Week 3: Parting the continents - continental rifting

Content:

Introduction to continental rifting

Intro comment: Ocean basins, and the underlying oceanic crust, are often bordered by continental rocks. Also consider the mental exercise of reversing seafloor spreading - it brings the continents back together. For example, running seafloor spreading backward for the North Atlantic brings the west coast of northern Africa back to joint with the east coast of North America. This would suggest the continents come apart to make new oceanic basins. In this section we will study that process of continental 'pulling apart' - rifting. If it continues the continents can only be stretched so thin, and seafloor spreading processes take over and an oceanic basin is born. This transition is known as "rift to drift". The rifting can also be insufficient to trigger seafloor spreading processes, and instead forms a rift basin or basins within continental crust. Continental rifts are locations of continental crustal extension/divergence, crustal thinning, sedimentary basin formation, and often thermal and igneous activity. They show a complex and diverse set of patterns, sometimes concentrated, sometimes more diffuse, and localized rifting can occur in what may initially seem unlikely positions - on top of convergent mountain belts for example. So we need to look at continental rifting broadly.

Suggested 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 rifted continental fragments flanking an oceanic basin that is opening. A classic case example is the Appalachian orogen (see adjacent diagram). In another example, the North Atlantic rifting also followed the path of the orogenic axis of the Norwegian and Greenland Caledonides. The basic idea is as follows. As oceanic crust gets older and colder, subduction likely starts and the ocean basin closes, with the bounding continents eventually colliding to form a new mountain belt. The old orogens are preferential weakness zones, prone to reactivation during the next rifting episode. It is basically a cycle of oceanic basin opening and closing. There are certainly 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) for the Wilson Cycle.

Long term fates of continental rifts?

Some examples of presently active continental rifts

East African Rift zone (EAR):

Rio Grande Rift (E side of Colorado Plateau):

Modern hot springs deposits (calcium carbonate) along a fault on the flanks of the Jemez caldera taken from on the 2005 UNO field trip to the New Mexico Rift. Elevated geothermal gradients, and magma chambers and recent intrusions all serve to increase the heat flow of the thinned crust leading to such hydrothermal activity. For this reason rift zones can have hydrothermal related mineralization and be ore exploration targets.

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 lens for scale.

Rhine Graben, in northern Europe.

Basin and Range province covering a good bit of the southwest U.S..

Keweenawan rift and the mid-continent gravity high - in the interior of the U.S..

Extensional accommodation mechanisms

The diagram to the right attempts show various mechanisms associated with continental extension:

Lithospheric thinning (5) is observed with modern rifts. What are the responsible mechanisms?

What is low angle normal faulting? You may remember from your Physical Geology or Structural Geology course that normal faults have a dip of ≈ 60° and many such faults do. However, there are also normal faults that dip at very low angles, in some cases sub-horizontal. These are often linked to normal faults with more 'standard' dips as evident in the diagram below. There was a significant debate as to the mechanics of such low-angle normal faults (e.g. Collettini 2011). Some people argued that they started out in a steeper position and then were rotated into a low-angle position, where they were inactive. However, evidence has accumulated that actively slipping low-angle normal faults occur. Understanding them mechanically involves the concept of very low strength surfaces, possibly with high fluid pressures. Extensive subhorizontal faults are known as detachments (with the upper plate detached from the underlying lower plate). A natural question to ask is what localizes the detachment at a given level?

Extensional detachments as an expression of thin-skinned tectonics: A mini-revolution within plate tectonics was realizing that detachments played an important role in the architecture of plate boundaries, both at convergent and divergent margins. This architecture of a deformed upper plate above a detachment with a less or differently deformed lower plate below was given the name of 'thin-skinned' tectonics. Detachments can have areas of 10,000s of square kilometers.When we describe convergent margins the concept of detachments and thin-skinned tectonics will be explored again.

Schematic image from USGS based on Basin and Range rifting showing the subhorizontal detachment and the position of mineralization in this tectonic context. Image from https://pubs.usgs.gov/bul/b2004/html/bull2004detachmentfaultrelated_mineraliz.htm accessed 9/5/2018.

Metamorphic core complexes and lower crustal flow, associated with extreme extension, more info here.

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

Traits:

Goat Mountain in Big Bend National Park in Texas. These pyroclastics 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:

Basin analysis: holistic look at a basin and the factors that contributed to its character. What factors in particular might influence rift basins?

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. This detachment is part of a metamorphic core complex. 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 for continental rifting

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

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.

Rift tip propagation, Vink and Courtillot model:

Orogenic topographic collapse: localized rifting within orogenic welts can be 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: a possible model for Carboniferous basins on the Barents Shelf.

Some general references on continental extensional tectonics:

References on the Basin and Range province:

References on core complexes, gravitational collapse, extensional detachments, and sundry:

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.