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RESEARCH Mesozoic Denudation History of the Lower Orange River

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RESEARCH Mesozoic Denudation History of the Lower Orange River RESEARCH Mesozoic denudation history of the lower Orange River and eastward migration of erosion across the southern African Plateau Jessica R. Stanley1 and Rebecca M. Flowers2 1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF IDAHO, 875 PERIMETER DRIVE, MS3022, MOSCOW, IDAHO 83844, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF COLORADO, 2200 COLORADO AVENUE, UCB399, BOULDER, COLORADO 80309, USA ABSTRACT Topographic uplift of the southern African Plateau is commonly attributed to mantle causes, but the links between mantle processes, uplift, and erosion patterns are not necessarily straightforward. We acquired apatite (U-Th)/He (AHe) dates from eight kimberlite and basement samples from the lower reaches of the large westward-draining Orange River system with the goal of evaluating the roles of lithospheric modification and river incision on the erosion history here. Average AHe dates range from 79 to 118 Ma and thermal history models sug- gest that most samples are consistent with a main erosion phase at ca. 120–100 Ma, with some variability across the region indicating a complex erosion history. Major erosion overlaps with the timing of strong lithospheric thermochemical modification as recorded in xeno- liths from the studied kimberlites, but the denudation pattern does not mimic the northward progression of lithospheric alteration across the study region. We attribute this area’s denudation history to a combination of mantle effects, rifting, establishment of the Orange River outlet at its current location, and later faulting. When considering these results with other kimberlite-derived surface histories from an ~1000-km-long E-W transect across the plateau, an eastward-younging trend in denudation is evident. The interplay of mantle processes and the shape of the large, west-draining Orange River basin likely control this first order-pattern. LITHOSPHERE; v. 12; no. 1; p. 74–87; GSA Data Repository Item 2020089 | Published online 30 January 2020 https:// doi .org /10 .1130 /L1121 .1 INTRODUCTION but proposed mechanisms are mostly mantle-induced (e.g., Lithgow- Bertelloni and Silver, 1998; Nyblade and Sleep, 2003; Burke and Gunnell, The processes that control the topography and denudation of conti- 2008). Thus, it is an ideal location to study the links between mantle pro- nental interiors are incompletely understood. While these regions are less cesses and erosion. Apatite fission-track (AFT) and more limited apatite tectonically active than those along plate boundaries, they comprise the (U-Th)/He (AHe) thermochronology studies have focused mostly on majority of continental areas. The significance of these regions for global Precambrian basement samples across and seaward of the escarpment that sediment fluxes and their importance in the coupling between erosion separates the plateau from the coastal plain (e.g., Gallagher and Brown, and climate cycles are debated (e.g., Willenbring et al., 2013; Warrick 1999; Brown et al., 2002; Tinker et al., 2008b; Kounov et al., 2009; Flow- et al., 2014). Better quantification of long-term denudation patterns and ers and Schoene, 2010; Wildman et al., 2015), with more limited work insight into what drives erosion rate change are key for understanding on basement samples from the plateau interior (Wildman et al., 2017). the evolution of these regions. Buoyancy change in the mantle likely These studies broadly document two periods of intensified erosion in exerts stronger influence on surface uplift and topographic change here southern Africa at ca. 150–120 Ma and ca. 100–80 Ma, which correlate than at classic plate boundaries (e.g., Abbott et al., 1997; Pysklywec and with increased sedimentation in the major offshore basins (e.g., Tinker Mitrovica, 1998; Braun, 2010). However, the degree to which this topo- et al., 2008a; Rouby et al., 2009; Guillocheau et al., 2012; Richardson graphic evolution is modulated by surface processes is an open question, et al., 2017; Baby et al., 2018b). because the relationships among mantle processes, lithospheric architec- Recent work using AHe on kimberlites has provided additional insight ture, surface uplift, fluvial network evolution, and the erosional response into the erosion history of the plateau interior (Fig. 1B) (Stanley et al., is complex (e.g., Pazzaglia and Gardner, 1994; Guillou-Frottier et al., 2013, 2015). The abundant Mesozoic kimberlites and related rocks across 2007; Molin et al., 2012). the plateau have shorter, simpler thermal histories that are easier to resolve Southern Africa is an example of a cratonic interior region that was than those of the ancient cratonic basement. Moreover, the kimberlites elevated from sea level to >1000 m elevation in post-Paleozoic time while commonly contain records of lithospheric processes active at the time of distal from convergent plate boundaries and with little surface deforma- their eruption through their xenoliths and chemistries, which provides the tion (Fig. 1A). The timing and mechanisms of surface uplift are debated, opportunity to directly evaluate causal links between mantle and surface processes. Our past work found different temporal relationships between Jessica Stanley http://orcid.org /0000 -0001 -8463 -9271 thermochemical modification of the lithosphere and surface erosion at Geological© 2020 The Society Authors. of Gold America Open |Access: LITHOSPHERE This paper | Volume is published 12 | underNumber the 1 terms| www.gsapubs.org of the CC-BY-NC license. 