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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 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 indicating a complex erosion history. Major erosion overlaps with the timing of strong lithospheric thermochemical modification as recorded in xeno- liths from the studied , 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.

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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 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 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 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

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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 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 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 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 , 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 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

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lithospheric modification if lithospheric processes dominate. Alternatively, . These cratons are surrounded by Proterozoic mobile belts and if fluvial processes more strongly modulate the erosion patterns at the base (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. 300–180 Ma Karoo erosion reflective of river network propagation. We use the data to evalu- Supergroup and the Cenozoic Kalahari Basin (Fig. 1B). ate these hypotheses, assess how the results fit with broader erosional and Southern Africa was affected by two major episodes of flood basalt lithospheric modification patterns across the plateau, and consider their magmatism during the breakup of . The ca. 183 Ma Karoo implications for connections among mantle, fluvial network, and erosional large igneous province (LIP) was centered on the east coast and basalts processes during uplift of the southern African Plateau. and associated dolerite sills are found throughout southern Africa (e.g., Marsh et al., 1997; Jourdan et al., 2005). Rocks from the ca. 132 Ma GEOLOGIC BACKGROUND OF SOUTHERN AFRICA Etendeka LIP (Renne et al., 1996) are dominantly found on the west coast. Significant kimberlite and related alkaline magmatism affected southern Topography and Orange River Evolution Africa from ca. 1800 to 45 Ma (Jelsma et al., 2004). Two major episodes of kimberlite volcanism occurred in the Cretaceous at ca. 200 Ma to Southern Africa is characterized by a distinctive topography. Eleva- 110 Ma and ca. 100 Ma to 70 Ma, with a peak at ca. 90 Ma (Jelsma et al., tions of the generally low-relief plateau interior average ~1000 m and in 2004, 2009; Moore et al., 2008). These kimberlites contain a wealth of places are >3000 m (Fig. 1A). The plateau is surrounded on three sides mantle and crustal xenoliths, which record snapshots of the state of the by the high relief “great escarpment,” where the elevation drops abruptly lithosphere and crustal cover at the time of eruption. to the coastal plain. This high topography is postulated to extend into the Southern Africa has one of the best-studied suites of mantle xenoliths bathymetry of the southeastern Atlantic Ocean; together these topographi- in the due to the abundance of diamondiferous kimberlites. Since cally elevated areas have been termed the “African Superswell” (Nyblade southern African kimberlites (and related alkaline ultramafic rocks like and Robinson, 1994). melilitites and lamprophyres) erupted over a considerable age range, it The Orange River currently drains most of the plateau interior (nearly is possible to study the evolution of the mantle lithosphere through geo- 106 km2), from its headwaters in the eastern highlands to where it enters logic time. Thermobarometry on xenolith and xenocryst suites from both the south Atlantic on the west coast (Fig. 1A). All paleo-drainage recon- the lithospheric mantle (Konzett et al., 2000; Bell et al., 2003; Griffin, structions for southern Africa suggest that the majority of the subcontinent 2003; Kobussen et al., 2008) and the lower crust (Schmitz and Bowring, has been westward-draining since the Cretaceous, likely since Gondwana 2003) have been interpreted to document a lithospheric thermal pulse breakup (Dingle and Hendey, 1984; Partridge and Maud, 1987; de Wit, sweeping westward across the continent from ca. 150 Ma to ca. 70 Ma. 1999). Evidence for this comes from reconstructed post-135 Ma sedimentary This thermal pulse was accompanied by metasomatism and chemical fluxes to the adjacent marine basins, with fluxes offshore to the west that modification (Griffin, 2003; Kobussen et al., 2009; Janney et al., 2010), are an order of magnitude higher than to the east and south (Tinker et al., which affected the Proterozoic off-craton lithosphere more substantially 2008a; Guillocheau et al., 2012; Baby et al., 2018b, 2018a). Additionally, than the Archean cratonic lithosphere (Schmitz and Bowring, 2003; Bell large deposits of alluvial diamonds are found only in river terrace deposits et al., 2003). The thermal event appears earliest in xenoliths contained of the lower Orange River and along the west coast, with their diamond- in the ca. 150 Ma East Griqualand pipes southeast of the Kaapvaal cra- iferous kimberlite sources located in the central and eastern plateau (de ton, while contemporaneous kimberlites on-craton and southwest of Wit, 1999; Bluck et al., 2005; Nakashole et al., 2018; Phillips et al., 2018). the craton were unaffected (Fig. 1B; Bell et al., 2003). By ca. 100 Ma, These relationships indicate that the headwaters for major rivers draining lithospheric thermal and metasomatic alteration reached southwest of to the Atlantic were persistently located on the eastern side of the continent. the craton, with xenolith-derived geothermal gradients elevated by up Several drainage reorganization histories have been proposed for the to 100 °C (Bell et al., 2003; Kobussen et al., 2008). This thermochemi- plateau interior, all of which infer that the mouth of the proto–Orange River cal modification pulse reached our study area west of the craton toward shifted through time (Dingle and Hendey, 1984; Partridge and Maud, 1987; the end of this event. de Wit, 1999; Moore, 1999; Moore et al., 2009). Recent work on the Atlan- tic offshore marine stratigraphy suggests that the main river depocenter LOWER ORANGE RIVER STUDY AREA was located south of the current outlet near the present-day Olifants River mouth prior to ca. 113 Ma (Baby et al., 2018b). Deposition then moved Geology and Previous Thermochronology to a proto–Orange River delta slightly south of the modern Orange River outlet at 113 Ma, with the current Orange River delta becoming active by Our study area is located in the western plateau in the lower reaches 93 Ma (Brown et al., 1995; Baby et al., 2018b). Detrital diamond studies of the Orange River system (Figs. 1 and 2A). Here, the Orange River has support this drainage reconstruction. Only pre–90 Ma detrital diamonds carved ~600 m of relief. The Proterozoic Namaqua-Natal belt metamor- occur between the Olifants and Orange River mouths, while post–90 Ma phic basement is overlain by the Ediacaran to Cambrian Nama Group detrital diamonds have been found north of here (Phillips and Harris, 2009; and the Carboniferous to Permian . Phillips et al., 2018). These relationships would be expected given the Previous thermochronology studies from the western region of south- strong northbound current along the coast if the source of the detrital ern Africa mostly focused on using AFT dating of basement samples to diamonds, the Orange River, had migrated north by ca. 93 Ma. Overall constrain fault reactivation and erosion associated with latest Jurassic to these observations document a northward migration of the main river stem Early Cretaceous continental breakup along this rifted margin. From the between 113 and 93 Ma with the current outlet established at that time. approximate latitude of the Orange River and southward, AFT data and a handful of AHe dates were used to argue for two erosional pulses in Geology, Magmatism, and Mantle Modification the middle and Late Cretaceous (Gallagher and Brown, 1999; Kounov et al., 2009, 2013; Wildman et al., 2015). More recent work highlighted Southern Africa is composed dominantly of thick, Precambrian conti- the importance of Late Cretaceous reactivation of individual structures nental lithosphere centered around the Archean Kaapvaal and in locally enhancing erosion in this area (Wildman et al., 2016, 2017).

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This is similar to previous AFT data from farther north along the rifted 16°E 18°E 20°E coast of northern that constrained the initiation of rapid cool- A SA13-31 Karoo dolerite ing and erosion at ca. 70 Ma, with total magnitudes that varied based on (848 m) location relative to reactivated structural lineaments (Raab et al., 2002). Empl: ca. 181—184 Ma In addition, two zircon (U-Th)/He (ZHe) and three zircon fission-track 26°S Rietfontein kimberlite (836 m) AHe: 108 ± 20 Ma (ZFT) dates from several samples in the lower Orange River range from Empl: 135 ± 9 Ma AHe: 62 ± 10 Ma (date-eU) 237 to 340 Ma and 367 to 439 Ma, respectively (Kounov et al., 2013). B'

