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The general aim of this thesis is to investigate heterogeneity in the structure of the crust and uppermost mantle of Archaean and Proterozoic terrains in southern Africa and to use the findings to advance our understanding of Precambrian crustal genesis. Teleseismic, regional and local seismic recordings by the broadband stations of the Southern African Seismic Experiment (SASE), Kimberley array, South African National Seismograph Network (SANSN) and the Global Seismic Network (GSN) are used in the inversion procedures to address the aim of this thesis. In the first part of the thesis, the nature of the lower crust across the southern African shield is investigated by jointly inverting receiver functions and Rayleigh wave group velocities. The resultant Vs models show that much of southern Africa has a lower crust that is mafic in composition, whereas the western parts of the Kaapvaal and Zimbabwe Cratons have a lower crust that is intermediate-to-felsic in composition probably due to rifting. The second part of the thesis evaluates the “dipping-sheet” and “continuous-sheet” models of the Bushveld Complex using better-resolved seismic models derived in a two-step approach, employing high-frequency Rayleigh wave group velocity tomography and the joint inversion of high-frequency receiver functions and 2–60 sec Rayleigh wave group velocities. The resultant seismic models favor a “continuous-sheet” model of the Bushveld Complex, although detailed modelling near the centre of the Complex shows that the subsurface mafic layering could be disrupted. The third part of the thesis, is focused on jointly inverting high-frequency teleseismic receiver functions and 10–60 sec Rayleigh wave group velocities to place shear wave velocity constraints on the source of the Beattie Magnetic Anomaly (BMA) at depth and to evaluate existing geophysical models of the BMA source. The resultant Vs models across the BMA suggest the BMA source to be at upper to middle crustal depths (5–20 km) with high velocity layers (≥ 3.5 km/s). Further to this, is a lower crust that is highly mafic (Vs ≥ 4.0 km/s) and a crust beneath the BMA that is on average thicker than 40 km. Plausible models of the BMA source are massive sulphide ore bodies and/or mineralized granulite-facies mid-crustal rocks and/or mineralized Proterozoic anorthosites. v Overall, the findings in this research project are consistent with the broad features of a previous model of Precambrian lithospheric evolution but allows for refinements of that model.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2017. / Massif-type anorthosites formed in the Proterozoic Eon are the most voluminous anorthosite occurrences on Earth, reaching tens of thousands of square kilometers in aerial extent. While they formed throughout the Proterozoic, most formed during a 700 Ma period between 1800 and 1100 Ma. The rocks are dominated by plagioclase (typically 70 – 95 volume %) of intermediate composition (An40-65). Olivine, orthopyroxene, clinopyroxene and Fe-Ti oxides make up the minor mafic proportion. While most researchers agree that the anorthosites formed from a high-alumina basaltic parental magma, there are disparate views on how that parental magma was generated. Whether the parental magma formed by partial melting of the lower crust, or by mantle melting, is a topic of much debate. The anorthosites commonly have crust-like isotopic signatures, but this could be produced by melting of the lower crust, or by crustal contamination of mantle-derived magmas. Many Proterozoic anorthosite complexes consist of both olivine-bearing and orthopyroxene-bearing anorthosites. This has been attributed to variable amounts of crustal contamination of mantle-derived magmas, based on evidence from isotopes and field relations. While geochemical and petrologic evidence for crustal contamination is plentiful, existing experimental work shows that a thermal divide exists for high-alumina basalts fractionating at lower crustal depths, casting doubts on whether fractionation of a mantle melt could produce anorthosite. Here I use high-pressure experiments to test whether the fractionation of high-alumina basalt can form anorthosites, and to what extent crustal contamination affects the fractionation sequence. The results are compared to new geochemical and petrologic data from the Kunene Anorthosite Complex (KAC), in Angola and Namibia. The KAC is one of the largest anorthosite complexes in the world, with an area of ~18 000 km2. The KAC (1438 – 1319 Ma) has an elongate shape and intruded into Palaeoproterozoic to Mesoproterozoic country rocks (~2200 to 1635 Ma) at the southern margin of the Congo craton. It is associated with a suite of granitoid rocks of variable composition, which are akin to the granitoids associated with nearly all Proterozoic anorthosites. The granitoids have been shown to be coeval with the anorthosites, but were from a chemically independent magma series. The most