Structures in extensional terranes

Lecture index:

Image of the Afar triple junction area where seafloor spreading on the Red Sea (tip of which is in upper portion of image) and the Gulf of Aden, is linked to continental rifting of the East Africa Rift zone. Image courtesy of NASA.

Suggested reading:

Key terms and concepts:


Listric normal faulting

Listric normal faults: The recognition that many high-angle normal faults (with dips around 60°) where attached to low-angle normal faults and detachments at depth led to the understanding that extensional provinces also exhibit a thin-skinned tectonic style.

Structures in cross section associated with listric normal faulting:

rollover anticline or hanging wall antiform:

hangingwall synform:

domino fault pattern:

Small scale imbricate normal faults within basement rocks in footwall of turtle-back shear zone of Death Valley. The basement rocks were ductilely sheared , and so this is a brittle overprint on mylonites due to the changing conditions as the extensional fault zone developed with time.

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

Cartoon of some of the mechanisms of crustal extension.1= thin-skinned normal faulting, 2 = dike swarm injection, 3) pervasive ductile stretching in lower crust, 4) magma intrusion into accommodation space, 5) perasive ductile stretching of lithosphere (remember that ultramafic rocks relatively strong) 6) ductile faulting, 7) thermal erosion of lithosphere (e.g. from a plume).


Sedimentologic response to rifting

half grabens and associated basins:

What would you expect the sedimentary facies patterns and architecture to be?

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

From USGS site - Stratigraphic code, depicting simple graben fill. This would be distinctly different from a half graben situation. Source: http://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html .

This is a view from the toe of an alluvial fan on the active fault margin of the main rift in Death Valley National Park, Note the person in the lower right for scale. Also note the topographic scarp that runs along the mountain front near the mouth of the small, steep canyon - this is a fault scarp from a relatively recent movement episode. The coarse material on the alluvial fan is mainly transported as debris flows that occur during rare times of intense rainfall and consists of sediment that is known as fanglomerate. This alluvial fan is sourced from the small steep canyon at its apex and fault movement renews the potential energy that helps drive sediment transport.

This is a close-up view of fanglomerate exposed in a small erosional gully in Death Valley. In the background you can see that sediment surfaces locally approach the angle of repose.

These brown and red sediments are earlier rift fill sediments, including fanglomerates, later tilted by continued faulting as Death Valley evoloved.. From the tilt you can predict in which direction the major border fault lies.

The view here is across the Death Valley rift to the west near the Bad Water area. In the foreground are salt-pans, part of the evaporitic playa deposits. Across the way a very large alluvial fan can be seen. The larger alluvial fan is developed on the tilt side of this overall half-graben structure.

The view here is to the west and at the foot of one of the large alluvial fans on the tilt side of the rift. Note the abrupt transition from playa muds to fanglomerate material that occurs here.

The view here is 180 degrees from that above, looking east across the rift basin to the active fault block with playa muds in the foreground.

The basin during one of the rare times rain fills the salt flats.


Map patterns and segmentation of extensional systems

Concept of along strike segmentation of the rift border: along a fault zone's length distinct segments occur.

Simplified map of Lake Baikal rift from USGS site http://marine.usgs.gov/fact-sheets/baikal/. This is a fairly good image for thinking about along strike changes and segmentation of a rift.


Role of igneous activity in extension

Igneous activity, e.g. dikes, can be an important part of a structural picture.

 

Experimental investigation of crack pattern associated with the intrusion of a blunt tipped dike, suggestive of the small scale structures that might be found near dike tips. Image source: http://lvo.wr.usgs.gov/InyoEruption/fault_caption.html .


Examples of well known rift zones

East African rift zone - crustal extension, incipient continental rifting

Image to right is a satellite image of the part of the East Africa Rift in Kenya near Lake Begoria. Image source is: http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA11082 . One can clearly make out the myriad of fault scarps and how they are segmented. Notice how the direction of the faults in the upper left portion is different from that in the eastern (right portion of the image. The area covered is 40.5 by 32 km in size.

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

a) propagating 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 modified understanding of triple junction geometry evolution, where reactivation of previous structure plays a crucial role.
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.

Map and cross section diagram showing evolution of two of the east coast Triassic rift basins. Note the well developed half-graben character. Image source and more details at http://3dparks.wr.usgs.gov/nyc/mesozoic/mesozoicbasins.htm .

Rhine Bresse grabens - crustal extension in foreland of and in association with convergent Alpine orogeny to south.

Keweenawan province:

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

Lake Baikal

Seismic section and interpretation of Baikal rift basin geometry. Image source: http://marine.usgs.gov/fact-sheets/baikal/ .

 

Basin and Range, SW U.S.

Right: Combined geologic map and DEM of basin and range province from USGS site tapestry of time: http://tapestry.usgs.gov/features/22basinrange.html . The striped pattern that looks a bit like stretch marks are stretch marks

Components of larger event:

Schematic cross section of a metamorphic core complex. Core complexes occur as thin-skinned extension of the brittle carapace reaches a point where the ductile deformation of the underlying metamorphic rocks (and often magmas) forms a large scale arch. Both lateral channel flow of the ductile crustal material and isostasy play a role. The result is a anticlinorium with a metamorphic core, and separated by a brittley extended carapace by a complex zone of mylonites and brittle fault rocks. In reality metamorphic core complexes are fairly diverse in their character.

Complex history of development of Basin and Range features:

Cross section image (greatly simplified) showing low angle and consistent dip direction of normal faulting in Death Valley area. Image source: http://geomaps.wr.usgs.gov/parks/deva/ftdan1.html.

Models for formation:

Seismic risk associated with Basin and Range faulting is significant. Suggests, along with GPS geodesy that distributed Basin and Range strain still occurring.

Map produced by USGS as part of study of seismic risk in Oregon and parts of Nevada.


One thing is clear - normal faulting occurs in a great diversity of tectonic settings.


Some general references on continental extensional tectonics:

References on the Basin and Range province:


Copyright Harmon D. Maher Jr., This may be used for non-profit educational purposes as long as proper attribution is given. Otherwise, please contact me. Thank you.