Kimberlites and Mantle Modification Stolzenfels kimberlite (560 m) SA13-19 (865 m) Dated kimberlites, lamprophyres, and melilitites in our study area range Empl: 201 ± 18 Ma Basement AHe: 107 ± 11 Ma from ca. 60 to 520 Ma in age (Allsopp et al., 1989; Jelsma et al., 2004; 28°S AHe: 118 ± 14 Ma Wu et al., 2010; Griffin et al., 2014), although their non-diamond­iferous Ondermatjie kimberlite character means they are less well-studied than their diamondiferous coun- (681 m) Empl: 515 ± 6 Ma terparts on the Kaapvaal craton. Nonetheless, the state of the lithospheric AHe: 93 ± 10 Ma mantle is constrained at key snapshots in time at three localities: Hoedkop, SA13-22 (628 m) Rietfontein, and Gibeon. Mantle xenolith data from these pipes can be Hoedkop lamprophyre (831 m) Basement compared with the relatively cool, cratonic Kalahari geotherm derived Empl: 79 ± 3 Ma AHe: 87 ± 8 Ma AHe: 48 111 Ma (date-eU) from mantle xenolith P-T arrays in the Kaapvaal and Zimbabwe cratons — B (Rudnick and Nyblade, 1999). 30°S Schuitdrift kimberlite (636 m) In the southern study area (Fig. 1), the Hoedkop ultramafic lampro- Empl: Cretaceous? phyre contains peridotite xenoliths that record metasomatic alteration and AHe: 108 ± 17.3 Ma a geotherm elevated by ~100 °C at all depths over the Kalahari geotherm (Bell et al., 2003; Janney et al., 2010). This is also observed in xenoliths B SSW NNE from the nearby Pofadder kimberlite (Shiimi and Janney, 2017). Hoed- SA13-19 SA13-31 900 kop is dated at 79 ± 3 Ma by perovskite U-Pb (Griffin et al., 2014) and (m) Ondermatjie 40 39 Hoedkop Rietfontein 80 ± 2 by Ar/ Ar on phlogopite (G.B. Kiviets, 2000, unpublished, but 700 atio n Schuitdrift v Avg. AHe Date (Ma) reported in Janney et al., 2010). This indicates that mantle modification SA13-22 had reached our southern study area by 80 Ma or earlier (Bell et al., 2003; El e 500 Stolzenfels 70 80 90 100 110 Kobussen et al., 2008). 0 100 200 300 In the northeastern part of our study area (Fig. 1), eclogite and perido- B Distance along B-B' (km) B' tite xenoliths in the Rietfontein kimberlite suggest a craton-like geotherm similar to the Kalahari geotherm (Appleyard et al., 2007; Janney et al., Figure 2. (A) Digital elevation model of lower Orange River study area show- ing sample locations, AHe dates, and locations of samples with previously 2010; Shiimi and Janney, 2017). The Rietfontein pipe has generally been published AHe data. Symbology same as Figure 1A. B–B′ marks location thought to be ca. 70 Ma (71.9 Ma, zircon U-Pb, Davis, 1977), but recent of transect in B and the plots in Figures 4A and 4C. Emplacement (Empl) perovskite U-Pb data indicate that it is likely older (135 ± 9 Ma, Griffin age references can be found in Tables 1 and 2. (B) SSW-NNE topographic et al., 2014). We favor the perovskite U-Pb date because it is a more recent profile along the B–B′ transect line showing the Orange River canyon and measurement that includes six single-grain dates (Griffin et al., 2014) sample locations, where sample symbols are color-coded by mean AHe rather than only one single-grain zircon U-Pb analyses (Davis, 1977). date. Hoedkop is not colored by mean date because the sample standard deviation is >20%. Avg.—average. Together the data suggest that the lithospheric mantle of the northeastern study area had not yet been modified by 135 Ma. The Gibeon kimberlite cluster in the northern part of our study area contains granular peridotite xenoliths that reveal a Kalahari-like geo- Thus, the results indicate a northward progression of lithospheric heating therm at depths <~100 km, while deformed peridotite xenoliths record and thermochemical modification across the lower Orange River region. a geotherm up to >200 °C hotter than the Kalahari gradient at greater depths (Franz et al., 1996; Boyd et al., 2004). Xenoliths show chemical (U-Th)/He THERMOCHRONOLOGY BACKGROUND evidence for metasomatism and a thinner lithosphere than compatible with a steady-state Kalahari geotherm (Boyd et al., 2004). These observations (U-Th)/He thermochronology is based on the radioactive decay of U, suggest recent thinning at the time of Gibeon eruption (Bell et al., 2003; Th, and Sm to 4He and thermally controlled volume diffusion of the He Boyd et al., 2004). The Gibeon cluster has generally been considered to atoms. At high temperatures He escapes rapidly from the crystal and at be 65–75 Ma (Fig. 1; Allsopp et al., 1989), but a younger U-Pb date of low temperatures it is quantitatively retained. The transition from He loss 57.6 ± 0.6 Ma was recently reported for a zircon megacryst from Mukurob, to retention occurs over a temperature range called the He partial reten- Namibia, one of the Gibeon pipes (Woodhead et al., 2017). We therefore tion zone (PRZ). Different minerals have a range of temperatures over adopt 58–75 Ma as the age of the Gibeon cluster. which this transition occurs. Apatite has one of the lowest temperature In summary, the mantle xenolith thermobarometry results and kim- sensitivities, ~30–90 °C (Farley, 2000; Shuster et al., 2006), making it berlite emplacement ages impose three important constraints on mantle useful for detecting upper crustal and erosional processes. modification in our study area. The data suggest that (1) in the south the Thermochronology is used to constrain the possible time-temperature lithosphere was modified by 80 Ma, (2) in the northeast the lithospheric (tT) paths experienced by a rock. For a single low-temperature thermo- mantle was still unperturbed at 135 Ma, and (3) in the north the litho- chronometer there are numerous thermal histories that can produce a sphere was actively being modified at ca. 58–75 Ma (Bell et al., 2003). given date, so it is common practice to use external geologic information,

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multiple types of thermochronometers, and/or clever spatial sampling diamondiferous kimberlites that we targeted for much of our previous strategies to better constrain thermal histories (e.g., Ehlers and Farley, work (Stanley et al., 2015). Our recent study showed that the (U-Th)/He 2003; Reiners and Brandon, 2006, and references therein). In the AHe technique in zircon (ZHe) and perovskite (PHe) can be effective at dat- system, accumulated radiation damage increases the apatite He retentivity ing kimberlites and serve as a valuable complementary method to other (Shuster et al., 2006), which presents another opportunity to better deci- techniques (Stanley and Flowers, 2016). To better interpret our AHe data pher tT paths by dating apatites with a range of temperature sensitivities and inform the overall history of the region, we therefore dated two of (Flowers et al., 2007, 2009). For rocks that resided near the surface long these kimberlites, Stolzenfels and Ondermatjie, with ZHe or PHe to better enough to accumulate damage, apatite with higher U and Th concentra- constrain the eruption dates. Our other kimberlites did not yield high- tions accumulate radiation damage more rapidly and will have higher quality zircon or perovskite for (U-Th)/He dating. effective closure temperatures than apatite with lower U and Th. Thermal histories where apatite reside in the 30–90 °C temperature window for Methods and Results substantial time can generate positive correlations between AHe date and effective uranium concentration (eU, [U] + 0.235[Th], Shuster et al., Apatite, zircon, and perovskite crystals were separated using standard 2006; Flowers et al., 2007), while tT paths where the apatite cools rap- density and magnetic separation techniques. Individual crystals were idly through the PRZ yield similar AHe dates at all eU (Flowers, 2009). selected for (U-Th)/He analysis based on crystal form and size. Apatite Apatite from basement rocks in cratonic interiors commonly underwent crystals were additionally selected to be free of inclusions under cross- protracted thermal histories that increase the AHe temperature sensitivity polarized light. Grains were photographed and their dimensions measured and cause date-eU correlations (Flowers, 2009; e.g., Flowers and Kelley, prior to loading in Nb packets for analysis. Helium measurements were 2011; Ault et al., 2013; Emberley and Schneider, 2017). made on an Australian Scientific Instruments AlphachronTM in the Uni- In addition to variations in temperature sensitivity due to radiation versity of Colorado Boulder (CU) Thermochronology Research and damage, several other factors can cause dispersion in AHe dates within Instrumentation Laboratory (CU TRaIL), Boulder, Colorado, USA. The a single sample. These include variability in grain size (Reiners and Far- samples were then dissolved and measured for U, Th, and Sm on a Thermo ley, 2001), grain fragmentation during mineral separation (Brown et al., Electron Corporation FinniganTM ELEMENT2 sector field inductively 2013), and injection of α-particles from neighboring minerals with high- coupled plasma–mass spectrometer at CU. Details of the experimental eU (Spiegel et al., 2009) or mineral coatings (Murray et al., 2014). These methods are the same as those described in Stanley et al. (2015) and effects can be enhanced in samples that spent prolonged time in the PRZ Stanley and Flowers (2016). The α-ejection correction of Ketcham et al. or that underwent reheating and partial resetting, thus causing complex (2011) was used for all apatite and zircon crystals. For perovskite, the AHe data sets in settings with such thermal histories (Flowers and Kelley, stopping distances of Stanley and Flowers (2016) were used with the 2011). He particles can travel significant distances when they are emitted, geometries presented in Ketcham et al. (2011). so a geometric correction is applied to account for He that is ejected out All data are reported in Table 1. Grains with α-ejection corrections

of the crystal, which adds additional uncertainty and prevents the use of (FT) corrections of less than 0.57 or eU of less than 5 ppm were excluded.

very small grain sizes (Farley et al., 1996; Ketcham et al., 2011). A large FT correction leads to high uncertainty, and grains with very low eU are more susceptible to bias toward older dates due to He injection SAMPLES, METHODS, AND RESULTS from high-eU neighboring phases. In addition, two grains that are outli- ers from the main population are included in italics in Table 1. These are Strategy and Samples plotted but not included in sample averages or the interpretation. Average (U-Th)/He dates and the associated 1σ standard deviation are reported Our sampling strategy aimed for a combination of kimberlites, Karoo for samples with <20% dispersion. dolerite sills, and gneissic basement rocks of differing age in a N-S tran- Five ZHe dates for the Stolzenfels kimberlite and five PHe dates for sect across the Orange River at a range of elevations (Fig. 2). These the Ondermatjie kimberlite yield average dates of 201 ± 18 Ma and 397 samples were selected with the goal of obtaining a suite of apatite char- ± 30 Ma, respectively (Table 1; Fig. 3). We also report 40 individual AHe acterized by a range of chemistries and crystallization ages that would dates from eight samples with 3–8 grains per sample (Table 1). Seven enable us to best reconstruct detailed thermal histories and regional exhu- of the samples are characterized by <20% dispersion and have mean mation patterns with the (U-Th)/He method. We specifically targeted the dates ranging from 62 ± 12 Ma to 118 ± 14 Ma, while the eighth sample Hoedkop lamprophyre, Rietfontein kimberlite, and Gibeon kimberlite (Hoedkop) has greater dispersion (35%). The sample AHe data do not that have mantle xenolith constraints on the lithospheric geotherm and show obvious correlations with elevation or proximity to the Orange River the degree of lithospheric metasomatic alteration at the time of their erup- (Figs. 2 and 4). Figure 5 shows AHe date-eU plots for all samples, where tion (as described above). Unfortunately, our sample from Gibeon did not individual grain dates are plotted with their 2σ analytical uncertainties. yield suitable apatite crystals for dating. In total we acquired AHe data for In most samples the dates do not vary strongly with eU. Exceptions are eight samples ranging in elevation from 560 m in the Orange River valley the Hoedkop lamprophyre (Fig. 5A) and Rietfontein kimberlite (Fig. 5B), to 865 m on the plateau surface. These samples consisted of the Rietfon- which yield younger AHe dates than the other samples and show slight tein, Schuitdrift, Stolzenfels, and Ondermatjie kimberlites, the Hoedkop positive date-eU correlations with the oldest dates overlapping the pub- lamprophyre, one Jurassic Karoo dolerite sill, and two Proterozoic base- lished eruption age for Hoedkop. ment gneisses (Fig. 2A; Table 1). The kimberlites and lamprophyre have published eruption ages from Cambrian to Late Cretaceous. DISCUSSION Accurate eruption dates for the kimberlites are important for compari- son with the AHe cooling date and for interpreting the xenolith record, Significance of ZHe and PHe Dates for Kimberlite Eruption Ages but kimberlites can be challenging to date. The kimberlites, lampro- phyres, and other small-volume volcanic rocks of this off-craton region The new ZHe and PHe dates better constrain the timing of kimberlite are undiamondiferous and therefore less well-dated than the on-craton volcanism in this region. The ZHe date from the Stolzenfels kimberlite

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TABLE 1. (U-Th)/He DATA FROM SOUTHERN AFRICA

a b c d e f Sample Mass r Term He U Th Sm eU Th/U Raw date FT Cor. date 2σ (µg) (µm) (ncc) (ppm) (ppm) (ppm) (ppm) (Ma) (Ma) (Ma)