distinctive granitoids in the KAC are the Red Granites, which outcrop around the southern margins of the complex, and also cross-cut the complex in a NE-SW linear belt, dividing the complex roughly into northern and southern domains. The rocks of the KAC are highly variable in terms of mode, mineral chemistry, and texture, but there is a general trend of more olivine-bearing anorthosites north of the granite belt, and orthopyroxene-bearing anorthosites to the south. The olivine-bearing rocks (or leucotroctolites) typically contain plagioclase and cumulus and/or intercumulus olivine, with lesser interstitial orthopyroxene and/or clinopyroxene, Fe-Ti oxides, and biotite. The orthopyroxene-bearing anorthosites (or leuconorites) contain cumulus plagioclase ± cumulus orthopyroxene, and interstitial orthopyroxene, clinopyroxene, oxides and biotite. The leucotroctolites are characterized by more calcic plagioclase (An56-75), while the leuconorites contain more intermediate plagioclase (An48-56). The variability of the rocks across the complex suggests that the KAC consists of several coalesced plutons with different histories. The petrologic data and field observations in this study are consistent with the leuconorites of the complex being derived from a mantle-derived magma that experienced contamination by silica-rich rocks, crystallizing orthopyroxene rather than olivine, and less calcic plagioclase. The leucotroctolites experienced less or no contamination. To test whether the mineral dichotomy and the variations in plagioclase chemistry observed in Proterozoic anorthosites are due to variably contaminated mantle-derived magma, piston cylinder experiments were conducted on a synthetic high-alumina basalt (HAB) composition, as well as a mixture of this HAB with 30% of a Red Granite composition. Experiments were conducted at 10 kbar, to simulate the depth at which anorthosite differentiation most likely begins (based on Al-in-orthopyroxene geobarometry of highly aluminous orthopyroxene megacrysts that occur in many massifs). The uncontaminated experiments produced olivine as the first liquidus phase, followed by plagioclase (An65-68), and then by clinopyroxene, pigeonite and ilmenite at progressively lower temperatures. Residual liquids evolve towards more silica-rich compositions with decreasing temperature. The contamination experiments produced liquidus orthopyroxene, followed by plagioclase (An51-56), and then by pigeonite at lower temperatures. The experiments show that contamination of a primitive HAB magma by granitic material, most likely produced by partial melting of the lower crust during anorthosite formation, can shift the mineral assemblages of the crystallizing anorthosite from olivinebearing to orthopyroxene-bearing, and produce less calcic plagioclase than the uncontaminated HAB magma. This could explain the observation of olivine-bearing and orthopyroxene-bearing anorthosites in the KAC and many other Proterozoic anorthosites. Previous high-pressure experimental studies, using a slightly more evolved HAB composition, indicated the presence of a thermal divide, which causes liquids to evolve to more Si-poor compositions. The experimental results presented in this study however, do not show a thermal divide, indicating that small variations in experimental starting composition can cause large differences in the liquid line of descent. The results of this study indicate that partial melting of the mantle can produce anorthosite parental magmas, and that the range in mineral assemblages of the anorthosites can be accounted for by crustal contamination of a mantle-derived magma. Fractionation of the experimental starting compositions was also modeled using the MELTS algorithm. These calculations produce a close match to the experimental liquid trends. This allows for modeling of a variety of compositional and environmental variables. The MELTS modeling shows that as little as 10% contamination of HAB magma with a granitic composition may position the magma in the orthopyroxene stability field, forming orthopyroxene-bearing anorthosites. The modeling also shows that a variety of silica-rich contaminants, including granites, granodiorites and tonalities, produce similar results and liquid evolution trends, so a range of granitoid compositions may successfully produce the shift in mineral assemblages of the anorthosites. This suggests that crustal contamination of mantle-derived HAB could be a widespread process and the primary mechanism that produces the distinctive crust-like signatures in Proterozoic anorthosites. In summary, the mineralogical and chemical diversity observed in Proterozoic anorthosites can be produced by variable amounts of crustal contamination of mantle-derived, highalumina basaltic magma. The experimental results in this study combined with field observations, and geochemical and isotopic data, provide evidence for a model of massif-type anorthosite petrogenesis. Orthopyroxene-bearing rocks formed from an originally