Forty years of TTG research

Mountains are key features of the Earth’s surface and host a substantial proportion of the world’s species. However, the links between the evolution and distribution of biodiversity and the formation of mountains remain poorly understood. Here, we integrate multiple datasets to assess the relationships between species richness in mountains, geology and climate at global and regional scales. Specifically, we analyse how erosion, relief, soil and climate relate to the geographical distribution of terrestrial tetrapods, which include amphibians, birds and mammals. We find that centres of species richness correlate with areas of high temperatures, annual rainfall and topographic relief, supporting previous studies. We unveil additional links between mountain-building processes and biodiversity: species richness correlates with erosion rates and heterogeneity of soil types, with a varying response across continents. These additional links are prominent but under-explored, and probably relate to the interplay between surface uplift, climate change and atmospheric circulation through time. They are also influenced by the location and orientation of mountain ranges in relation to air circulation patterns, and how species diversification, dispersal and refugia respond to climate change. A better understanding of biosphere–lithosphere interactions is needed to understand the patterns and evolution of mountain biodiversity across space and time.

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Geological and climatic influences on mountain biodiversity

Passive margins through earth history

The study of source‐to‐sink systems relates long‐term variations in sediment flux to morphogenic evolution of erosional–depositional systems. These variations are caused by an intricate combination of autogenic and allogenic forcing mechanisms that operate on multiple time scales – from individual transport events to large‐scale filling of basins. In order to achieve a better understanding of how these mechanisms influence morphological characteristics on different scales, 29 submodern source‐to‐sink systems have been investigated. The study is based on measurements of morphological parameters from catchments, shelves and slopes derived from a ∼1 km global digital elevation model dataset, in combination with data on basin floor fans, sediment supply, water discharge and deposition rates derived from published literature. By comparing various morphological and sedimentological parameters within and between individual systems, a number of relationships governing system evolution and behaviour are identified. The results suggest that the amount of low‐gradient floodplain area and river channel gradient are good indicators for catchment storage potential. Catchment area and river channel length is also related to shelf area and shelf width, respectively. Similarly to the floodplain area, these parameters are important for long‐term storage of sediment on the shelf platform. Additionally, the basin floor fan area is correlative to the long‐term deposition rate and the slope length. The slope length thus proves to be a useful parameter linking proximal and distal segments in source‐to‐sink systems. The relationships observed in this study provide insight into segment scale development of source‐to‐sink systems, and an understanding of these relationships in modern systems may result in improved knowledge on internal and external development of source‐to‐sink systems over geological time scales. They also allow for the development of a set of semi‐quantitative guidelines that can be used to predict similar relationships in other systems where data from individual system segments are missing or lacking.

Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systems

Subduction drives plate tectonics and builds continental crust, and as such is one of the most important processes for shaping the present-day Earth. Here we review both theory and observations for the viability and style of Archean subduction. High Archean mantle temperature gave low mantle viscosity and affected plate strength and plate buoyancy. This resulted in slower or intermittent subduction, either of which resulted in Earth cooling profiles that fit available data. Some geological observations are interpreted as subduction related, including an “arc” signature in various igneous rocks (suggesting burial of surface material to depths of 50–100 km), structural thrust belts and dipping seismic reflectors, and high-pressure–low-temperature and low-pressure–high-temperature paired metamorphic belts. Combined geodynamical and geochemical evidence suggests that subduction operated in the Archean, although not, as often assumed, as shallow flat subduction. Instead, subduction was more episodic in nature,...

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Archean Subduction: Fact or Fiction?

Mesozoic exhumation of the southern Cape, South Africa, quantified using apatite fission track thermochronology

Linking source and sink: Evaluating the balance between onshore erosion and offshore sediment accumulation since Gondwana break-up, South Africa

The Kaapvaal Craton is partially covered by the supracrustal sequences of remnants of the Witwatersrand, Ventersdorp and Transvaal-correlated Griqualand West Basins that span about 800 million years (~3.0 to 2.2 Ga). This study describes and interprets eight newly available seismic reflection profiles of the Ventersdorp and younger cover sequences, acquired by the vibroseis method to 6 seconds TWT (~ 17km), and totalling ~720km in length. New stratigraphic and structural features are identified across three main regions: the Kaapvaal Craton’s western margin, the craton’s northern margin, (southwards from the Thabazimbi-Murchison Lineament across the western extremity of the Bushveld Igneous Complex) and the craton’s interior.

Across the craton’s western margin, the volcano-sedimentary succession is shown to thicken towards its edge from 0 km to >17km over 150km. The Ventersdorp Supergroup underlies the Transvaal Sequence right to the western extremity of the craton. Close to the margin, we recognise a normal fault, here named the Moshaweng fault, with >6km normal displacement and an unknown distance of strike-slip displacement. Three prominent unconformities are identified, as well as ~300% tectonic thickening of the Hartley Formation in response to east-verging, Paleoproterozoic (~1.8 Ga) thrusting across the craton. Folding and thrusting of Griqualand West and Olifantshoek Supergroups across the craton are also apparent.

A tectonic model for the evolution of the craton’s western margin involves the formation of an Atlantic-type margin by extension, volcanism and rifting over ~70 Ma, followed by 160 Myrs of thermal subsidence. Subsequently, Paleoproterozoic (1.7 to 1.9 Ga) compression involved folding and thrusting eastward across the margin.

Along the northern margin of the craton, the seismic profiles traverse a syncline which trends east to west over several hundred kilometers and involves mainly Transvaal-aged rocks and sporadic Bushveld outcrop. Evident from these profiles are discrete Ventersdorp half grabens, up to 3km in depth, with bounding listric normal faults that trend east to west, subparallel to the Thabazimbi-Murchison Lineament (TML), and downthrow predominantly to the north. These faults are truncated by the well-imaged Black Reef unconformity.