Zircon and Perovskite (U-Th)/He data

SA13-17: Stolzenfels-Wes kimberlite, zircon: 19.65818°E, 28.5048°S, 560 m elev. z1 8 54 2T 69.80 77.5 53.2 N/A 90.0 0.7 142 0.78 181 5 z3 4 43 2T 94.90 112.3 55.1 N/A 125.2 0.5 139 0.73 189 6 z3 4 45 2T 146.70 152.0 42.7 N/A 162.0 0.3 166 0.75 221 5 z4 10 60 2T 47.80 42.2 29.7 N/A 49.1 0.7 178 0.81 221 7 z5 8 58 2T 119.90 109.8 131.3 N/A 140.6 1.2 156 0.81 193 3 Mean ZHe: 201 ± 18 Ma, 9% standard deviation Published age: 60.3 ± 3.9 Ma, perovskite U-Pb (Griffin et al., 2014)

SA13-15: Ondermatjie kimberlite, perovskite: 19.43458°E, 28.36494°S, 681 m elev. p1 3 48 F 11.30 29.6 174.0 27.0 70.5 5.9 435 N/A 435 25 p2 7 63 F 22.20 25.0 174.4 17.5 66.0 7.0 410 N/A 410 20 p3 3 49 F 10.20 26.4 172.2 35.3 66.8 6.5 392 N/A 392 67 p4 4 53 F 9.80 24.8 113.2 20.0 51.4 4.6 393 N/A 393 27 p5 3 50 F 13.20 43.7 209.9 30.3 93.0 4.8 354 N/A 354 78 Mean PHe: 397 ± 30 Ma, 8% standard deviation Published age: 515 ± 6 Ma, perovskite U-Pb (Wu et al., 2010)

Apatite (U-Th)/He data

SA13-13: Hoedkop lamprophyre, 18.70811°E, 29.28263°S, 831 m elev. a2 1 36 0T 0.06 4.5 13.8 47.0 7.8 3.0 48 0.60 79 22 a6 2 40 0T 0.03 4.2 3.4 26.0 5.0 0.8 32 0.65 49 4 a8 3 54 2T 0.77 21.6 38.3 10.9 30.6 1.8 82 0.73 112 2 a10 1 35 0T 0.09 11.7 26.5 5.3 17.9 2.3 39 0.60 66 2 Date-eU correlation, 35% standard deviation of AHe dates Published age: 79 ± 3 Ma (Griffin et al., 2014)

SA13-30: Rietfontein kimberlite: 20.03708°E, 26.74332°S, 835 m elev. a1 1 35 0T 0.04 6.4 18.0 52.3 10.6 2.8 37 0.58 62 17 a3 1 38 0T 0.09 8.0 28.6 58.3 14.7 3.6 44 0.61 71 11 a4 1 35 1T 0.02 6.6 3.4 3.9 7.4 0.5 26 0.60 43 20 a5 1 35 0T 0.08 8.9 45.4 50.4 19.5 5.1 36 0.57 63 8 a6 1 36 0T 0.08 10.0 41.5 71.9 19.8 4.1 44 0.59 73 12 Mean: Date-eU trend. 62 ± 12 Ma, 19% standard deviation Published ages: 135 ± 9 Ma, perovskite U-Pb (Griffin et al., 2014); 71.9 Ma, zircon U-Pb (Davis, 1977)

SA13-21: Schuitdrift kimberlite: 19.80731°E, 28.55292°S, 636 m elev. a1 4 53 0T 0.93 12.4 58.4 20.4 26.1 4.7 83 0.72 116 5 a2 1 39 1T 0.65 46.5 14.3 36.8 49.9 0.3 78 0.64 121 5 a7 1 36 1T 0.18 25.7 35.9 8.2 34.1 1.4 54 0.61 89 1 Mean: 108 ± 17 Ma, 16% standard deviation Published age: No published age for this locality

SA13-17: Stolzenfels kimberlite: 19.65818°E, 28.5048°S, 560 m elev. a1 3 52 2T 2.45 84.1 40.4 49.4 93.6 0.5 84 0.69 122 3 a2 2 45 1T 0.83 59.0 15.3 50.9 62.6 0.3 65 0.67 97 3 a4 3 56 2T 1.40 41.5 15.6 7.9 45.2 0.4 81 0.72 113 2 a5 1 44 2T 0.30 21.8 32.4 23.1 29.4 1.5 60 0.64 95 3 a6 1 39 1T 0.85 86.9 22.0 64.7 92.0 0.3 69 0.63 111 3 Mean: 107 ± 11 Ma, 10% standard deviation Published ages: 201 ± 18 Ma, ZHe (this study); 60.3 ± 3.9 Ma, Perovskite U-Pb (Griffin et al., 2014).

SA13-31: Karoo dolerite, 20.31352°E, 26.74322°S, 848 m elev. a1 2 42 0T 0.09 5.0 16.7 54.7 8.9 3.4 55 0.65 83 3 a2 1 34 0T 0.07 5.3 17.8 76.5 9.5 3.3 82 0.57 138 7 a3 1 37 0T 0.06 4.5 13.4 58.0 7.6 3.0 70 0.61 111 6 a4 1 38 0T 0.17 9.8 33.0 81.6 17.5 3.4 64 0.61 102 3 a5 1 37 0T 0.07 5.8 15.3 55.7 9.4 2.6 65 0.62 103 5 Mean: 108 ± 20 Ma, 18% standard deviation Age: ca. 181–184 Ma (Jourdan et al., 2008) (continued)

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TABLE 1. (U-Th)/He DATA FROM SOUTHERN AFRICA (continued)

a b c d e f Sample Mass r Term He U Th Sm eU Th/U Raw date FT Cor. date 2σ (µg) (µm) (ncc) (ppm) (ppm) (ppm) (ppm) (Ma) (Ma) (Ma)

Apatite (U-Th)/He data (continued)

SA13-15: Ondermatjie kimberlite: 19.43458°E, 28.36494°S, 681 m elev. a1 2 43 1T 0.25 18.4 23.6 5.9 24.0 1.3 56 0.67 83 7 a2 2 47 0T 0.47 25.0 31.0 8.5 32.3 1.2 68 0.70 97 6 a6 5 54 0T 0.23 2.5 11.7 10.3 5.3 4.7 68 0.72 94 3 a7 2 50 2T 0.10 3.1 11.5 11.2 5.8 3.7 76 0.70 108 8 a9 1 33 0T 0.09 11.5 13.9 3.4 14.7 1.2 56 0.58 96 4 a10 2 45 2T 0.08 4.1 13.0 15.9 7.1 3.2 54 0.67 81 5 a12 2.1 37.2 0T 0.014 7.3 0.9 1.7 7.5 0.1 7 0.63 11 0.6 Mean: 93 ± 10 Ma. 11% standard deviation Published Age: 515 ± 6 Ma, Perovskite U-Pb (Wu et al., 2010)

SA13-22: Biotite gneiss: 19.81271°E, 28.55596°S, 628 m elev. a1 2 48 2T 0.99 40.5 71.6 6.9 57.4 1.8 59 0.70 85 2 a2 1 37 0T 0.59 70.2 116.3 13.5 97.5 1.7 60 0.62 96 4 a3 1 34 2T 0.35 52.2 74.7 8.4 69.8 1.4 47 0.58 81 4 a4 2 47 1T 1.18 57.0 105.1 18.1 81.7 1.8 65 0.70 94 3 a5 1 40 2T 0.48 45.1 68.9 8.2 61.3 1.5 49 0.65 75 3 a6 4 57 1T 1.75 40.4 66.6 7.6 56.1 1.6 67 0.75 89 2 Mean: 87 ± 8 Ma, 9% standard deviation Age: ca. 1.2 Ga, Proterozoic Namaqua-Natal belt basement

SA13-19: Biotite gneiss, 19.75778°E, 28.39045°S, 865 m elev. a1 1 39 0T 0.25 22.9 7.0 46.5 24.5 0.3 80 0.64 122 5 a2 1 38 2T 0.30 18.7 4.9 33.8 19.9 0.3 87 0.63 135 3 a3 1 41 0T 0.28 20.4 5.1 35.6 21.6 0.2 75 0.66 113 3 a4 0.9 34.8 2T 0.192 12.1 6.1 36.9 13.5 0.5 127 0.60 208 11.2 a5 1 40 0T 0.17 15.7 3.0 34.1 16.4 0.2 68 0.66 102 4 Mean: 118 ± 14 Ma, 12% standard deviation Age: ca. 1.2 Ga, Proterozoic Namaqua-Natal belt basement Note: Rows in italics indicate two grains that are outliers from the main population and not included in averages. Cor.—corrected; elev.— elevation; N/A—not applicable. aMean and 1s standard deviation of corrected dates reported for samples with <20% standard deviation. No mean reported for samples with >20% standard deviation. bEquivalent spherical radius (r), the radius of a sphere with the same surface area to volume ratio. c Type of grain terminations: 2T—whole grain; 1T—one tip broken off; 0T—both tips broken off; F—interior fragment (no FT correction applied). deU: effective uranium concentration, weights U and Th for their alpha productivity, computed as [U] + 0.235 * [Th]. e FT is alpha-ejection correction of Ketcham et al. (2011). fAnalytical uncertainty based on U, Th, He, and grain length measurements.

A Stolzenfels zircon B Ondermatjie perovskite

400 400

300 300 (Ma) (Ma) dat e dat e 200 200 ZH e PH e 100 100

0 0 0 50 100 150 0 50 100 150 eU (ppm) eU (ppm) Figure 3. Date-eU plots for ZHe data from the Stolzenfels kimberlite (A) and PHe data from the Ondermatjie kimberlite (B), southern African Plateau. Data can be found in Table 1. Error bars represent the 2σ analytical uncertainty (in some cases smaller than the marker).

