Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Rebecca M. Flowers

Second Advisor

Shijie Zhong

Third Advisor

Robert S. Anderson

Fourth Advisor

Peter H. Molnar

Fifth Advisor

G. Lang Farmer

Abstract

Southern African topography is enigmatic because much of it rose from sea level to >1000 m elevation while distal from convergent boundaries with little surface deformation. Most mechanisms proposed to explain elevation gain originate in the mantle; however documenting the surface response to deep-seated processes is a major challenge in tectonics. Here I show that apatite (U-Th)/He thermochronometry (AHe) on kimberlites is an innovative technique to directly link surface erosion and deep processes. Kimberlites contain a record of the state of the lithosphere at time of eruption through their xenolith suites. When accurately dated, xenoliths can be used to decipher temporal trends in mantle evolution. Since kimberlites can be challenging to date conventionally, I show that (U-Th)/He on kimberlitic zircon (ZHe) and perovskite (PHe) both reproducibly confirm eruption ages for previously dated kimberlites, demonstrating that ZHe and PHe can date kimberlite eruption.

Xenoliths from southern African kimberlites document a lithospheric thermochemical modification event in the Cretaceous. Heating and metasomatism was pervasive off-craton but more subtle on-craton. AHe dating of off-craton kimberlites documents a period of erosion from ~110 to 90 Ma, synchronous with mantle modification, thereby directly documenting the surface response to mantle change. On-craton, AHe documents a spatially variable wave of erosion from ~120 Ma to

A landscape evolution model was used to further explore the complex relationship between topographic development and erosion. First attempts to link thermochronology with topography and sedimentary flux observations show that the thermochronology dataset’s spatiotemporal information is advantageous for constraining the physical parameters of the model.

(U-Th)/He thermochronometry is powerful for interpreting the rich geologic record of kimberlites. This enhanced thermal and geologic record is capable of discerning detailed erosion patterns in continental interiors and linking them to deep earth processes.

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