highalumina basaltic magma that experienced contamination by granitic partial melts of the lower crust, during ponding of the magma at the Moho. This process preconditioned the surrounding crust and possibly prevented further anatexis. Following emplacement of orthopyroxene-bearing anorthosites, subsequent magma pulses ponded at the Moho did not assimilate any/as much granitic material, as they were interacting with preconditioned crust, and formed olivine-bearing anorthosites. With better constraints on the parental magma composition, magma source, and crustal contamination processes, addressing aspects such as the tectonic setting and emplacement mechanisms of these massive intrusions should be prioritized. Understanding these enigmatic aspects of anorthosite petrogenesis is leading the anorthosite community towards answering the ultimate questions of why massif-type anorthosites are restricted to the Proterozoic. / XL2018

Several seismic reflection surveys were conducted in the late 1980s and early 1990s under the auspices of the SA National Geophysics Programme. These surveys targeted the Bushveld Complex, Limpopo Mobile Belt (Limpopo Province), Witwatersrand Basin, Vredefort Dome and the Beattie magnetic anomaly (BMA) in the Southern Karoo. The ~100 km seismic reflection profile described in this study (SAGS-03-92) covers the BMA, the Southern Cape Conductive Belt (SCCB) and the Karoo/Cape Fold Belt boundary. The profile runs from approximately Droëkloof in the south to Beaufort West in the north along the N12 national road. The profile was acquired in 1992, but the complete profile was not interpreted or published prior to this study. The purpose of this study is to successfully reprocess the data and to do a structural and stratigraphic interpretation in order to try and understand the geological history and processes that led up to the formation of the rocks in that area. SAGS-03-92 reveals a clear image of the crust in the southern Karoo. The crust is interpreted to be around 37 km thick in the area of investigation and can be classed into three parts: upper crust, middle crust and lower crust. The upper crust consists of the Karoo and Cape Supergroup rocks that dip slightly to the south. This interpretation has been confirmed by two deep boreholes (BH No. 3 and KW 1/67). The seismic fabric shows quite a strong character in the upper crust and the interpreted boundaries between the different lithologies (The Table Mountain, Bokkeveld and Witteberg Groups of the Cape Supergroup and the Dwyka, Ecca and Beaufort Groups of the Karoo Supergroup) are for the most part quite easy to identify. Within the Cape Fold Belt (CFB), however, the seismic character becomes distorted in such a way that it is very difficult to make out any features. This is possibly due to the severe faulting and folding that occurred when the CFB formed. An unconformity that can continually be followed throughout the profile (although it disappears in the south of the profile possibly due to deformation when the CFB formed) separates the upper crust from the middle crust and the unconformity is clearly indicated by a strong series of reflectors on the seismic profile. The middle crust is interpreted to consist of granitic-gneisses belonging to the Bushmanland Terrane (part of the Namaqua-Natal Belt (NNB)). The seismic profile suggests that the NNB gneisses continue beneath the Cape Fold Belt. The seismic fabric dips steeply to the north. The middle crust also hosts the source of the Beattie Magnetic Anomaly (BMA). There is an area of high reflectivity under the BMA on the seismic profile that differs significantly from the surrounding seismic character. This area is characterised by a beanshaped cluster of strong reflections dipping north and south. It is ~10 km wide, with a thickness of ~8 km and occurs at a depth of ~6 km to ~10 km. The lower crust is interpreted to consist of either granites belonging to the Areachap Terrane, Richtersveld or Kheis Province (NNB) or rocks belonging to the Kheis Province. The seismic fabric of the lower crust dips moderately to the south. The Moho is recognised at ~37 km depth at ~68 km from the south of the profile, but for the rest of the profile, it is unclear where the Moho is encountered. The research done for this study correlates well with work done under the auspices of Inkaba yeAfrica, especially the work done by Ansa Lindeque