Across the craton interior, horst and graben structures, involving the Witwatersrand and lower Ventersdorp Formations, dominate the seismic profiles. Bounding listric normal faults (one example with minimum throw of 9.5km), trend east to west and northeast to southwest and downthrow to the north, south, northwest and southeast. The Black Reef unconformity is again distinctive, as is a Paleozoic unconformity (the Beaufort unconformity).

The orientation of boundary faults of Ventersdorp-aged half-grabens along the craton’s margins and interior, south of the TML, suggests a complex process of extension tectonism for ~70 Myrs that overlaps in time with northward thrusting of volcano-sedimentary sequences to the north of the TML. On a regional scale, the data presented are consistent with a model for the Kaapvaal Craton that resembles, both in space and timing, the evolution of the Argentine Atlantic margin. This margin developed during an initial 40 to 70 Ma period of regional extension across a ~600 km wide volcano-tectonic rift zone behind an active (Andean) subduction zone, followed by regional lithospheric cooling and subsidence.

Our results also confirm that the Kaapvaal Craton was part of a larger Meso-Archean landmass and thus supports growing realisation that models of Archean continental growth rates based on the present day aerial extent of Archean cratons, represent only minimum values.

Seismic Stratigraphic Constraints on Neoarchean - Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa

Eight deep-seismic reflection profiles (16 seconds TWT; generated by the vibroseis method) are presented and interpreted. The profiles cover a total length of 500km and are projected into a common section that strikes northwest to southeast across the central part of the Kaapvaal Craton, including the Colesburg Magnetic Lineament (CML). The profiles clearly image the crust-mantle transition (Moho) between 11 and 13 seconds TWT (~ 36 to 41km).

The sections link some of the brittle Ventersdorp (~2.7 Ga) faults near surface to more subhorizontal ductile shear zones and related features that transect the upper crust of the Kaapvaal Craton. Sharp shallow-dipping reflectors that predate the Ventersdorp faults subdivide the entire crust into four elongate panels (or slivers; >200km by 10 to 20km). These panels extend from below Vryburg in the west, across the CML, to the crystalline basement below the Witwatersrand Basin in the east. The upper-most crustal reflectors dip towards the west and indicate extensive east-directed tectonic accretion and emplacement of a ~10km-thick allochthonous upper crustal panel comprising the Amalia-Kraaipan granite-greenstone terrane, across a Witwatersrand basement panel. The latter, in turn, is underlain by an elongate crustal panel that dips west from the surface near Welkom, reaching the lower crust below the centre of the section. The CML that subdivides the craton into an eastern block and a western block at surface is not a feature that can be easily linked to an underlying suture in the lower crust and mantle. Instead, this lineament has a shallow origin related either (1) to the 3.0 to 2.8 Ga Amalia-Kraaipan greenstone belt lithologies (that include extensive BIF, and/or (2) to possible thin back-thrust wedges of highly magnetic shales of the lower Witwatersrand sequences with (as previously suggested by Corner et al. , 1990) shallow east-dipping magnetite-rich basement granites.

The entire crustal section resembles a tectonically stacked series of crustal slivers derived from the (north-) west that amalgamated in the Neoarchean (2.92 to 2.76 Ga) along east-propagating thrusts that cut up-section from middle to lower crust to reach the surface next to the upper Witwatersrand foreland basin. Distinct shallow dipping reflectors below the Moho suggest that the upper mantle was involved in this “shingling” tectonics ( c.f. Cook, 1986). Any related suture zone must have been positioned to the west of the present craton boundary. The lowermost crustal panel is probably composed of old TTG- basement (>3.2 Ga) that was extended and partially melted during the Ventersdorp event (~2.7 Ga), leaving a near-flat smooth and annealed Moho.

A seismic profile across the southern margin of the Kaapvaal craton images the tectonic boundary with the Mesoproterozoic Natal Namaqua Mobile Belt (NNMB), and is consistent with existing geological and geochemical models in which the Kaapvaal craton is overthrust by the NNMB. The data suggest that cratonic crust and probably mantle lithosphere underlie a significant section of the NNMB beneath Natal and the Eastern Cape.

Crustal structures across the central Kaapvaal craton from deep-seismic reflection data

Low elastic strength of ancient lithosphere based on flexural analyses has been interpreted to reflect elevated regional geothermal gradients in response to higher global heat production in the past. Here we present a flexural analysis of Archean/Palaeoproterozoic sediment cover along the western margin of the Archaean Kaapvaal craton based on seismic stratigraphy. Our results show that between ~1.93 and ~1.75 Ga, the Archaean margin of the craton had an effective elastic thickness of 7.5 to 10km compared to its present day value of 60 to 70km. Because the Kaapvaal craton had already stabilized by ~2.7 Ga and was underlain by 150 to 300km thick strong mantle lithosphere, it is unlikely that the relatively thin elastic thickness along this old margin reflects a change in secular cooling of the Earth. Instead, we interpret the low elastic strength to be a transient marginal tectonic effect similar to that recorded along modern continental margins.

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Justine Tinker

Papers

Mesozoic exhumation of the southern Cape, South Africa, quantified using apatite fission track thermochronology

Linking source and sink: Evaluating the balance between onshore erosion and offshore sediment accumulation since Gondwana break-up, South Africa

Seismic Stratigraphic Constraints on Neoarchean - Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa

Crustal structures across the central Kaapvaal craton from deep-seismic reflection data

Old, strong continental lithosphere with weak Archaean margin at ~1.8 Ga, Kaapvaal Craton, South Africa