent settings and are major orogenic belts, a few 100s km wide and 100s to >1000 km long.
[Figure. Fold-‐and-‐thrust belts in (A) the Appalachians, and (B) the Canadian Rockies. From Twiss & Moores (2007). From Twiss & Moores (2007)]
23. Fold-‐and-‐Thrust Belts: Thrust faults in these belts typically sole into a basal décollement that dips gently towards the hinterland. So the orogenic belt thickens in this direction. Thrust ramps dip at ~30° and form an imbricate fan. These faults may or may not reach the surface (the latter are blind thrusts).
[Figure. Fold-‐and-‐thrust belts in (A) the Appalachians, and (B) the Canadian Rockies. From Twiss & Moores (2007). From Twiss & Moores (2007)]