Many aspects of Proterozoic massif-type anorthosite petrogenesis have been, and remain, controversial. Mafic lower crust and depleted mantle have both been proposed as mutually exclusive sources of these near-monomineralic, temporally restricted batholiths. The debate surrounding the magma source has also led to uncertainty regarding the tectonic setting of these massifs, with a range of possibilities including convergent, divergent and anorogenic settings. The dramatic geochemical effects of crustal contamination in these massifs are well known and strong crustal signatures are evident in most, if not all, Proterozoic anorthosite massifs. The source debate, in the simplest sense, reduces to whether the ubiquitous crustal signature is derived principally from melting of a lower crust or is an effect of crustal assimilation. The origin of this crustal signature, and whether it obscures the original isotopic composition of the magmas or not, has fuelled the debate surrounding the source of the anorthosites. Using major element, trace element and isotopic compositions, as well as energyconstrained assimilation-fractional-crystallisation (EC-AFC) modelling from samples representing various stages of the polybaric crystallisation history of the magmas, including high-pressure megacrysts, anorthosites and their internal mineral phases, I remove the obfuscating effects of possible crustal contamination and probe the source of the magmas. In order to assess the effects of crustal contamination, if any, anorthosites from three massifs – the Mealy Mountains Intrusive Suite, Nain Plutonic Suite (both in eastern Canada) and Rogaland Anorthosite Province (Norway), have been analysed – all of which intrude into crust of significantly different age and chemical character. Sm-Nd geochronology of high-Al, high-pressure orthopyroxene megacrysts, as well as the comagmatic, host anorthosites, indicate that the magmatic system is long-lived, with an age difference between the megacrysts and hosts of ~110-130 million years. Isotopic compositions of primitive megacrysts qualitatively show that the magmas were derived from melting of the depleted mantle. Strong links between the isotopic offset from depleted mantle evolution and the age and composition of the surrounding crust confirm that the geochemical nature of the crustal contaminant plays a significant role in the petrogenesis of the anorthositic rocks. The geochronological indications of a long-lived magmatic system point to Proterozoic anorthosite formation in a continental magmatic arc – one of the only environments capable of supplying geographically-localised magma and heat to the base of the crust for over 100 million years. Proposed divergent or ‘anorogenic’ settings could not plausibly supply magma to the base of the crust for over 100 m.y. without initiating ocean formation or continental break-up. Anorthosite emplacement at mid-crustal levels may coincide with late- to post-orogenic events in several terranes, but evidence presented for a long-lived magmatic system is incongruent with this proposed setting. In this thesis, I propose that the petrogenesis of these intrusives must span both orogenic and post-orogenic periods. An overlap in megacryst crystallisation age with the onset of calc-alkaline orogenic magmatism in the Sveconorwegian Orogen, both occuring ~100 m.y. before anorthosite emplacement, confirms that initial magma and megacryst formation coincides with the main phase of magmatic and orogenic activity in a convergent magmatic arc. These geochronological constraints have implications for regional geodynamics in the Sveconorwegian Orogen (and the Labrador region) with the evidence providing corroboratory support for a long-lived accretionary orogen, as opposed to the widely-held view that the Sveconorwegian orogeny was predominantly collisional. Compositions of high-pressure megacrysts, anorthosites and analysis of internal isotopic disequilibrium indicates that lower crustal contamination has a significant influence on the isotopic composition of the rocks, with relatively minor contributions from the mid- to upper crust. Energy-constrained AFC modelling confirms that significant lower crustal contamination occurs during ponding of magmas at the Moho and is able to reproduce the observed isochronous isotopic compositions of the megacrysts as well as the compositions of the host anorthosites. Evidence of varying degrees of internal isotopic disequilibrium reinforces the significant role that assimilation of crust of different age and chemical nature have on the compositions of Proterozoic anorthosites. Unexpected patterns of isotopic disequilibrium show that anorthosite petrogenesis is not a “simple” case of progressive crustal contamination during polybaric ascent of viscous, partially-molten 4 magma mushes, but is more likely to involve significant differentiation and solidification at lower crust depths, followed by ascent of high-crystallinity bodies (> 50 % crystallinity) to upper crustal levels. Although the composition of the bulk continental crust is different to plagioclase-rich Proterozoic anorthosites, both are missing a mafic component. It is unclear how this missing mafic component was generated in the continental crust, because most of the evidence for these crustal differentiation processes is sequestered below or near the Moho. However, Proterozoic anorthosites, formed by viscous, plagioclase-rich mushes, entrain rare cumulate megacrysts from these depths and consequently preserve evidence of magmatic differentiation processes at the Moho. The evidence for the formation and