Lecture index: Listric normal faulting. / Sedimentologic response. / Map patterns and segmentation of extensional systems. / Role of igneous activity in extension. / Examples of well known rift zones. /

Reading:

• Lister, G. & others, 1986, Detachment faulting and the evolution of passive continental margins; Geology, vol. 14, p. 246-250.
• Scholz, C. H. & Contreras, J. C., 1998, Mechanics of continental rift architecture; Geology, v. 26, p. 967-970. Elegant paper with very testable assertions as to the 3-D geometry of rifts.

Key terms and concepts:

• half grabens
• growth faults
• listric normal faults and thin-skinned fault style
• rollover anticlines
• X and V geometries in association with detachments
• intrabasinal unconformities
• segmentation and arcuate traces
• relay and tear faults
• high angle normal faults cutting low-angle normal faults
• role of igneous activity in extension
• metamorphic core complexes
• Basin and Range as an example
• variety of tectonics settings

Lecture outline:

• I. non-Andersonian geometries and associated structures in cross section.
• II. the sedimentologic response.
• III. fault rotation in a Mohr perspective.
• IV. map patterns and segmentation.
• V. the role of igneous activity.
• VI. the role of isostasy and metamorphic core complexes.
• VII. Basin and Range and other examples.

Listric normal faults - the recognition of low-angle normal faults and detachments at depth -> thin-skinned tectonic style.

• 2 observations indicate normal faults shallow at depth
  • surface geometry - tilting of strata, i.e. block rotation and half graben development.
  • deeper seismic sections, which show entirety of geometry.
• interesting that while surface geometry was always available, it was seismic sections that convinced geologic community.
• many normal faults curved so that flatten out at depth.
• see same pattern on a small and accessible scale - where? earth slumps, curved slip surface + rotation.

Structures in cross section associated with listric normal faulting:

rollover anticline or hanging wall antiform:

• direct function of listric character.
• follows general theme of hanging wall deformation due to non-planar fault geometry.

hangingwall synform - more than one step., compaction syncline, or fault propagation fold

tri-shear zones and fault 'drag'

domino fault pattern:

• strong synthetic set.
• associated with thin-skinned extension - master detachment with shallow dip.
• history of later high-angle faults cutting low-angle faults in zones of severe extension.
  • View from the Mohr perspective: dominoe set characterized by counter slip rotation. Eventually lock up - rotate out of a favorable position for slip.
• direction of propagation (hanging wall collapse).

Scholz & Contreras model of X and V faults, for asymmetric and symmetric faults.

Major detachment often at ductile-brittle transition. Shallower detachment levels are possible.

half grabens and associated basins:

• active fault margin on one side, other side a rotating surface.
• intrabasinal or intraformational unconformities.

think of sedimentary facies patterns in this case?

• coarse clastic fault-related margin, fault scarp alluvial fans, fanglomerates
• fining trends to basin axis
• an axial paleocurrent system (river or turbidite if deep water).
• internal drainage and evaporites.

possibly unroofing sequences in clastics = important history of uplift + erosion

relay faults, tear faults.

concept of segmentation due to fault linkage, evident in slip gradients.

some evidence that in map view trace of normal faces are arcuate (J. Karson, EAR)

igneous activity, e.g. dike, part of structural picture. Kinematics of dike swarms: locally can accomodate several tens of persent of extension.

role at depth of intrusions (speculative). Tested by gravity models?

role in 'lubrication'.

East African rift zone - crustal extension, incipient continental rifting

description of EAR:

• 3000 km long, 45-65 km wide.
• Ethiopia, Kenya domes - about 700 m height, formed in lower-middle Miocene.
• fault throws - 2000 to 4000 m.
• many faults parallel older basement structures.
• negative Bouger gravity anomaly (mass deficiency) - remember an ongoing rift.
• Trap and Afar volcanic series
• fissure flood and plateau basalts, Tertiary to Recent age.
• Ethiopia area - 30 km extension, Kenya - 10 km, Tanzania 2-3km. N-S gradient
• recent data suggests it is thin-skinned.

Model for rift development: hotspot, triple junction pattern: Burke and Dewey reference, 1973

a) propogating crestal rifts with 120° geometry on top of thermal domes with concurrent igneous activity

b) linking of crestal rifts, failed rift arm = aulocogen, rearrangement of underlying mantle convection system.

c) extension concentrated in continuous (perhaps branching) continental rift zone, continental crust notably thinned, perhaps changed in character due to igneous activity.

d) igneous processes dominate along an axial zone, and get orthogonal spreading ridge system and passive margin development.(later workers point out this process may be diachronous along the length of the rift zone).