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A B 1000 150

800

100 ation (m) v

El e 600 AHe date (Ma)

50

400 0 100 200 300 50 100 150 SSW Distance along B−B' (km) NNE AHe date (Ma) C D 1000 150

800

100 ation (m) v

El e 600 AHe date (Ma)

50

400 0 100 200 300 50 100 150 SSW Distance along B−B' (km) NNE AHe date (Ma)

eU (ppm) 100 200 300 400

Figure 4. Plots of average AHe date and associated 1σ standard deviation for the seven samples, southern African Plateau, with <20% dispersion versus (A) distance north along the B–B′ transect line in Figure 2 and (B) elevation. The average for the eighth sample, Hoedkop, is not plotted or interpreted because the dispersion is >20%. (C-D) AHe date versus distance and AHe date versus elevation plots similar to those in A and B, but with the individual AHe dates from this study and previously published studies plotted. Data from this study are marked as circles with their 2σ analytical uncertainties and color-coded by eU. Triangles are data from Wildman et al. (2017), which are also color-coded by eU. Black ×s are data from Kounov et al. (2013) for which eU information is unavailable.

of 201 ± 18 Ma (Fig. 3A) is substantially older than the published U-Pb tectonic and thermal processes after eruption. The discrepancy between perovskite date of 60.3 ± 3.9 Ma (Griffin et al., 2014). While perovskite the kimberlite U-Pb and PHe dates for Ondermatjie suggests that heating U-Pb is generally the preferred dating method for kimberlites, potential partially reset the PHe system in the early Paleozoic and that tempera- complexities include high common Pb contents and multiple age domains tures subsequently were <300 °C. This is consistent with ZFT dates of that can make the ages less reliable (see Stanley and Flowers, 2016, for a 350–450 Ma from the area that limit temperatures to <200–300 °C since summary of kimberlite dating techniques). In this particular case we favor that time (Kounov et al., 2013). our ZHe date both because it is a collection of five dates from individual grains yielding consistent results and because the AHe dates from this Erosion History Along the Lower Orange River sample are reproducible and older than the perovskite U-Pb date (AHe average 107 ± 11 Ma). Our new AHe data for samples along the lower Orange River help con- The PHe date of 397 ± 30 Ma for the Ondermatjie kimberlite (Fig. 3B) strain the cooling and erosion history along a NNE-SSW transect through is younger than the published perovskite U-Pb date of 515 ± 6 Ma (Wu this region. The majority of the samples yield Cretaceous dates, with the et al., 2010). We favor the Cambrian U-Pb date as representing the erup- exception of several individual dates from the Hoedkop and Rietfontein tion age over our Devonian PHe date because the U-Pb date is the weighted pipes that are <65 Ma. Plots of AHe date versus distance and AHe date average of 20 reproducible measurements and because the perovskite versus elevation for our sample transect display no systematic patterns closure temperature for Pb is presumed to be higher than for He (PHe (Figs. 4A and 4B). Nor do obvious patterns emerge when previously pub- closure temperature likely >300 °C; Stanley and Flowers, 2016). PHe lished AHe data for Precambrian samples from this same region (Kounov is probably less reliable than U-Pb in perovskite for older kimberlites et al., 2013; Wildman et al., 2017) are added to these plots (Figs. 4C and because He is more likely to be lost than Pb due to mid-to-upper crustal 4D; colored by eU, where such data are available, to reveal dispersion

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A SA13−13: Hoedkop lamprophyre, Elev: 831 m B SA13−30: Rietfontein kimberlite, Elev: 835 m 0 0 200 200

150 50 150 50 (Ma) (Ma) C) C) ( ° ( ° 100 100 100 100 dat e dat e em p em p T T 50 150 50 150 AH e AH e

0 200 0 200 0 25 50 75 100 200 150 100 50 0 0 25 50 75 100 200 150 100 50 0 eU (ppm) Time (Ma) eU (ppm) Time (Ma) C SA13−21: Schuitdrift kimbertlite, Elev: 636 m D SA13−17: Stolzenfels kimberlite, Elev 560 m 0 0 200 200

150 50 150 50 (Ma) (Ma) C) C) ( ° ( ° 100 100 100 100 dat e dat e em p em p T T 50 150 50 150 AH e AH e

0 200 0 200 0 25 50 75 100 200 150 100 50 0 0 25 50 75 100 200 150 100 50 0 eU (ppm) Time (Ma) eU (ppm) Time (Ma) E SA13−31: Karoo dolerite, Elev: 848 m F SA13−15: Ondermatjie kimberlite, Elev: 681 m 0 0 200 200

150 50 150 50 (Ma) (Ma) C) C) ( ° ( ° 100 100 100 100 dat e dat e em p em p T T 50 150 50 150 AH e AH e

0 200 0 200 0 25 50 75 100 200 150 100 50 0 0 25 50 75 100 200 150 100 50 0 eU (ppm) Time (Ma) eU (ppm) Time (Ma) G SA13−22: basement gneiss, Elev: 628 m H SA13−19: basement gneiss, Elev: 865 m 0 0 200 200

150 50 150 50 (Ma) (Ma) C) C) ° ° ( ( 100 100 100 100 dat e dat e em p em p T T 50 150 50 150 AH e AH e

0 200 0 200 0 25 50 75 100 200 150 100 50 0 0 25 50 75 100 200 150 100 50 0 eU (ppm) Time (Ma) eU (ppm) Time (Ma)

Model Fit Acceptable Good Best Weighted Mean

Figure 5. (A–H) AHe date-eU plots and thermal history modeling results for all samples, southern African Plateau. Date-eU plots show kimberlite and lamprophyre emplacement ages and associated uncertainties as blue bars, eU bins used in the models as vertical dashed lines, and the date-eU pat- tern predicted by the best fit time-temperature (tT) path as the black curve. Errors are plotted as the 2σ analytical uncertainties, and in some cases are smaller than the symbol. Gray points are outliers not included in the modeling or interpretation. The tT plots show the thermal history constraints as dashed boxes, the weighted mean path in black, the best-fit path in dark gray, good-fit paths in gray, and acceptable-fit paths in light gray. The tT models for the Ondermatjie kimberlite and two Precambrian basement samples start at 450 Ma, but the plots are truncated at 230 Ma for visual clarity. Additional details of model inputs and their rationale are in Table 2 and full model outputs are in the Data Repository material (see text footnote 1). Elev—elevation; Temp—temperature.

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TABLE 2. THERMAL HISTORY MODEL INPUT FOR SOUTHERN AFRICAN SAMPLES

1. Thermochronologic data Samples and data used in simulations Sample Rock type eU bins for AHe dataa Emplacement age used in model

Mesozoic rocks SA13-13 Hoedkop lamprophyre <15ppm (2 grains), >15 ppm (2 grains) 79 ± 3 Ma (Griffin et al., 2014) SA13-30 Rietfontein kimberlite <15ppm (3 grains), >15 ppm (2 grains) 70–140 Ma (72 Ma, Davis, 1977; 135 Ma, Griffin et al., 2014) SA13-21 Schuitdrift kimberlite 1 bin (3 grains) 80-220 Ma (age unknown, range encompasses ages of other kimberlites in the region) SA13-17 Stolzenfels-Wes kimberlite <65ppm (3 grains), >65 ppm (2 grains) 201.0 ± 18 Ma (ZHe, this study) SA13-31 Karoo dolerite <15 ppm (4 grains), >15 ppm (1 grain) 183 ± 3 Ma (Jourdan et al., 2008) Paleozoic and older rocks SA13-15 Ondermatjie kimberlite <15ppm (4 grains), >15 ppm (2 grains) SA13-19 Basement gneiss 1 bin (5 grains) SA13-22 Basement gneiss <65 ppm (3 grains), >65 ppm (3grains) AHe data treatment: Dates, uncertainties, and other relevant constraints Treatment: Samples were binned into groups with similar eU values, and the mean of each group was modeled. He dates (Ma): Mean uncorrected He date of each bin. aEjection corrected in HeFTy using Ketcham et al. (2011). Error (Ma) applied in modeling: The 1s sample standard deviation of each bin was applied if ≥10%. If <10%, then 10% was applied. r (um): Mean equivalent spherical radius of each bin. eU (ppm): Mean U and Th for each bin. eU zonation: None assumed.

2. Additional geologic information Constraint Explanation and data source

Mesozoic rocks (1) >120 °C at time of eruption/emplacement Kimberlite and dolerite emplaced as hot magma. Eruption dates for each sample from part 1. (2) 20 °C at present day Present-day approximate surface temperature. Paleozoic and older rocks (1) >200° C between 350 and 450 Ma Based on PHe dates from the Ondermatjie kimberlites in this study (397 ± 30 Ma), as well as regional zircon fission-track dates from Kounov et al. (2013) that range from 362 to 439 Ma. (2) <80 °C around 300 Ma Dwyka sediments are preserved nearby, and the start of Dwyka deposition was at ca. 300 Ma (Catuneanu et al., 2005). (3) >110 °C between 180 and 110 Ma Based on regional thermal history models of basement AFT and AHe data from Wildman et al., (2017). Regional apatite fission-track dates are between 59 and 129 Ma (Wildman et al., 2017; Kounov et al., 2013) suggesting that they were reset in the Cretaceous. Karoo basin burial peaked at ca. 183 Ma with the eruption of the flood basalts. (4) 20 °C at present day Present-day approximate surface temperature.

3. System- and model-specific parameters He kinetic model: RDAAM (radiation damage accumulation and annealing model; Flowers et al., 2009). Statistical fitting criteria: Goodness of fit (GOF) values >0.5 for “good” fits. >0.05 for “acceptable” fits. The good-fits also must have a minimum GOF of 1/(N+1) where N is number of statistics used (Ketcham et al., 2005). Modeling code: AHe uses HeFTy v1.9.3. Number of time-temperature paths attempted: 30,000 for all models.

possibly due to radiation damage). The lack of systematic relationships basement and Cambrian Ondermatjie kimberlite samples. The detailed either indicates that there are no significant spatial differences in the tim- data and constraints used for each model, and their rationale, are summa- ing of cooling, or that the patterns are obscured by other causes of data rized in Table 2 (after Flowers et al., 2015) and the full model parameters dispersion, such as radiation damage and/or the sample crystallization age. and outputs can be found in the GSA Data Repository1. We used the HeFTy thermal history modeling software (Ketcham, The Hoedkop ultramafic lamprophyre (831 m) and Rietfontein kimber- 2005) to further evaluate any spatial patterns in the timing of cooling lite (835 m) are located at similar elevations at the southern and northern that might not be apparent from the AHe data alone. Inverse modeling in ends of the sample transect. The AHe dates for Hoedkop (Fig. 5A) are HeFTy simulates a series of random time-temperature (tT) paths conform- slightly dispersed but overlap the eruption age of 79 ± 3 Ma (Griffin et al., ing to user-defined tT constraints corresponding to geologic information, 2014). This suggests that the AHe date records cooling at the time of erup- and reports “good” and “acceptable” fit thermal histories that simultane- tion and the sample underwent little erosive cooling after eruption, which ously satisfy the date, eU, and equivalent spherical radius for each sample is corroborated by the tT models (Fig. 5A). Assuming that the Rietfontein (Ketcham, 2005). For kimberlite and Karoo dolerite samples, the only geologic constraints applied to the thermal history were that the rock had 1 GSA Data Repository Item 2020089, full HeFTy model parameters and output used to be >120 °C at the time of emplacement and at surface temperature of to make Figures 5 and 6, is available at http://www.geosociety​.org​/datarepository​ 20 °C today. Additional constraints were imposed on the Precambrian /2020, or on request from [email protected].