74 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/12/1/74/4952558/74.pdf by guest on 30 September 2021 STANLEY AND FLOWERS | Mesozoic denudation history of the lower Orange River RESEARCH AHe Data Coastal Internally Basins drained Kimberlites basin Limpopo This study A Basin Stanley et al. 2015 Stanley et al. 2013 Orange River Basement and Karoo Basin This study Study area (Fig 2) Wildman et al. 2017 25°S Gibeon Wildman et al. 2016 Kounov et al. 2013 Rietfontein er Riv er Molopo Africa iv R Orange River Fish aal Hoedkop River V A A' Figure 1. (A) Digital elevation model showing southern African topography, 30°S major river systems and river basins, 15° E Olifants kimberlite occurrences, and locations of samples in this study as well as those Elevation (m) River N from the Orange River catchment with 0 400 Km 500 35° E previously published AHe data. A–A′ Coastal Basins 1200 marks location of transect line used in Other kimberlites 1500 Figure 6. (B) Shaded relief map showing 2100 Drainage divides the present-day extent of the Kalahari 25° E 3000 35°S 20° E 30° E Rivers sedimentary rocks, Karoo sedimentary rocks, and Karoo flood basalts. Dated Study area (Fig 2) Mozambique kimberlites <118 Ma and >118 Ma are B Belt marked, with those dated by AHe in Rehoboth Terrane Gibeon this study or previous work (Stanley Cratonic at et al., 2013, 2015) denoted by the larger ca. 135 Ma Kaapvaal diamonds. Boxes indicate the location, Occurring at Craton 75-58 Ma timing, and intensity of lithospheric Gariep Rietfontein heating and metasomatism, which pro- gressed broadly from SE to NW (Bell B elt et al., 2003). LIP—large igneous province. Hoedkop Mild by ca. 90 Ma On-craton Kaapvaal Strong by study area (Stanley 100-80 Ma et al., 2015) Strong by 150 Ma based on kimberlite Off-craton Karoo study Namaqua-Natal Belt xenoliths area (Stanley et al., 2013) Cenozoic Kalahari Basin Cape Fold Belt Jurassic Karoo Kimberlites LIP Basalts <118 Ma Permian to >118 Ma Jurassic Karoo Basin different locations. In our off-craton Karoo study region, pervasive heat- Here we focus on the lower Orange River region where strong litho- ing and metasomatism of the Proterozoic lithosphere (Bell et al., 2003; spheric modification at the base of a major river network provides the Kobussen et al., 2008; Janney et al., 2010) occurred simultaneously with opportunity to examine the relative roles of lithospheric processes and a major phase of surface erosion that was most intense at ca. 100–90 Ma river network geometry on erosion patterns (Fig. 1). Previous work on (Fig. 1; Stanley et al., 2013). This relationship suggested that lithospheric exclusively basement samples from this region inferred erosion during modification directly triggered surface uplift and erosion. In contrast, on continental breakup and later exhumation along reactivated crustal struc- the Kaapvaal craton, more subtle lithospheric modification due to the tures driven by mantle processes (Wildman et al., 2016, 2017). Here we Archean lithosphere’s more resistant character (Bell et al., 2003; Griffin, present new (U-Th)/He data primarily on kimberlites and related bodies 2003; Kobussen et al., 2009) was accompanied by spatially variable ero- of varying age in a transect across the Orange River, with a focus on sion that migrated eastward across the continent from ca. 120 to <60 Ma deconvolving potential links between surface and lithospheric mantle (Fig. 1B; Stanley et al., 2015). This implied that buoyancy change in the processes in this region. Kimberlite-borne xenoliths in our studied pipes lithosphere contributed less to surface uplift here than in the off-craton show that the region underwent significant lithospheric modification first region, pointing to the need for additional deeper mantle dynamic pro- in the southern portion of the study area and later in the northern part cesses to explain the elevations. (Fig. 1B; Bell et al., 2003), so erosion patterns might mimic the pattern of Geological Society of America | LITHOSPHERE | Volume 12 | Number 1 | www.gsapubs.org 75 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/12/1/74/4952558/74.pdf by guest on 30 September 2021 STANLEY AND FLOWERS | Mesozoic denudation history of the lower Orange River RESEARCH lithospheric modification if lithospheric processes dominate. Alternatively, cratons. These cratons are surrounded by Proterozoic mobile belts and if fluvial processes more strongly modulate the erosion patterns at the base terranes (Fig. 1B). Basement terranes are variably overlain by Precambrian of this major river system, we might instead observe an eastward wave of volcanic and sedimentary basins, as well as by the ca.
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