sequestration of dense ultramafic cumulates in ponding magmas at the Moho can not only explain the missing mafic component in Proterozoic anorthosites, but also suggests that cumulate formation in crust-forming, arc environments is a significant process and should be taken into account in models dealing with evolution and differentiation of the continental crust. Sampling and petrographic and geochemical analysis of five pegmatitic segregations, or “pods”, from anorthosites of the Mealy Mountains Intrusive Suite reveal a diverse range of compositions from mafic, Fe-rich and Si-poor, to Fe-poor and Sirich felsic compositions and from monzogranite through quartz-monzodiorite and monzodiorite to Fe-P-rich gabbronorite. Each pod shows a range of noteworthy graphic, myrmekitic and symplectic textures on a variety of scales, along with distinctive mineralogical assemblages and highly-enriched trace element compositions. Derivitive minerals (e.g. apatite and zircon), high concentrations of Fe, Ti, P (and in some cases SiO2) and 10-1000 times chondrite enrichment suggest that many of the pods are highly fractionated. U-Pb zircon geochronology reveals that all the pods are the same age as the anorthositic hosts and confirms that the Mealy Mountains Intrusive Suite was emplaced between 1654 and 1628 Ma. Using the aforementioned evidence, I show that the pods represent the fluid-bearing, late-stage crystallisation products of a residual liquid in the massif anorthosite system and provide a window into the final stages of crystallisation in the anorthosite system. A range of rock types (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, have also been documented in other Proterozoic anorthosite massifs. These have, at one time or another, controversially been interpreted as the residual liquids of anorthosite crystallisation. The observation of in-situ pods with similar compositions to all of the aforementioned rock types and displaying textures indicative of late-stage crystallisation support the notion that these associated lithologic units are comagmatic with, but residual to, the anorthosites and are not residual liquids of other crustally-derived rocks, immiscible liquids, parental magmas or cumulates. Isotopic compositions of these highly-fractionated, late-stage pods also overlap with those of anorthosites, lending further evidence to the case that upper crustal contamination plays only a minor role in developing the chemical signature of the anorthosites. With these results I propose that the nature/composition of the residual liquids of Proterozoic anorthosite magmas can vary dramatically, depending on geochemical differences in the original magma pulses and by mixing of mobilised, independently-evolved segregations of residual liquids. This process could explain why so many varied rock types associated with Proterozoic anorthosites have been suggested as residual liquids: these rocks all represent residual liquids resulting from varying degrees of differentiation, subsequent mobilisation, mixing and final solidification as plutons or dykes. Proterozoic anorthosite petrogenesis is an inherently polybaric process and so by its very nature produces a range of complicated and contradictory features which have clouded interpretation of numerous aspects of the rocks formation. In analysing crystallisation products from numerous stages of the anorthosites polybaric history, I have been able to probe the magmatic processes operating at different stages of Proterozoic anorthosite petrogenesis. In doing so I show that the magmas are derived from melting of the depleted mantle in continental-arc-like settings – two controversial aspects of Proterozoic anorthosite petrogenesis. These constraints on the source and tectonic setting will allow renewed investigation into the ultimate question surrounding Proterozoic anorthosites: why are these rock types restricted to the Proterozoic and what clues does this temporal restriction offer about Earth’s geodynamic evolution during this period? The assertion in this thesis that 5 Proterozoic anorthosites formed in arc environments dictates that subduction processes or geodynamic conditions during the Proterozoic favoured the production of voluminous masses of plagioclase, because modern-day magmatic arc terranes show no evidence of anorthosites with similar compositions. However, calcic anorthositic inclusions and xenoliths are observed in modern-day volcanic and continental arcs suggesting that anorthosites may be forming in these environments, but that conditions such as water content or style of subduction are different to the Proterozoic, producing less and compositionally different plagioclase and anorthosite. The results of this thesis shed new light on and refine the petrogenesis of Proterozoic anorthosites, but the focus of research must now shift to explaining the temporal restriction of these intrusions and the implications of this restriction for the geodynamic evolution on Earth during the Proterozoic.