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kimberlite was emplaced at 135 ± 9 Ma (based on its perovskite U-Pb date; plateau in both the off-craton Karoo (Stanley et al., 2013) and on-craton Griffin et al., 2014), then its average AHe date of 62 ± 10 Ma indicates Kaapvaal (Stanley et al., 2015) regions. Here we can assess if erosion substantial post-eruptive cooling. If instead the Rietfontein kimberlite in our lower Orange River study area overlaps with the broad timing of was emplaced at 72 Ma (Davis, 1977) then the cooling history for Riet- lithospheric modification, and if it mimics the northward progression of fontein is similar to that of Hoedkop with little post-eruption cooling due lithospheric modification across the region. to erosion. Both the Hoedkop and Rietfontein samples contain low eU Most of our lower Orange River samples are consistent with major (<20 ppm) apatites that define a weak, positive correlation between date erosion at ca. 120–100 Ma, with both earlier and later erosion implied by and eU (Figs. 5A and 5B). For Rietfontein, this pattern allows for later, some samples. The main ca. 120–100 Ma unroofing phase thus overlaps low magnitude Cenozoic cooling in the tT simulation result (Fig. 5B). broadly with the timing of lithospheric modification. Xenoliths in the The Schuitdrift kimberlite (636 m) in the middle of the transect is part Hoedkop lamprophyre indicate that mantle metasomatism and thinning of the Ariemsvlei kimberlite cluster. This cluster is generally thought to be had occurred by 80 Ma, while Rietfontein kimberlite xenoliths indicate middle to Late Cretaceous in age (Jelsma et al., 2004), but the exact age that it had not started by 135 Ma, pinning the probable timing between of Schuitdrift is unknown. Its average AHe date of 108 ± 17 Ma imposes these dates. Lithospheric modification in the Karoo region to the east a minimum age on emplacement and could either represent cooling asso- is more tightly constrained to between 117 and 103 Ma (Fig. 1B; Bell ciated with eruption or post-eruptive cooling/exhumation (Fig. 5C). This et al., 2003; Kobussen et al., 2008; Janney et al., 2010). Since lithospheric kimberlite lacks low eU apatites, so the lowest temperature portion of its modification likely progressed from SE to NW (Bell et al., 2003), our history is not well-determined (Fig. 5C). Orange River study area located northwest of the Karoo (Fig. 1B) was The two Jurassic-aged samples, the Stolzenfels kimberlite (560 m, probably not affected prior to 117 Ma. This limits likely thermochemi- ZHe age of 201 ± 18 Ma) and a Karoo dolerite sill (SA13-31, 848 m, cal modification of Proterozoic lithosphere in the lower Orange River to ca. 181–184 Ma, Jourdan et al., 2008), yield overlapping AHe dates of 117–80 Ma. Metasomatism of the off-craton lithosphere here was sub- 107 ± 11 Ma and 108 ± 17 Ma that postdate emplacement (Figs. 5D and stantial and associated with lithospheric thinning (Janney et al., 2010). 5E). Consequently, tT models for both samples show a ca. 120–100 Ma The broad coincidence of this lithospheric event with major surface ero- cooling phase (Figs. 5D and 5E). The simulation for Stolzenfels allows sion from ca. 120 to 100 Ma suggests that lithospheric modification may additional later cooling but does not require it. have contributed to surface uplift and denudation. These relationships are The Cambrian Ondermatjie kimberlite and two Precambrian basement similar to those documented in our off-craton Karoo study area where samples also yield Cretaceous AHe dates that record Cretaceous cooling. regional erosion was simultaneous with strong lithospheric modification The Ondermatjie kimberlite (681 m) and a basement gneiss at similar (Stanley et al., 2013), but the timing of the mantle thermochemical event elevation (SA13-22, 628 m) yield overlapping AHe dates of 93 ± 10 Ma is less tightly constrained in the lower Orange River. and 87 ± 8 Ma (Figs. 5F and 5G). At slightly higher elevation, another If lithospheric modification alone influenced the erosion history, then basement gneiss (SA13-19) gives a slightly older AHe date of 118 ± 14 Ma we might expect the AHe data to young northward because mantle modifi- (Fig. 5H). The thermal history models show substantial Cretaceous cooling cation occurred earlier in the southern study area (between 117 and 80 Ma, of all three samples, with the higher elevation basement sample cooling as described above) than to the north where Gibeon pipe xenoliths indi- 10–40 m.y. earlier than the other two (Figs. 5F, 5G, and 5H). cate the lithosphere was actively undergoing modification at 75–58 Ma Taken together, the relationship between eruption ages and AHe dates (Fig. 1B; Bell et al., 2003; Boyd et al., 2004; Janney et al., 2010). However, for our samples suggests that there was significant cooling and erosion neither our AHe data nor previously published results suggest a system- in Early to mid-Cretaceous time that occurred prior to eruption of the atic northward progression of erosion. Thus, despite the broad overlap 70–80 Ma kimberlites. The shape of the cooling paths varies between in the timing of erosion and lithospheric modification, the relationships samples and with eruption age (Fig. 5). None of the good-fit tT paths imply additional influences on the denudation patterns across the region. reach surface temperature before 125 Ma. All samples cooled below ~70 °C by ca. 75 Ma, with the exception of the Hoedkop lamprophyre An Eastward-Migrating Wave of Erosion across the Southern that erupted around this time and the Rietfontein kimberlite (Figs. 5A and African Plateau and Its Causes 5B). Although most samples can be explained by a main cooling phase from ca. 120 to 100 Ma, both older (150–110 Ma; SA13-19, Fig. 5H) Thermochronologic data from the lower Orange River region suggest and younger (100–70 Ma; SA13-30, SA13-22, Figs. 5B and 5G) cooling a multiphase unroofing history. The Cretaceous AHe dates from this area is permitted by some samples. Variability in cooling was also detected are far more heterogeneous over short wavelengths than in our AHe data by previous AFT and AHe study of basement in this region, from which sets from elsewhere on the plateau (Stanley et al., 2013, 2015). This is two phases of Cretaceous erosion starting at 130–120 Ma and 110–70 Ma compatible with Late Cretaceous movement on faults causing spatial were inferred (Wildman et al., 2017). complexity in the thermochronologic data patterns (Wildman et al., 2017), and implies multiple influences on the erosion history. Opening of the Evaluating the Role of Lithospheric Thermochemical south Atlantic Ocean at ca. 130 Ma (Collier et al., 2017), establishment of Modification on the Erosion History of the Lower Orange River the mouth of the Orange River in its current location from 112 to 93 Ma (de Wit, 1999; Baby et al., 2018b), deeper mantle dynamic effects (e.g., Our AHe data set includes results for four kimberlites and one ultra- Lithgow-Bertelloni and Silver, 1998), as well as lithospheric modification mafic lamprophyre with crystallization ages as young as Late Cretaceous. (e.g., Bell et al., 2003) all may have influenced denudation here. These data complement previous work exclusively on ancient basement Distinctly different relationships between surface erosion patterns and samples in this region (Wildman et al., 2016, 2017), but our focus on lithospheric mantle evolution emerge from our kimberlite AHe studies in kimberlites allows us to more comprehensively evaluate temporal and three different plateau regions. Unlike the more complex erosion history in causal links with lithospheric thermochemical modification processes as our current off-craton study area in the lower Orange River, the off-craton recorded by mantle xenoliths in the same and nearby pipes. This approach Karoo study region farther east underwent a simpler history of substantial is like that in our past kimberlite thermochronology studies across the erosion at 100–90 Ma coincident with pervasive lithospheric modification

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and thinning (Fig. 1B; Stanley et al., 2013). And our kimberlite transect the mouth of the Orange River northward between ca. 112 and 93 Ma (de across the southern Kaapvaal craton revealed an eastward migrating wave Wit, 1999; Baby et al., 2018b). Similarly, lithospheric-induced surface of erosion from ca. 120–60 Ma associated with relatively weak modifica- uplift in the off-craton Karoo region enhanced denudation (Stanley et al., tion of the Archean lithosphere (Fig. 1B; Stanley et al., 2015). 2013) and would have further promoted rapid eastward propagation of To better evaluate the large-scale trends in this broader data set across erosion. In contrast, the wave of enhanced erosion and incision slowed an ~1000-km-long region, Figure 6 indicates the time when the weighted in the upstream, on-craton region to the east (as indicated by a more dra- mean tT path from the thermal history models for Mesozoic kimberlites matic eastward younging in the timing of cooling of samples here; Fig. 6) and related igneous rocks in an E-W transect across the plateau cool owing to less lithospheric-induced surface uplift and a smaller drainage through 70 °C (tT models in Fig. 5 of this paper and those in Stanley area. A thicker package of Karoo basalt in the east may have added a sec- et al., 2013, 2015). We only plot results for those kimberlites with post- ondary lithologic control. Thus, the interaction of mantle-derived uplift eruption AHe dates that capture cooling associated with major erosion. and river network evolution likely exerted key control on the Cretaceous The younger pipes that yield AHe dates coincident with pipe eruption are uplift, denudation, and topographic history of the southern African Plateau. excluded because they do not record the erosion event owing to kimber- lite emplacement after major erosion occurred. Figure 6 shows that the CONCLUSIONS western, off-craton lower Orange River samples record the earliest cool- ing/erosion, with slightly later cooling/erosion of the off-craton Karoo New AHe dates from kimberlites, other shallow intrusions, and base- samples, followed by a general eastward younging of cooling across the ment rocks from the lower Orange River region document a regional craton. The timing of lithospheric modification is also shown on Figure phase of cooling/erosion dominantly from ca. 120 to 100 Ma prior to the 6. In the off-craton regions and western craton, cooling overlaps with eruption of the youngest kimberlites, with some spatial complexity in the lithospheric modification, while dates trend younger to the east on the cooling/erosion history across the study area. The 120–100 Ma erosional craton where modification was mild. phase overlaps the time of strong lithospheric modification here but does We suggest that the timing and nature of mantle processes as well as not mimic the spatial patterns in detail, indicating that surface uplift and the location within the drainage network critically influenced the eastward erosion from this process alone cannot explain the data. Mantle processes progression of erosion across the plateau. Westward tilting of the continent and the northward shift of the Orange River mouth to its current location, initially established a large westward-draining river system (Braun et al., as well as previously proposed continental breakup and fault reactivation 2014). Latest Jurassic to Early Cretaceous rifting on the west coast and activity (Wildman et al., 2016, 2017), may all have influenced the Cre- the onset of seafloor spreading ca. 130 Ma (Collier et al., 2017) initiated taceous surface history of the study area. When data from kimberlites in Early Cretaceous cooling and erosion here, followed by a pulse of erosion this region are compared with those from across the plateau (Stanley et al., that propagated up the river network. The west-to-east cooling/erosion 2013; 2015), an eastward younging of Cretaceous cooling is apparent patterns in Figure 6 are broadly congruent with predictions of the stream (Fig. 6). We suggest westward tilting of the continent triggered an ero- power model (Howard and Kerby, 1983), where a continental uplift event sional wave that started in the west and propagated across the continent triggers a wave of incision that starts at the outlet of the river network, (Braun et al., 2014). Uplift due to lithospheric modification west of the migrates upstream (e.g., Rosenbloom and Anderson, 1994; Whipple and craton likely induced more rapid erosion here than on-craton. The pro- Tucker, 1999), and then decelerates due to the diminishing upstream gression of the main phase of erosion from west to east suggests that the drainage area (e.g., Berlin and Anderson, 2007). large, westward-draining river network played a key role in controlling Lithospheric modification, possibly driven by deeper dynamic pro- erosion patterns across the plateau. Together, kimberlite AHe data from cesses, may have further enhanced the eastward erosional pattern. Uplift the lower Orange River region and across the southern African Plateau associated with strong thermochemical alteration in the lower Orange imply that geomorphic processes, especially the large-scale drainage River region likely promoted rapid mid-Cretaceous erosion here, could network geometry, temper the denudational response to mantle processes. have helped drive mid- to Late Cretaceous exhumation in the west along Interplay between mantle and surface processes has a strong control on faults (Wildman et al., 2017), and may even have contributed to pushing erosion patterns and rates in continental interiors.