Dan Inman's senior thesis - triple junction boundary departs radically from 120°, hints at new possible understanding of triple junction geometry evolution

Interesting implication of model of cont. rifting to oceanic spreading evolution. All passive continental margins are underlain by rifted continental crust.

Triassic rift basins - east coast of U.S.

Rhine Bresse grabens - crustal extension in association with convergent Alpine orogeny.

Keweenawan province:

• > 10,000 m volcanic rocks, 6000 m of terrestrial sandstones and conglomerates.
• forms synclinal trough with fill overlapping any fault margin sides.
• 1.1 Ba age, 100 Ma time span.
• mid-continent gravity high -> basalt fill.
• layered intrusions -> Duluth gabbro lopolith.
• local compressive reativation.
• COCORP - thin-skinned basin.

Himalayas-Tibetan plateau N-S trending, E-W extension oblique to zone of concurrent thrusting and convergence.

Lake Baikal

Basin and Range

• divergent plate boundaries.
• gravitational spreading of thickened continental crust (in high portions of contractional orogens).
• foreland indentor tectonics (e.g. Rhine graben).
• relay or releasing bends in transcurrent boundaries.
• outer beam extrados position - e.g. with subducted slab.
• mass wasting - upper portion.
• continental shelves -> down to the coast faults (e.g. Gulf coast).
• on tops of salt domes.
  

Some general references on continental extensional tectonics:

• Baker, B. H. & others, 1972, Geology of the Eastern Rift System of Africa; Geological Society of America Special Paper 136, 67 p. Mainly a thorough descriptive effort.
• Burke, K. & J. Dewey, 1973, Plume-generated triple junctions: Key indicators in applying plate tectonics to old rocks; Journal of Geology, vol. 81, p. 406-433. A classic in plate tectonics describing a mechanical-historical model for continental rifting with many testable aspects (developed in the context of the EAR).
• Craddock, C., 1973, Structural evolution of the Keweenawan Province; Geology, vol. 1, # 4, p. 190. Describes the history of this Precambrian intracontinental rift.
• Courtillot, V. & Vink, G. E., 1983, How continents break-up: Scientific American, p. 43-49.
• Lister, G. & others, 1986, Detachment faulting and the evolution of passive continental margins; Geology, vol. 14, p. 246-250.
  Describes a variant on the Wernicke model and applies it to continental margins, discussing the effect of the inherent asymmetry of Wernicke's and their model on margin development.
• Mohr, P., 1987, Structural Style of Continental Rifting in Ethiopia: Reverse decollements: EOS, p. 721, 729-730.
• Rosendahl, B. R., 1987, Architecture of continental rifts with special reference to East Africa; Annual Review of Earth and Planetary Sciences, 15, 445-503. Discusses segmentation, and the relation to deeper underlying processes.
• Ruppel, C., 1996, Extensional Processes in Continental Lithosphere; Journal of Geophysical Research, v. 100, p. 24,187-24,215. An excellent summary of continental rifting - a very good place to start, and to look for additional references.
• Scholz, C. H. & Contreras, J. C., 1998, Mechanics of continental rift architecture; Geology, v. 26, p. 967-970. Elegant paper with very testable assertions as to the 3-D geometry of rifts.
• Serpa, L. & others, 1984, Structure of the southern Keweenawan rift from COCORP surveys across the midcontinent geophysical anomaly in northeastern Kansas; Tectonics, vol. 3, # 3, 367-384. Demonstrates the listric, low-angle character of this rift.
• Illies, J. H. (ed.), 1981, Mechanism of Graben Formation - Development in Geotectonics 17; Elsevier press, New York, 226 p.
• Wernicke, B., 1981, Low-angle normal faults in the basin and range province: nappe tectonics in an extending orogen; Nature, vol. 291, p. 645-647. A crucial paper reinforcing the idea of thin-skinned extension, and applying it to the entire plate thickness.

References on the Basin and Range province:

• Allmendinger & others, 1983, Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah from COCORP seismic reflection data; Geology, v. 11, 532-536.
• Davis, G. & Lister, G., 1988, Detachment faulting in continental extension; Perspectives from the southwestern U. S. Cordillera; GSA Special Paper 218, p. 133-161.
• Gans, P. B., Mahood, G. A., & Schermer, E., 1989, Synextensional magmatism in the Basin and Range Province: A case study from the eastern Great Basin: GSA Special Paper # 233, P. 53.
• Lister, G. & Davis, G., 1989, The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.
• Mayer, L. (ed.), 1986, Extensional Tectonics of the Southwestern United States: A perspective on Processes and Kinematics: GSA Special Paper # 208, 122 p.
• Wernicke, B., 1981, Low-angle normal faults in the Basin and Range province: Nappe tectonics in an extended orogen; Nature, v. 291, p. 645-648.