Surface and subsurface data from the Sulaiman thrust belt show that nearly all the 10 km thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to a minimum of 20 km in the hinterland of the Sulaiman fold belt without significant thrust faults at the surface. The balanced structural cross-:section suggests that the tectonic uplift in the Sulaiman fold belt is a result of thin-skinned, passive-roof duplex style of deformation. The duplex sequence of Jurassic and older rocks is separated from the roof sequence by a passive-back thrust in thick Cretaceous shales. The passive-roof sequence remains intact for about 150 km and becomes emergent along a passive-back thrust in the hinterland. The structures are expressed at the surface by fault-related folds in the foreland and out-of-sequence structures (secondary faults and related pop-ups) in the interior. The duplex structure varies from fault-bend folds to anticlinal stacks, and hinterland dipping duplexes. Progressive deformation reveals a series of structural and geometrical features including: (1) broad concentric folding at the fault tip; (2) development of a passive-roof and duplex sequence; (2) forward propagation of the duplex as critical taper is achieved; (4) tear faults and extensional normal faults within the overthrust wedge; and (5) out of sequence (secondary) thrusting. The 349 km long balanced cross-section from the Sulaiman fold belt restores to an original length of 727 km that provides 378 km of shortening in the cover strata of the Indian subcontinent. Minimum estimate of shortening is 328 km. Modelling of the Bouguer gravity profile from the Sulaiman foredeep across the Indian/ Afghan collision zone suggests the depth to the Moho at the Sulaiman deformation front is about 36 km. Depth to Moho increases northward with a gentle gradient of 1.1° (20 m/km) for 280 km to the hinterland where the depth to the Moho is about 42 km. About 150 km north across the Khojak flysch the Moho gradient steepens abruptly to about 7.8° (136 m/km) to attain an average depth of about 57 km in eastern Afghanistan. This suggests that the Sulaiman fold belt is underlain by transitional crust associated with the western passive margin of the Indian subcontinent. / Graduation date: 1992

At collisional mountain ranges the tectonic history of crustal shortening and subsequent post-collisional erosion is preserved in the form of the presently observed gravity anomalies. In this study, models of erosion and isostatic rebound at various stages of collision illustrate the evolution of crustal structure, topography, and resulting gravity anomalies. The Ouachita Mountains of Arkansas, which show a low/high Bouguer gravity couple characteristic of the initial stages of collision, have undergone just 8 km of erosion during the process of completely rebounding the syn-orogenic crustal root. This minor rebound means that the Ouachitas retain a crustal geometry similar to the continental margin prior to collision, including thin transitional and oceanic crust. At more advances stages of collision Bouguer gravity anomalies show a broad low reflecting a thickened crustal root. The width of this low, which relates directly to the amount of crustal shortening, is retained during subsequent erosion and elastic rebound, but the amplitude decays gradually. Thus, the width and amplitude of the low can be used to estimate the degree of convergence and amount of erosion, respectively, for a specific mountain range. For the Scandinavian Caledonides results are consistent with 20 km of erosion following 200 km of crustal shortening. Following a larger magnitude of convergence, about 300 km, the southern Appalachians are estimated to have undergone 28 km of post-collisional erosion. Bouguer gravity profiles across the recently-active Alps compare with a model of 200 km of crustal shortening and 8 to 12 km of erosion. While the Alps have undergone a similar amount of shortening as that estimated for the Caledonides, erosion and post-collisional rebound is at an initial stage, such that a thick section of exotic crust still overlies the underthrusted European Platform. The results of these model comparisons suggest that the crustal geometry ofa collisional mountain range should be viewed as a consequence of the degree of crustal shortening as well as the amount of erosion and isostatic rebound. In models at moderate to advanced stages of shortening ( 200 km), and mature stages of erosion (e.g., Caledonides, Appalachians), the geometry of the crustal "suture" between overthrusting and underthrusting crusts is present as a shallow, subhorizontal de collement beneath the foreland. In the hinterland the suture abruptly steepens, a result of differential uplift during isostatic rebound. This crustal geometry, characteristic of seismic-reflection profiles across many ancient mountain belts, suggests: (1) that the "low angle detachment" observed beneath collisional mountain ranges was originally much deeper and steeper than it is at present; and (2) that steep-dipping seismic reflectors towards the hinterland represent the thrusted contact between converging crustal blocks, but have been steepened as a result of isostatic uplift following erosion. / Graduation date: 1992