Figure 6. Cooling of weighted mean path through 70 °C for Meso- Karoo study area zoic kimberlites and igneous rocks (this study; Stanley et al., 2013, (Stanley et This study al. 2013) Kaapvaal study area (Stanley et al. 2015) 2015) versus distance across the southern African Plateau along the east-to-west A–A′ transect line in Figure 1A. Vertical dashed 120 line indicates the edge of the Kaapvaal craton. Only samples with AHe dates younger than eruption are plotted, because pipes Lithospheric that yield AHe dates coincident with pipe eruption do not record

(Ma) modification 90 the erosion event owing to pipe emplacement after unroofing mild occurred. Samples from west to east: Stolzenfels, Schuitdrift, Riet-

Tim e strong fontein, SA13-31, Markt, Uintjiesberg, Melton Wold, Makganyene, Newlands, New Elands, Star, Monastery. Full HeFTy output can be 60 found in the Data Repository (see footnote 1). Pink shading indi- cates whether lithospheric modification was strong (pervasive Off-craton On-craton heating and metasomatism) or mild (heating and metasomatism more limited), with the vertical and horizontal dimensions of the 0 250 500 750 1000 W E pink boxes denoting the constraints on the timing and lateral Distance along A-A' (km) extent of the event. Lithospheric modification information from Bell et al. (2003), Kobussen et al. (2008, 2009), Janney et al. (2010).

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ACKNOWLEDGMENTS Ehlers, T.A., and Farley, K.A., 2003, Apatite (U-Th)/He thermochronometry: Methods and ap- This work was supported by National Science Foundation (NSF) grants EAR-0951518 to RMF plications to problems in tectonic and surface processes: and Planetary Science and NSF GRFP grant DGE-1144083 to JRS. NSF grants EAR-1126991 and EAR-1559306 to RMF Letters, v. 206, p. 1–14, https://​doi​.org​/10​.1016​/S0012​-821X​(02)01069​-5. supported the instrumentation in the CU TRaIL with which the (U-Th)/He data were acquired. Emberley, J.M., and Schneider, D.A., 2017, Tracking low-temperature tectonism of the St. We are grateful to Maria Shipapo for assistance during fieldwork, David Bell for information Lawrence Platform and Humber Zone, southern Quebec Appalachians, through apa- on kimberlite localities, Maarten de Wit and Jay Barton for many insightful discussions, and tite and zircon (U-Th)/He thermochronology: Canadian Journal of Earth Sciences, v. 54, numerous helpful land owners in and Namibia for allowing us to sample on their p. 827–849, https://​doi​.org​/10​.1139​/cjes​-2016​-0199. land. We thank Phil Janney and an anonymous reviewer for helpful comments that improved Farley, K.A., 2000, Helium diffusion from apatite: General behavior as illustrated by Durango the manuscript and Damien Nance for effective editorial handling. Publication of this article fluorapatite: Journal of Geophysical Research, Solid Earth, v. 105, p. 2903–2914, https://​ was funded by the University of Idaho Open Access Publishing Fund. doi​.org​/10​.1029​/1999JB900348. Farley, K.A., Wolf, R.A., and Silver, L.T., 1996, The effects of long alpha-stopping distances on (U-Th)/He ages: Geochimica et Cosmochimica Acta, v. 60, p. 4223–4229, https://doi​ ​.org​ REFERENCES CITED /10​.1016​/S0016​-7037​(96)00193​-7. Abbott, D.H., Drury, R., and Mooney, W.D., 1997, as lithological icebergs: The Flowers, R.M., 2009, Exploiting radiation damage control on apatite (U-Th)/He dates in cra- importance of buoyant lithospheric roots: Earth and Planetary Science Letters, v. 149, tonic regions: Earth and Planetary Science Letters, v. 277, p. 148–155, https://doi​ ​.org​/10​ p. 15–27, https://​doi​.org​/10​.1016​/S0012​-821X​(97)00065​-4. .1016​/j​.epsl​.2008​.10​.005. Allsopp, H.L., Bristow, J.W., Smith, C.B., Brown, R.W., Gleadow, A.J.W., Kramers, J.D., and Flowers, R.M., and Kelley, S.A., 2011, Interpreting data dispersion and “inverted” dates in apa- Garvie, O.G., 1989, A summary of radiometric dating methods applicable to kimberlites tite (U-Th)/He and fission-track datasets: An example from the US midcontinent: Geochi- and related rocks: Proceedings of the Fourth International Kimberlite Conference, Perth, mica et Cosmochimica Acta, v. 75, p. 5169–5186, https://​doi​.org​/10​.1016​/j​.gca​.2011​.06​.016. , Geological Society of Australia Carlton, v. 1, p. 343–357. Flowers, R.M., and Schoene, B., 2010, (U-Th)/He thermochronometry constraints on unroofing Appleyard, C.M., Bell, D.R., and Roex, A.P., 2007, Petrology and geochemistry of eclogite xe- of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau: noliths from the Rietfontein kimberlite, Northern Cape, South Africa: Contributions to Geology, v. 38, p. 827–830, https://​doi​.org​/10​.1130​/G30980​.1. Mineralogy and Petrology, v. 154, p. 309–333, https://​doi​.org​/10​.1007​/s00410​-007​-0195​-7. Flowers, R.M., Shuster, D.L., Wernicke, B.P., and Farley, K.A., 2007, Radiation damage control Ault, A.K., Flowers, R.M., and Bowring, S.A., 2013, Phanerozoic surface history of the Slave on apatite (U-Th)/He dates from the Grand Canyon region, Colorado Plateau: Geology, craton: Tectonics, v. 32, p. 1066–1083, https://​doi​.org​/10​.1002​/tect​.20069. v. 35, p. 450–447, https://​doi​.org​/10​.1130​/G23471A​.1. Baby, G., Guillocheau, F., Boulogne, C., Robin, C., and Dall’Asta, M., 2018a, Uplift history of a Flowers, R.M., Ketcham, R.A., Shuster, D.L., and Farley, K.A., 2009, Apatite (U-Th)/He ther- transform continental margin revealed by the stratigraphic record: The case of the Agul- mochronometry using a radiation damage accumulation and annealing model: Geochi- has transform margin along the Southern African Plateau: Tectonophysics, v. 731–732, mica et Cosmochimica Acta, v. 73, p. 2347–2365, https://doi​ .org​ /10​ .1016​ /j​ .gca​ .2009​ .01​ .015.​ p. 104–130, https://​doi​.org​/10​.1016​/j​.tecto​.2018​.03​.014. Flowers, R.M., Farley, K.A, and Ketcham, R.A., 2015, A reporting protocol for thermochrono- Baby, G., Guillocheau, F., Morin, J., Ressouche, J., Robin, C., Broucke, O., and Dall’Asta, M., logic modeling illustrated with data from the Grand Canyon: Earth and Planetary Science 2018b, Post- stratigraphic evolution of the Atlantic margin of Namibia and South Af- Letters, v. 432, p. 425–435, https://​doi​.org​/10​.1016​/j​.epsl​.2015​.09​.053. rica: Implications for the vertical movements of the margin and the uplift history of the Franz, L., Brey, G.P., and Okrusch, M., 1996, Steady state geotherm, thermal disturbances, South African Plateau: Marine and Petroleum Geology, v. 97, p. 169–191, https://​doi​.org​ and tectonic development of the lower lithosphere underneath the Gibeon Kimberlite /10​.1016​/j​.marpetgeo​.2018​.06​.030. Province, Namibia: Contributions to Mineralogy and Petrology, v. 126, p. 181–198, https://​ Bell, D.R., Schmitz, M.D., and Janney, P.E., 2003, Mesozoic thermal evolution of the southern Afri- doi​.org​/10​.1007​/s004100050243. can mantle lithosphere: Lithos, v. 71, p. 273–287, https://doi​ .org​ /10​ .1016​ /S0024​ -4937​ (03)00117​ -8.