- Title
- A seismic characterization of the crustal structure underlying the offshore Natal valley within the south-west Indian Ocean
- Creator
- Moorcroft, David
- Subject
- Seismology
- Subject
- Seismology -- Kwa Zulu Natal -- Research Coasts -- Kwa Zulu Natal
- Date Issued
- 2018
- Date
- 2018
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10948/22387
- Identifier
- vital:29965
- Description
- The Natal Valley is a ~1000 km long, elongated depression in the seafloor, located within the south-west Indian Ocean, between the coastline of south-east Africa and the offshore aseismic Mozambique Ridge. Knowledge of its present-day crustal structure is important to reconstruct the Gondwana supercontinent break-up history and the concurrent opening of the Indian Ocean, which started during the Jurassic, some ~180 – 150 million years ago. Major geological events relating to this supercontinent break-up including continental rifting, magmatism, the emplacement of extensive oceanic flood basalt as submarine plateaus, vertical processes associated with tectonic uplift, flexural subsidence of the margin and relative global sea-level change are recorded within the crust underlying the south-west Indian Ocean. Although these events have been well modelled, fundamental questions remain regarding the nature and tectonic structure of the crust. Today, the Natal Valley region represents a frontier for geophysical exploration. Early attempts at imaging the crust beneath the seafloor within the south-west Indian Ocean made use of now primitive seismic reflection data acquisition and processing techniques available to them at the time (1960’s - 1970’s). Considering the advancements made in marine seismic reflection imaging, these vintage datasets lacked the horizontal and vertical resolution necessary to most accurately account for and describe features of the upper crust underlying the northern Natal Valley. This study is based on the acquisition, processing and preliminary interpretation of a newly acquired, regional seismic reflection profile, Profile Mz5-007, acquired across the northernmost region of the Natal Valley during the PAMELA-Moz5 cruise in 2016. The methods of marine seismic reflection data acquisition, processing and interpretation are described. The seismic reflection data presented in this dissertation provides a regional-scaled appraisal of the upper crust underlying the northern Natal Valley for the first time. The imagery allows a comprehensive description of crustal structures and features from the very northern Natal Valley, close to the Mozambique coastline, southwards into the southern Natal Valley offshore of Durban. The findings presented here support the notion that the crust underlying the northern Natal Valley experienced roughly N-S orientated trans-tensional stresses in relation to the breakup of West Gondwana. At least two major phases of volcanism are imaged within the seismic stratigraphy. The lower-most event underlies the Limpopo Fan in the northernmost regions of the Natal Valley at a depth of 2 seconds TWT and is interpreted as a seismic expression of the upper surface of the Lower – Middle Jurassic Karoo LIP. The progressive disappearance of this horizon towards the south coincides with the emergence of a second, overlying, as yet unreported volcanic horizon, which extends southwards throughout the Natal Valley. On the basis of its regional distribution, spatial relationship with the underlying event boundary and its deformation history, this volcanic horizon is interpreted to represent a series of basaltic lava outpourings during the Lower Cretaceous, initiated in response to trans-tensional crustal deformation associated with the breakup of West Gondwana. In addition to these two major phases of flood basalt outpouring, a number of localized volcanic flows are imaged throughout the stratigraphy, most notably over the Central Terrace. In addition, at least four major unconformities are identified, all of which are well developed within the Limpopo Fan. One of these is interpreted to represent the well-developed Mid-Cretaceous hiatus in sedimentation, providing a relative seismo-stratigraphic constraint to overlying and underlying seismic units. A review of the structural elements of the crust reveals how the northern Natal Valley is partitioned into two sub-basins separated by a prominent basement high, the Central Terrace, imaged across the central portions of Profile Mz5-007. To the north of the Central Terrace, structural deformation appears largely extensional, characterized by a large (~37 km wide) asymmetrical depression directly to the north of the Central Terrace and only minor structural faulting. To the south of the Central Terrace, trans-tensional stresses associated with withdrawal of the Falkland Malvinas Plateau have deformed the upper crust into a series of apparently southwards tilted crustal blocks, producing a series of deep (3.5 s TWT) half graben-like structures, within which syn-rift fill has an apparent dip towards the south. There is evidence to suggest ongoing reactivation of these fault structures. This zone of deformation is shown to bear resemblances to the faulting mapped onshore of the Kwa-Zulu Natal coastline, providing reason to consider the two regions as a continuous zone of deformation associated with the withdrawal of the Falkland-Malvinas Plateau during the breakup of West Gondwana. Furthermore, a number of post-sedimentary intrusive features are imaged, the largest of which is the Naude Ridge; a large pinnacle-like feature which has penetrated the upper crust towards the southern limit of the profile. This feature corresponds to an abrupt change in seafloor depth and upper crustal structure, suggesting that this feature may mark the transition between subsided continental crust interpreted to underlie the northern Natal Valley to the north and oceanic crust known to underlie the southern Natal Valley to the south.
- Format
- ix, 162 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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