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 rifts:

Presently active continental rifts

East African Rift zone:

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.

Death Valley.

Lake Baikal Russia.

Salton trough, California (transtensive).

Dead Sea rift (transtensive).

Some better known past continental rifts

Triassic grabens along U.S. east coast.

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 Europe.

Basin and Range province of SW U.S..

Keweenawan rift and mid-continent gravity high - interior U.S.. UNO field trip.

 


Extensional accommodation mechanisms

Diagram to right shows the various mechanism:

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

Taits:

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

Traits:

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


Map pattern evolution and models of continental rifting

Hot spots (mantle plumes), triple junctions and aulocogens (Burke & Dewey (1973) model).


Exercise: 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?

What might be factors that determine the pattern of continental rifting?


Other models to explain continental rifting:


Some general references on continental extensional tectonics:

References on the Basin and Range province:


Course 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.