​ Gallagher, K., and Brown, R.W., 1999, Denudation and uplift at passive margins: The record Berlin, M.M., and Anderson, R.S., 2007, Modeling of knickpoint retreat on the Roan Plateau, on the Atlantic Margin of southern Africa: Philosophical Transactions of the Royal Soci- western Colorado: Journal of Geophysical Research, Solid Earth, v. 112, https://​doi​.org​ ety A: Mathematical, Physical and Engineering Sciences, v. 357, p. 835–859, https://​doi​ /10​.1029​/2006JF000553. .org​/10​.1098​/rsta​.1999​.0354. Bluck, B.J., Ward, J.D., and De Wit, M.C.J., 2005, Diamond mega-placers: Southern Africa and Griffin, W., 2003, The evolution of lithospheric mantle beneath the Kalahari Craton and its the Kaapvaal craton in a global context, in McDonald, I., Boyce, A.J., Butler, I.B., Herrington, margins: Lithos, v. 71, p. 215–241, https://​doi​.org​/10​.1016​/j​.lithos​.2003​.07​.006. R.J., and Polya, D.A., eds., Mineral Deposits and Earth Evolution: Geological Society, Lon- Griffin, W.L., Batumike, J.M., Greau, Y., Pearson, N.J., Shee, S.R., and O’Reilly, S.Y., 2014, Em- don, Special Publication 248, p. 213–245, https://​doi​.org​/10​.1144​/GSL​.SP​.2005​.248​.01​.12. placement ages and sources of kimberlites and related rocks in southern Africa: U-Pb Boyd, F.R., Pearson, D.G., Hoal, K.O., Hoal, B.G., Nixon, P.H., Kingston, M.J., and Mertzman, ages and Sr-Nd isotopes of groundmass perovskite: Contributions to Mineralogy and S.A., 2004, Garnet lherzolites from Louwrensia, Namibia: Bulk composition and P/T rela- Petrology, v. 168, https://​doi​.org​/10​.1007​/s00410​-014​-1032​-4. tions: Lithos, v. 77, p. 573–592, https://​doi​.org​/10​.1016​/j​.lithos​.2004​.03​.010. Guillocheau, F., Rouby, D., Robin, C., Helm, C., Rolland, N., Le Carlier de Veslud, C., and Braun, Braun, J., 2010, The many surface expressions of mantle dynamics: Nature Geoscience, v. 3, J., 2012, Quantification and causes of the terrigeneous sediment budget at the scale of p. 825–833, https://​doi​.org​/10​.1038​/ngeo1020. a continental margin: A new method applied to the Namibia-South Africa margin: Basin Braun, J., Guillocheau, F., Robin, C., Baby, G., and Jelsma, H.A., 2014, Rapid erosion of the Research, v. 24, p. 3–30, https://​doi​.org​/10​.1111​/j​.1365​-2117​.2011​.00511​.x. Southern African Plateau as it climbs over a mantle superswell: Journal of Geophysical Guillou-Frottier, L., Burov, E., Nehlig, P., and Wyns, R., 2007, Deciphering plume-lithosphere Research, Solid Earth, v. 119, p. 6093–6112, https://​doi​.org​/10​.1002​/2014JB010998. interactions beneath from topographic signatures: Global and Planetary Change, Brown, L.F., Benson, J.M., Brink, G.J., Doherty, S., Jollands, A., Jungslager, E.H.A., Keenan, v. 58, p. 119–140, https://​doi​.org​/10​.1016​/j​.gloplacha​.2006​.10​.003. J.H.G., Muntingh, A., and van Wyk, N.J.S., 1995, Sequence Stratigraphy in Offshore South Howard, A.D., and Kerby, G., 1983, Channel changes in badlands: Geological Society of African Divergent Basins: An Atlas on Exploration for Cretaceous Lowstand Traps by America Bulletin, v. 94, p. 739–752, https://doi​ .org​ /10​ .1130​ /0016​ -7606​ (1983)94​ <739:​ CCIB>2​ ​ Soekor (Pty) Ltd.: AAPG Studies in Geology, v. 41, 184 p., https://​doi​.org/10​ ​.1306​/St41600. .0​.CO;2. Brown, R.W., Summerfield, M.A., and Gleadow, A.J.W., 2002, Denudational history along Janney, P.E., Shirey, S.B., Carlson, R.W., Pearson, D.G., Bell, D.R., Le Roex, A.P., Ishikawa, A., a transect across the Escarpment of southern Africa derived from apatite Nixon, P.H., and Boyd, F.R., 2010, Age, composition and thermal characteristics of South fission track thermochronology: Journal of Geophysical Research, Solid Earth, v. 107, African off-craton mantle lithosphere: Evidence for a multi-stage history: Journal of Pe- https://​doi​.org​/10​.1029​/2001JB000745. trology, v. 51, p. 1849–1890, https://​doi​.org​/10​.1093​/petrology​/egq041. Brown, R.W., Beucher, R., Roper, S., Persano, C., Stuart, F., and Fitzgerald, P., 2013, Natural Jelsma, H., Barnett, W., Richards, S., and Lister, G., 2009, Tectonic setting of kimberlites: Lithos, age dispersion arising from the analysis of broken crystals. Part I: Theoretical basis and v. 112, p. 155–165, https://​doi​.org​/10​.1016​/j​.lithos​.2009​.06​.030. implications for the apatite (U-Th)/He thermochronometer: Geochimica et Cosmochimica Jelsma, H.A., de Wit, M.J., Thiart, C., Dirks, P.H.G.M., Viola, G., Basson, I.J., and Anckar, E., Acta, v. 122, p. 478–497, https://​doi​.org​/10​.1016​/j​.gca​.2013​.05​.041. 2004, Preferential distribution along transcontinental corridors of kimberlites and related Burke, K., and Gunnell, Y., 2008, The African Erosion Surface: A Continental-Scale Synthe- rocks of Southern Africa: South African Journal of Geology, v. 107, p. 301–324, https://​ sis of Geomorphology, Tectonics, and Environmental Change over the Past 180 Million doi​.org​/10​.2113​/107​.1​-2​.301. Years: Geological Society of America Memoir 201, 66 p., https://​doi.org​ ​/10.1130​ /2008​ .1201.​ Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., and Le Catuneanu, O., Wopfner, H., Eriksson, P.G., Cairncross, B., Rubidge, B.S., Smith, R.M.H., and Gall, B., 2005, Karoo large igneous province: Brevity, origin, and relation to mass ex- Hancox, P.J., 2005, The Karoo basins of south-central Africa: Journal of African Earth Sci- tinction questioned by new 40Ar/39Ar age data: Geology, v. 33, p. 745–748, https://​doi​.org​ ences, v. 43, p. 211–253, https://​doi​.org​/10​.1016​/j​.jafrearsci​.2005​.07​.007. /10​.1130​/G21632​.1. Collier, J.S., McDermott, C., Warner, G., Gyori, N., Schnabel, M., McDermott, K., and Horn, Jourdan, F., Féraud, G., Bertrand, H., Watkeys, M.K., and Renne, P.R., 2008, The 40Ar/39Ar ages B.W., 2017, New constraints on the age and style of continental breakup in the South At- of the sill complex of the Karoo large igneous province: Implications for the Pliensba- lantic from magnetic anomaly data: Earth and Planetary Science Letters, v. 477, p. 27–40, chian‐Toarcian climate change: Geochemistry, Geophysics, Geosystems, v. 9, https://doi​ ​ https://​doi​.org​/10​.1016​/j​.epsl​.2017​.08​.007. .org​/10​.1029​/2008GC001994. Davis, G.L., 1977, The ages and uranium contents of zircons from kimberlites and associ- Ketcham, R.A., 2005, Forward and inverse modeling of low-temperature thermochronom- ated rocks: Geophysical Laboratory, Carnegie Institution of Washington Yearbook, v. 76, etry data: Reviews in Mineralogy and Geochemistry, v. 58, p. 275–314, https://​doi​.org​ p. 631–654. /10.2138​/rmg​.2005​.58​.11. de Wit, M.C.J., 1999, Post-Gondwana drainage and the development of diamond placers in Ketcham, R.A., Gautheron, C., and Tassan-Got, L., 2011, Accounting for long alpha-particle western South Africa: Economic Geology and the Bulletin of the Society of Economic stopping distances in (U-Th-Sm)/He geochronology: Refinement of the baseline case: Geo- Geologists, v. 94, p. 721–740, https://​doi​.org​/10​.2113​/gsecongeo​.94​.5​.721. chimica et Cosmochimica Acta, v. 75, p. 7779–7791, https://doi​ .org​ /10​ .1016​ /j​ .gca​ .2011​ .10​ .011.​ Dingle, R.V., and Hendey, Q.B., 1984, Late Mesozoic and Tertiary sediment supply to the east- Kobussen, A.F., Griffin, W.L., O’Reilly, S.Y., and Shee, S.R., 2008, Ghosts of lithospheres past: ern Cape Basin (SE Atlantic) and paleo-drainage systems in Southwestern Africa: Marine Imaging an evolving lithospheric mantle in southern Africa: Geology, v. 36, p. 515, https://​ Geology, v. 56, p. 13–26, https://​doi​.org​/10​.1016​/0025​-3227​(84)90003​-3. doi​.org​/10​.1130​/G24868A​.1.

