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Extensional Regimes
Ch. 17, p. 333-‐
1. Normal Faults: Extension is accommodated by normal faults (FW moves up, HW moves down). Normal faults typically form with dips of ~60° but can be vertical at the surface (often reactivating existing vertical features, such as joints). At depth, they can curve in a concave-‐upward manner (listric) and become almost horizontal. This is called a detachment fault.
[Fig. 17.1. Normal faults can have a range of dips but typically form with dips of ~60°]
2. Listric Normal Faults: Detachments: Listric faults may form rollover anticlines. The detachment portion of the fault usually contains mylonitic fault rocks, implying a transition to ductile behavior.
[Figure. Detachment fault interpreted from seismic reflection data. From Twiss & Moores (2007)]
3. Normal Fault Surface Geometry: At the surface, normal fault traces are usually sinuous, reflecting the linkages between individual segments during fault growth.
[Figure. Segmented normal fault, Stillwater Range, NV. From Twiss & Moores (2007)]
4. Extended Environments: Extended regions contain systems of normal faults with similar strikes but may dip in mutually opposite directions, forming a conjugate set.
A shared hanging wall between a conjugate set forms a down-‐dropped graben. A shared footwall between a conjugate set forms an uplifted horst.
[Figure. Components of extended environments. From Twiss & Moores (2007)]
5. Extended Environments: Subsidiary faults dipping in the same direction as the major fault are synthetic. Those dipping opposite to the major fault are antithetic. 6. Extended Environments: Numerous synthetic major faults may form a series of half-‐grabens. 7. Extended Environments: Detachment faults have deformed hanging walls comprised of imbricate faults. They may terminate at the detachment fault or curve into it. 8. Extended Environments: Imbricate faults rotate into shallower dips through time, rolling over like dominoes (the “domino model”) and causing progressive steepening of beds.
[Fig. 17.3. Rigid domino model of fault rotations, also called bookshelf faulting]
9. Bookshelf Faulting: In the domino model (bookshelf faulting), all faults have the same dip, slip sense, slip rate, rotation rate, and cumulative slip. Simplistically, there is no internal deformation of the blocks; however, such deformation usually exists (i.e., not really rigid).
[Box 17.2. North Sea domino model for imbricate faults]
10. Kinematic Issues: Cross sections across normal faults must make geologic sense. When restored, a viable result must occur.
[Figure. Restoring slip on a normal fault cross section. From Twiss & Moores (2007)]
11. Kinematic Issues: Listric faults provide some challenges in this regard as geometric space problems exist. A rollover anticline may form, or the HW is imbricated. Apparent gaps along the detachment would need to be removed by ductile deformation or a mobile layer (e.g., salt).
[Fig. 17.4. Accommodating space problems along listric faults] [Figure. Space problems in the hanging wall of a listric fault. From Twiss & Moores (2007)]
12. Formation of Low-‐Angle Normal Faults: Low-‐angle normal faults provide somewhat of a mechanical enigma as they form at a high angle to the maximum compressive stress direction (s1), which is vertical in extending environments. Their existence was undocumented before the 1970s and their mode of formation is still debated.
[Fig. 17.7. Low-‐angle detachment fault in the Norwegian Caledonides]
13. Metamorphic Core Complexes: One possibility for low-‐angle detachment faults is that extension and thinning of the crust causes uplift, which rotates older imbricate faults into shallow dips and renders them inactive. New faults form; ongoing uplift repeats the process until a domino-‐like style of imbricate faults is produced.
Uplift exposes the deeper crustal rocks and is called a metamorphic core complex. They usually show mylonitic rocks with superposed late-‐stage brittle deformation postdating uplift.
[Fig. 17.8. Progressive formation of a metamorphic core complex]
14. Metamorphic Core Complexes: Metamorphic core complexes were first described from the Basin and Range of the western USA but have subsequently been found around the world, including along the mid-‐ocean ridge systems.
[Box 17.4. Metamorphic core complexes in the western USA]
15. Metamorphic Core Complexes:
[Figure. Metamorphic core complexes in the Whipple Mountains region, NV. From Twiss & Moores (2007)]
