Week 3: Parting the
continents - continental rifting.
Introduction to continental rifting
Intro comment: Ocean basins, and the oceanic crust within, are bounded by continental rocks. Running seafloor spreading backwards also brings the continents back together, and so it is clear that continents come apart to make new oceanic basins. In this section we will study what happens when continents come apart. Sometimes they come apart so far that an oceanic basin is born. But it turns out that the behavior is much more complex, and sometimes the rifting aborts, sometimes it is concentrated, sometimes it is more diffuse, and localized rifting can occur in what may initially seem unlikely positions - on top of convergent mountain belts for example. So we will look at continental rifting more broadly here.
Reading: Rift Basin Architecture and Evolution by Roy W. Schlische & Martha Oliver Withjack at http://www.ldeo.columbia.edu/~polsen/nbcp/breakupintro.html.
Wilson cycle concept.
The basic idea in the Wilson cycle is that continental rifting
tends to occur along old orogenic axes, so that these become the
edges of the rifted continental fragments next to an oceanic basin that is opening. Eventually subduction starts in the ocean, the ocean basin closes, and the bounding continents eventually collide
to form a newmountain belt. These are preferential weakness zones, setting up the axis for the next rifting
episode. It is basically a cycle of open basin opening and closing. The classic case example is the Appalachian orogen (see
adjacent diagram). There are plenty of examples where the Wilson
cycle is not followed faithfully - it is much more of a schema.
Continental rifting is a convenient
starting point (but not a simple one). Continental rifts are locations
of continental crustal extension/divergence, crustal thinning,
sedimentary basin formation, and often thermal and igneous activity.
As indicated, they are diverse, complex and polygenetic. Continental crust can only be thinned so far and at some point and in some way seafloor spreading takes over.
Eventual fates of continental
- ocean basin growth and passive
- the implication is that old rift basin structures underlie passive continental margins such as the east coast of North and South America.
- aulocogens (failed rift arms).
- interior rift basin (prone
to reactivation of multiple types).
Presently active continental
East African Rift zone:
- rift basins and lakes, volcanics, domes.
Rio Grande Rift (E side of Colorado Plateau):
Modern hot springs deposits (calcium carbonate) along a fault on the flanks of the Jemez calera taken from on the 2005 UNO field trip to the New Mexico Rift.
Same field trip, same group as above, but in the recesses of a lava tube from a recent basalt flow from the Mal Pais area of New Mexico.
Lake Baikal Russia.
Salton trough, California
Dead Sea rift (transtensive).
Some better known past continental
Triassic grabens along U.S.
- Formed during the rifting of Africa and North America leading to the formation of the North Atlantic Oceanic basin.
- Two well known rift basins: the Connecticut River Valley and the Newark Basin.
- Border faults pose seismic risk, especially the Ramapo fault in lower New York state.
- Cross sections to right show the evolution of a major basin centered on New Jersey. Image and additional information source: http://3dparks.wr.usgs.gov/nyc/mesozoic/mesozoicbasins.htm
- Note the faults and classic half-graben geometry, with a major curved (listric) fault on one side, and an overall wedge shape of the basin fill (yellow colore here).
- CAMP = Central Atlantic Magmatic Province: this is one of the larger LIP provinces, and is thought to hae contributed to the break up of Pangea that the Triassic grabens are part of. A short description of LIPs in general featuring CAMP can be found at http://www.ldeo.columbia.edu/~polsen/nbcp/lipmc.html .
Image showing the footprint of the LIP CAMP. Image source: http://en.wikipedia.org/wiki/File:CAMP_Magmatism_in_the_context_of_Pangea.jpg .
Palisades Sill on the west side of the Hudson River, below Peekskill. This large mafic sill can be traced down to NYC, and is part of the LIP.
Particularly coarse phase of the mafic (basaltic) sill with plagioclase and clinopyroxene grains. Camera lense for scale.
Rhine Graben, in northern
Basin and Range province
of SW U.S..
Map below is from the USGS tapestry site. The striped pattern is due to the Basin and Range province. The map to the right shows the larger extent of the Basin and Range province and is from http://geomaps.wr.usgs.gov/parks/deva/ftdan1.html . Note the overall map distribution form of the province - it is not a linear parting of the ways. This very much speaks to the origin of the rifting.
- Also site of metamorphic core complexes, a distinctive feature associated with extreme continental rifting.
Keweenawan rift and mid-continent
gravity high - interior U.S.. UNO
Below is a map taken from the USGS site: http://pubs.usgs.gov/info/mwni_cu/ . where the interest is in possible nickel-copper deposits. Rifts can not only be the sites of ore mineralization, but can also harbor fossil fuel deposits, and so rifts are a common exploration target - a hot spot.
Diagram to right shows the various mechanism:
- normal faulting.
- high angle normal faulting
- low angle normal faulting
- brittle ductile transition on site of detachment.
- common half graben geometry.
- dike swarms (component 2).
- lower crustal attenuation
- lower crustal intrusion (component
- lithospheric attenuation
is observed with modern rifts. What
are the responsible mechanisms?
- metamamorphic core complexes and lower crustal flow, associated with extreme extension.
Schematic cross section sketch of a metamorphic core complex, which is a large antiformal feature where metamorphic basement is in the core, and separated from overlying, extended cover rocks and sedimentary fill by a fault zone composed of both ductile fault rocks (mylonites) and brittle fault rocks (often with brittle overprinting ductile features). There is discussion on the role of isostatic rebound and/or lower crustal ductile flow play in the development of the large elongate domal feature.
