Thursday, December 8, 2016 GSA/AGS/SPE Joint Luncheon

"Controls on Continental Rift Evolution: The East African Rift in Kenya and Tanzania"

Noon Luncheon 11:30-1:00 pm

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LUNCHEON ABSTRACT

Continental rifts are a critical part of Earth’s plate tectonic system. They mark the dissection of thick continental lithosphere, which can ultimately result in complete separation of the continents (or breakup) and the creation of new ocean basins. The manner in which continental rifting is initiated and subsequently evolves, however, is still not fully understood. We observe that there are
two end-member rift types: amagmatic (or magma-poor) and magmatic (or magma-rich), with the latter possibly being an evolutionary product of the former. Initially discrete border-fault segments are the dominant features early in the rifting process, but mechanically interact and ultimately become connected by transfer faults oriented obliquely to the strikes of border faults. During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults.

This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the <7 Ma Natron and Magadi basins (Kenya- Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release.
Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift, fault-dominated strain accommodation. A conceptual model for the evolution of earlystage rifting is as follows: in the first 3 m.y., border faults accommodate the majority of regional extension (1.24–1.78 mm yr–1 in Natron at a slip rate ranging 1.93–3.56 mm yr–1), with a significant portion of intra-rift faulting (38%–96%) driven by flexure of the border fault hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40–0.66 mm yr–1 in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr–1), and regional extension is primarily accommodated in the intra-rift fault population (1.34–1.60 mm yr–1), with an accompanying transition of magmatic volatile release into the rift center. The migration of extension to the intra-rift region is facilitated by transverse structures above inherited crustal weaknesses that allow border fault slip to be transferred to the intra-rift fault population, possibly in response to changing stress fields during the first few million years of rift evolution. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. The flow of magmatic fluids within fault systems thus plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

SPEAKER BIOGRAPHY

Speaker: Simon Katterhorn, Department of Geological Sciences, University of Alaska Anchorage

Dr Kattenhorn is a Professor and Director in the Department of Geological Sciences. He teaches courses in structural geology, geomechanics, and geohazards. His research considers tectonic and volcanic processes in developing extensional fault systems such as continental rifts and mid-ocean spreading centers, with application to hydrocarbon systems and geohazards. His planetary geoscience research considers the tectonic evolution of solid surfaces throughout the Solar System, including Mars and its moon Phobos as well as outer solar system icy moons such as Europa, Enceladus, Titan, Dione, and Triton. Dr Kattenhorn is a former Chair of the Planetary Geology Division of the Geological Society of America and lead investigator of the CRAFTI (Continental Rifting in Africa: Fluid-Tectonic Interaction) collaborative research group that was developed with funding from the National Science Foundation and now involves researchers from UAA, University of Rochester, University of New Mexico, Rensselaer Polytechnic Institute, and Syracuse University.


Tuesday, December 13, 2016 GSA/AGS Joint Luncheon

"Smith Bay Discovery"

Noon Luncheon 11:30-1:00 pm

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LUNCHEON ABSTRACT

TBA

SPEAKER BIOGRAPHY

Speaker: Gerry Sullentrop, Caelus Energy

 

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