Integrating paleoseismic studies with geochronology and thermochronology to understand the timing of rifting, volcanism and uplift in the Rukwa Rift Basin, Tanzania

Hilbert-Wolf, Hannah Louise (2016) Integrating paleoseismic studies with geochronology and thermochronology to understand the timing of rifting, volcanism and uplift in the Rukwa Rift Basin, Tanzania. PhD thesis, James Cook University.

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Although continental rifting has contributed to shaping the Earth's landmasses and landscapes for arguably a large part of our planet's history, the archetypal East African Rift System is one of very few active continental rifts on the Earth today. Research to better understand rifting mechanisms and rift evolution is of great interest because rifts are natural laboratories for understanding how continents break up. Furthermore, rifts result in the accumulation of thick sediments with the potential to form both hydrocarbon resources and rich fossil archives of past ecosystems and climates. The break-up of lithosphere also leads to significant disturbances of the Earth's surface, and consequently rift regions are regarded as geologic hazards, involving large magnitude earthquakes, volcanic eruptions, debris flows, and more. In an effort to elucidate some of the complexities of the active East African Rift System, this research has focused on a lesser-studied segment of the western branch of the East African Rift System, the Rukwa Rift Basin, located in southwestern Tanzania. Developed within the Paleoproterozoic Ubendian orogenic belt between the margins of Archean cratons, the Rukwa Rift Basin is unique because it preserves one of the thickest (~9-11 km) and best-exposed sedimentary sequences in East Africa, containing sedimentary units deposited from the Permian – Recent times, including a rare window into the late Oligocene and late Miocene continental records that are not exposed anywhere else in subequatorial Africa.

The common objective of the studies presented here was to decipher how a sedimentary basin within a continental rift zone records the complex relationships between the generation of accommodation space, uplift, volcanism, faulting, and sediment drainage and depositional patterns in response to rifting. Previous work on the rift flanks, volcanic rocks, and stratigraphy provided only a limited view into this important basin, and so an integrated approach to basin analysis was devised in order to understand the above associations via a sedimentary perspective. U-Pb geochronology has been combined with (U-Th)/He and fission track low-temperature thermochronology and applied to detrital zircons and apatites from the sedimentary sequences of the Rukwa Rift. U-Pb dating of newly discovered tuffs and detrital zircons from well cuttings revealed that after initial rifting in the late Oligocene, renewed volcanism and sediment accumulation began again by at least 8.7 Ma in the Rukwa Rift Basin. Provenance studies of the Lake Beds succession that contains these late Miocene tuffs reveal that the Rukwa Rift Basin was internally draining through to at least the Pliocene, implying that rift flank and volcanic topography impeded drainage inlets and outlets.

A new approach for utilizing hydrocarbon exploration well cuttings for detrital zircon analysis, specifically to obtain maximum depositional age control, was developed as a part of this study on the Lake Beds, and has the potential to become a powerful new tool for stratigraphic dating and correlation for hydrocarbon exploration. Detrital lowtemperature thermochronology data suggests that there was minimal uplift associated with either pulse of rifting and sedimentation (at ~25 and ~9 Ma) recorded in the Rukwa Rift Basin, and therefore much of the high topography and incised landscape observed today in this part of eastern Africa likely developed during the late Pliocene to Quaternary. Multiple populations of cooling ages spanning the Paleoproterozic to Late Cretaceous obtained from detrital zircon and apatite low-temperature thermochronology suggests that the Rukwa Rift and vicinity was subject to repeated far-field tectonic stresses.

The sedimentary record in the Rukwa Rift also preserves a rare archive of seismicity, in the form of soft-sediment deformation features (seismites). This research has documented several new seismite morphologies, expanding the database of seismogenic sedimentary structures, which is important for future seismic hazard preparations and predictions. Extensive seismites from the Late Quaternary to Recent upper Lake Beds have been documented, including a decameter-scale clastic 'megablock complex' correlative to similarly sized recumbent folds over 35 km away; evidence for Late Pleistocene large magnitude earthquakes in the Rukwa Rift Basin. In addition, new seismogenic sedimentary structures from the Cretaceous Namba Member of the Galula Formation, termed 'balloon-shaped inflation structures', formed primarily by gas-escape as opposed to the more commonly called upon mechanism, water-escape. This discovery not only has shed light on the longevity of seismic activity within the Rukwa Rift Basin, but also serves as an important discovery for the re-examination of the classification scheme of soft-sediment deformation structures.

The results of combined zircon and apatite detrital- and tephro-geochronology and thermochronology have been integrated with sedimentology-based paleoseismic investigations to provide new insights into the history of the Rukwa Rift Basin, which has important implications for documenting the evolutionary record and for documenting previously unknown seismic activity. Constraining the magnitude and timing of rifting events, magmatism, faulting, and uplift is critical because these processes created and fundamentally changed the landscape, redirected drainages, affected local climate, and more, all of which have major implications for the flora and fauna, including hominins, that evolved along the East African Rift valleys.

Item ID: 48296
Item Type: Thesis (PhD)
Keywords: Africa, calibration standard, earthquake, gas-escape, geology, Great Lakes, paleoseismology, paleoseismology, rift, Rukwa Rift Basin, Rukwa, sedimentology, seismites, Sm–Nd isotopes, soft-sediment deformation, sphene, titanite, trace elements, U–Pb dating, volcanism, water-escape
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For this thesis, Hannah Hilbert-Wolf received the Dean's Award for Excellence 2017.

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter One & Appendix 1: Hilbert-Wolf, Hannah Louise, and Roberts, Eric M. (2015) Giant seismites and megablock uplift in the East African Rift: evidence for Late Pleistocene large magnitude earthquakes. PLoS ONE, 10 (6). pp. 1-18.

Chapter Two & Appendix 3: Hilbert-Wolf, Hannah L., Roberts, Eric M., and Simpson, Edward L. (2016) New sedimentary structures in seismites from SW Tanzania: evaluating gas- vs. water-escape mechanisms of soft-sediment deformation. Sedimentary Geology, 344. pp. 253-262.

Chapter Three: Hilbert-Wolf, Hannah, Roberts, Eric, Downie, Bob, Mtelela, Cassy, Stevens, Nancy J., and O'Connor, Patrick (2017) Application of U-Pb detrital zircon geochronology to drill cuttings for age control in hydrocarbon exploration wells: a case example from the Rukwa Rift Basin, Tanzania. AAPG Bulletin, 101 (2). pp. 143-159.

Appendix 2: Spandler, Carl, Hammerli, Johannes, Sha, Peng, Hilbert-Wolf, Hannah, Hu, Yi, Roberts, Eric, and Schmitz, Mark (2016) MKED1: a new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology. Chemical Geology, 425. pp. 110-126.

Appendix 4: Roberts, Eric M., Todd, Christopher N., Aanen, Duur K., Nobre, Tânia, Hilbert-Wolf, Hannah L., O'Connor, Patrick M., Tapanila, Leif, Mtelela, Cassy, and Stevens, Nancy J. (2016) Oligocene termite nests with in situ fungus gardens from the Rukwa Rift Basin, Tanzania, support a paleogene African origin for insect agriculture. PLoS ONE, 11 (6). pp. 1-17.

Date Deposited: 06 Apr 2017 01:04
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040301 Basin Analysis @ 30%
04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 30%
04 EARTH SCIENCES > 0403 Geology > 040310 Sedimentology @ 40%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 80%
96 ENVIRONMENT > 9610 Natural Hazards > 961099 Natural Hazards not elsewhere classified @ 10%
85 ENERGY > 8501 Energy Exploration > 850102 Geothermal Exploration @ 10%
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