More information, diagrams and photos on the structural geology of rift zones in general, and on the Basin and Range in particular can be found at: http://maps.unomaha.edu/maher/GEOL3300/week14/rift.html .
Rift-related igneous activity
- bimodal signature with respect to composition - lost of basalts and some rhyolites.
- trace element geochemistry:
the template/signature endeavor.
are all factors that determine the chemical composition of a
- contintental rift basalts
enriched in alkalis (K, Ba, Rb), incompatible elements, LIL.
- deep mantle plume contribution.
- mantle fluids and metasomatism.
- lithospheric mantle contribution.
Goat Mountain in Big Bend National Park in Texas. These pyrolcastics from explosive volcanism in the past are part of the Basin and Range rift basin that the National Park is centered on. These are silicic in composition and represent part of the rhyolite end of the spectrum associated with continental rifts. The area is no longer volcanically active, but was about 38-32 million years ago. The image below is from the park and shows the geologic interpretation of Goat Mountain.
Rift basin sedimentation
- sediment traps, accomodation
space (miles deep).
- often arkoses, immature sediments.
- half grabens.
- fault driven sedimentation:
alluvial fans and debris flows (fanglomerates).
- along strike changes in fault and basin architecture produces segmentation,
and seperate depocenters.
- every basin unique: Carboniferous
rift on Bjørnøya.
Below: cross section of rift sediments in the Newark Basin from the USGS. Note the relationship between the sediment type and its position in the half graben.
Death Valley is an excellent place to study the sediments being deposited in an active rift basin, and the images below show some of the relationships.
In the foreground is an alluvial fan in Death Valley (note the person to the right for scale) that consists mainly of debris flow deposits that emanate from the canyon mouth at the apex of the fan. These deposits are fanglomerates. A careful look will show a fault scarp that displaces these deposits and runs parallel to the mountain front. This is part of the rift bounding normal fault (likely with a strike-slip component) that has caused the mountain uplift. The rock basin above is the source of the debris flows that mainly happen during the rare (from a human perspective) and intense rain falls that mobilize debris flows in this part of the world.
Fanglomerate and other Death Valley rift deposits faulted against mylonitized, brecciated and chloritized (from retrograde metamorphism) basement rocks. Note the low angle orientation and low angle truncation of the hanging wall sediments against the fault.
Evaporite (salt deposits) that occupy the middle part of the Death Valley rift, with alluvial fan deposits across the way on the other side.
Thermal and uplift history
Mechanisms and geometry
- higher heat flow due to advective/magmatic
fluid transfer in crust.
- higher heat flow due to thinner
crust and lithosphere.
- thermal and isostatic uplift components.
- off axis thermal and igneous activity, and its relationship to a thin-skinnned tectonic style.
- geothermal map source: http://www1.eere.energy.gov/geothermal/geomap.html
. Given what we have discussed so far, which areas can be attributed to ongoing or recent rifting and which to something else?
Map pattern evolution and
models of continental rifting
Hot spots (mantle plumes), triple junctions
and aulocogens (Burke & Dewey (1973) model).
- stationary plumes -> continental doming,
igneous activity and thinning and weakening of lithosphere ->
development of triple junction pattern of rifts -> propagation
and continued spreading and rift linkage -> domination of
igneous processes over faulting and development of seafloor spreading
center for some rifts, while other rifts cease activity.
- application to EAR, and south
- some questions:
- inadequacy of doming stresses to create rifts.
- validity of triple junction
pattern (as 120 degree angle between arms), and role of previously existing crustal anistropy.
- transition from triple junction
to orthogonal seafloor spreading geometry.
- map from USGS showing a simplified the EAR rift zone:
- Influence of previous structure on rift patterns? One broad example of this is how the Norwegian-Greenland oceanic
basin follows the axis of the Norwegian-Greenland Caledonides.
- Ordovician-Silurian major
contractional mountain building followed by:
- Devonian collapse basins.
- limited Carboniferous rifting.
- Jurassic-Cretaceous rifting.
- Tertiary formation of the
- Episodic rifting along the
Caledonide axis in this area over a course of 300 Ma.
- Rift tip propagation, Vink
and Courtillot model.
- propogating rift tip with
seafloor spreading on one side, and continental rifting on other.
- application to Gulf of Aden
and Gulf of California (key trait, slight obliquity between seafloor
magnetic anomalies and continental margins)
- locked zones
- implications for fits of
- beyond pole-of-rotation compression.
- studies in wax of rift propagation/evolution.
- passive vs. active rifting.
map patterns of continental rifts: How consistent is the direction of fault trends?
How consistent is the direction of downthrow? Are the faults curved
or angular in map plan? Are any other map patterns discernable
(en echelon (staggered), radial, triple junction)? Is segmentation
spacing consistent? Frequency and pattern of branching?
might be factors that determine the pattern of continental rifting?
Other models to explain
- orogenic topographic collapse
- main questions is why localized
rifting within orogenic welts is coeval and spatially related
to crustal thickening in convergent settings.
- best example is the Himalayas.
- rigid indentor models.
- transtensive releasing
bends: will take about
much more later.
- intraplate tectonism: good model for Carboniferous basins.
Some general references on continental extensional
- 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
- 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
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
- 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.
materials for Plate Tectonics, GEOL 3700, University of Nebraska
at Omaha. Instructor: H. D. Maher Jr., copyright. This material
may be used for non-profit educational purposes with appropriate
attribution of authorship. Otherwise please contact author.