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Kobussen, A.F., Griffin, W.L., and O’Reilly, S.Y., 2009, Cretaceous thermo-chemical modifica- Rosenbloom, N.A., and Anderson, R.S., 1994, Hillslope and channel evolution in a marine tion of the Kaapvaal cratonic lithosphere, South Africa: Lithos, v. 112, p. 886–895, https://​ terraced landscape, Santa Cruz, California: Journal of Geophysical Research, Solid Earth, doi​.org​/10​.1016​/j​.lithos​.2009​.06​.031. v. 99, p. 14013–14029, https://​doi​.org​/10​.1029​/94JB00048. Konzett, J., Armstrong, R.A., and Günther, D., 2000, Modal metasomatism in the Kaapvaal Rouby, D., Bonnet, S., Guillocheau, F., Gallagher, K., Robin, C., Biancotto, F., Dauteuil, O., and craton lithosphere: Constraints on timing and genesis from U-Pb zircon dating of meta- Braun, J., 2009, Sediment supply to the Orange sedimentary system over the last 150 My: somatized peridotites and MARID-type xenoliths: Contributions to Mineralogy and Pe- An evaluation from sedimentation/denudation balance: Marine and Petroleum Geology, trology, v. 139, p. 704–719, https://​doi​.org​/10​.1007​/s004100000160. v. 26, p. 782–794, https://​doi​.org​/10​.1016​/j​.marpetgeo​.2008​.08​.004. Kounov, A., Viola, G., de Wit, M., and Andreoli, M.A.G., 2009, Denudation along the Atlantic Rudnick, R.L., and Nyblade, A.A., 1999, The thickness and heat production of Archean litho- passive margin: New insights from apatite fission-track analysis on the western coast of sphere: Constraints from xenolith thermobarometry and surface heat flow,in Fei, Y., South Africa, in Lisker, F., Ventura, B., and Glasmacher, U.A., eds., Thermochronological Bertka, C.M., and Mysen, B.O., eds., Mantle Petrology: Field Observations and High Methods: From Palaeotemperature Constraints to Landscape Evolution Models: Geologi- Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd: The Geochemical Society, cal Society of London, Special Publication 324, p. 287–306, https://doi​ .org​ /10​ .1144​ /SP324​ .19.​ Special Publication 6, p. 3–12. Kounov, A., Viola, G., Dunkl, I., and Frimmel, H.E., 2013, Southern African perspectives on Schmitz, M.D., and Bowring, S.A., 2003, Constraints on the thermal evolution of continental the long-term morpho-tectonic evolution of cratonic interiors: Tectonophysics, v. 601, lithosphere from U-Pb accessory mineral thermochronometry of lower crustal xenoliths, p. 177–191, https://​doi​.org​/10​.1016​/j​.tecto​.2013​.05​.009. southern Africa: Contributions to Mineralogy and Petrology, v. 144, p. 592–618, https://​ Lithgow-Bertelloni, C., and Silver, P.G., 1998, Dynamic topography, plate driving forces and doi​.org​/10​.1007​/s00410​-002​-0419​-9. the African superswell: Nature, v. 395, p. 269–272, https://​doi​.org​/10​.1038​/26212. Shiimi, E.T., and Janney, P.E., 2017, Contrasting thermal structure, melt depletion and meta- Marsh, J.S., Hooper, P.R., Rehacek, J., Duncan, R.A., and Duncan, A.R., 1997, Stratigraphy and somatism of mantle lithosphere beneath two Proterozoic terranes west of the Kaapvaal age of Karoo basalts of Lesotho and implications for correlations within the Karoo igneous craton, southern Africa: International Kimberlite Conference Extended Abstracts, v. 11, province, in Mahoney, J.J., and Coffin, M.F., eds., Large Igneous Provinces: Continental, p. 1–3, https://​doi​.org​/10​.29173​/ikc3898. Oceanic, and Planetary Flood Volcanism: American Geophysical Union, Geophysical Shuster, D.L., Flowers, R.M., and Farley, K.A., 2006, The influence of natural radiation dam- Monograph Series, v. 100, p. 247–272, https://​doi​.org​/10​.1029​/GM100p0247. age on helium diffusion kinetics in apatite: Earth and Planetary Science Letters, v. 249, Molin, P., Fubelli, G., Nocentini, M., Sperini, S., Ignat, P., Grecu, F., and Dramis, F., 2012, Inter- p. 148–161, https://​doi​.org​/10​.1016​/j​.epsl​.2006​.07​.028. action of mantle dynamics, crustal tectonics, and surface processes in the topography of Spiegel, C., Kohn, B., Belton, D., Berner, Z., and Gleadow, A., 2009, Apatite (U-Th-Sm)/He the Romanian Carpathians: A geomorphological approach: Global and Planetary Change, thermochronology of rapidly cooled samples: The effect of He implantation: Earth and v. 90–91, p. 58–72, https://​doi​.org​/10​.1016​/j​.gloplacha​.2011​.05​.005. Planetary Science Letters, v. 285, p. 105–114, https://​doi​.org​/10​.1016​/j​.epsl​.2009​.05​.045. Moore, A.E., 1999, A reappraisal of epeirogenic flexure axes in southern Africa: South African Stanley, J.R., and Flowers, R.M., 2016, Dating kimberlite emplacement with zircon and Journal of Geology, v. 102, p. 363–376. perovskite (U-Th)/He geochronology: Geochemistry, Geophysics, Geosystems, v. 17, Moore, A., Blenkinsop, T., and Cotterill, F. (Woody), 2008, Controls on post-Gondwana alka- https://​doi​.org​/10​.1002​/2016GC006519. line volcanism in Southern Africa: Earth and Planetary Science Letters, v. 268, p. 151–164, Stanley, J.R., Flowers, R.M., and Bell, D.R., 2013, Kimberlite (U-Th)/He dating links surface https://​doi​.org​/10​.1016​/j​.epsl​.2008​.01​.007. erosion with lithospheric heating, thinning, and metasomatism in the southern African Moore, A., Blenkinsop, T., and Cotterill, F. (Woody), 2009, Southern African topography and Plateau: Geology, v. 41, p. 1243–1246, https://​doi​.org​/10​.1130​/G34797​.1. erosion history: Plumes or ?: Terra Nova, v. 21, p. 310–315, https://doi​ ​.org​ Stanley, J.R., Flowers, R.M., and Bell, D.R., 2015, Erosion patterns and mantle sources of /10​.1111​/j​.1365​-3121​.2009​.00887​.x. topographic change across the southern African Plateau derived from the shallow and Murray, K.E., Orme, D.A., and Reiners, P.W., 2014, Effects of U-Th-rich grain boundary phases deep records of kimberlites: Geochemistry, Geophysics, Geosystems, v. 16, p. 3235–3256, on apatite helium ages: Chemical Geology, v. 390, p. 135–151, https://doi​ ​.org​/10​.1016​/j​ https://​doi​.org​/10​.1002​/2015GC005969. .chemgeo​.2014​.09​.023. Tinker, J., de Wit, M., and Brown, R.W., 2008a, Linking source and sink: Evaluating the balance Nakashole, A.N., Hodgson, D.M., Chapman, R.J., Morgan, D.J., and Jacob, R.J., 2018, Long- between onshore erosion and offshore sediment accumulation since Gondwana break-up, term controls on continental-scale bedrock river terrace deposition from integrated clast South Africa: Tectonophysics, v. 455, p. 94–103, https://doi​ .org​ /10​ .1016​ /j​ .tecto​ .2007​ .11​ .040.​ and heavy mineral assemblage analysis: An example from the lower Orange River, Na- Tinker, J., de Wit, M., and Brown, R.W., 2008b, Mesozoic exhumation of the southern Cape, mibia: Sedimentary Geology, v. 364, p. 103–120, https://​doi​.org​/10​.1016​/j​.sedgeo​.2017​ South Africa, quantified using apatite fission track thermochronology: Tectonophysics, .12​.010. v. 455, p. 77–93, https://​doi​.org​/10​.1016​/j​.tecto​.2007​.10​.009. Nyblade, A.A., and Robinson, S.W., 1994, The African superswell: Geophysical Research Let- Warrick, J.A., Milliman, J.D., Walling, D.E., Wasson, R.J., Syvitski, J.P.M., and Aalto, R.E., 2014, ters, v. 21, p. 765–768, https://​doi​.org​/10​.1029​/94GL00631. Comment: Earth is (mostly) flat: Apportionment of the flux of continental sediment over Nyblade, A.A., and Sleep, N.H., 2003, Long lasting epeirogenic uplift from mantle plumes millennial time scales: Geology, v. 42, no. 1, https://​doi​.org​/10​.1130​/G34846C​.1. and the origin of the Southern African Plateau: Geochemistry, Geophysics, Geosystems, Whipple, K.X., and Tucker, G.E., 1999, Dynamics of the stream‐power river incision model: v. 4, https://​doi​.org​/10​.1029​/2003GC000573. Implications for height limits of mountain ranges, landscape response timescales, and Partridge, T.C., and Maud, R.R., 1987, Geomorphic evolution of southern Africa since the Me- research needs: Journal of Geophysical Research, Solid Earth, v. 104, p. 17661–17674, sozoic: South African Journal of Geology, v. 90, p. 179–208. https://​doi​.org​/10​.1029​/1999JB900120. Pazzaglia, F.J., and Gardner, W., 1994, Late Cenozoic flexural deformation of the middle U.S. Wildman, M., Brown, R.W., Watkins, R., Carter, A., Gleadow, A., and Summerfield, M., 2015, Atlantic passive margin: Journal of Geophysical Research, Solid Earth, v. 99, p. 12143– Post break-up tectonic inversion across the southwestern cape of South Africa: New in- 12157, https://​doi​.org​/10​.1029​/93JB03130. sights from apatite and zircon fission track thermochronometry: Tectonophysics, v. 654, Phillips, D., and Harris, J.W., 2009, Diamond provenance studies from 40Ar/39Ar dating of p. 30–55, https://​doi​.org​/10​.1016​/j​.tecto​.2015​.04​.012. clinopyroxene inclusions: An example from the west coast of Namibia: Lithos, v. 112, Wildman, M., Brown, R.W., Beucher, R., Persano, C., Stuart, F., Gallagher, K., Schwanethal, p. 793–805, https://​doi​.org​/10​.1016​/j​.lithos​.2009​.05​.003. J., and Cater, A., 2016, The chronology and tectonic style of landscape evolution along Phillips, D., Harris, J.W., de Wit, M.C.J., and Matchan, E.L., 2018, Provenance history of detrital the elevated Atlantic continental margin of South Africa resolved by joint apatite fis- diamond deposits, West Coast of Namaqualand, South Africa: Mineralogy and Petrology, sion track and (U-Th-Sm)/He thermochronology: Tectonics, v. 35, https://doi​ ​.org​/10​.1002​ v. 112, p. 259–273, https://​doi​.org​/10​.1007​/s00710​-018​-0568​-9. /2015TC004042 Pysklywec, R.N., and Mitrovica, J.X., 1998, Mantle flow mechanisms for the large-scale sub- Wildman, M., Brown, R.W., Persano, C., Beucher, R., Stuart, F.M., Mackintosh, V., Gallagher, K., sidence of continental interiors: Geology, v. 26, p. 687–690, https://​doi​.org​/10​.1130​/0091​ Schwanethal, J., and Carter, A., 2017, Contrasting Mesozoic evolution across the boundary -7613​(1998)026​<0687:​MFMFTL>2​.3​.CO;2. between on and off craton regions of the South African plateau inferred from apatite fis- Raab, M.J., Brown, R.W., Gallagher, K., Carter, A., and Weber, K., 2002, Late Cretaceous re- sion track and (U-Th-Sm)/He thermochronology: Journal of Geophysical Research, Solid activation of major crustal shear zones in northern Namibia: Constraints from apatite Earth, v. 122, p. 1517–1547, https://​doi​.org​/10​.1002​/2016JB013478. fission track analysis: Tectonophysics, v. 349, p. 75–92, https://doi​ ​.org​/10​.1016​/S0040​ Willenbring, J.K., Codilean, A.T., and McElroy, B., 2013, Earth is (mostly) flat: Apportionment -1951​(02)00047​-1. of the flux of continental sediment over millennial time scales: Geology, v. 41, p. 343–346, Reiners, P.W., and Brandon, M.T., 2006, Using thermochronology to understand orogenic ero- https://​doi​.org​/10​.1130​/G33918​.1. sion: Annual Review of Earth and Planetary Sciences, v. 34, p. 419–466, https://doi​ ​.org​/10​ Woodhead, J.D., Hergt, J.M., Giuliani, D., Phillips, D., and Maas, R., 2017, Tracking continental- .1146​/annurev​.earth​.34​.031405​.125202. scale modification of the Earth’s mantle using zircon megacrysts: Geochemical Perspec- Reiners, P.W., and Farley, K.A., 2001, Influence of crystal size on apatite (U-Th)/He thermochro- tives Letters, v. 4, https://​doi​.org​/10​.7185​/geochemlet​.1727. nology: An example from the Bighorn Mountains, Wyoming: Earth and Planetary Science Wu, F.-Y., Yang, Y.-H., Mitchell, R.H., Li, Q.-L., Yang, J.-H., and Zhang, Y.-B., 2010, In situ U-Pb Letters, v. 188, p. 413–420, https://​doi​.org​/10​.1016​/S0012​-821X​(01)00341​-7. age determination and Nd isotopic analysis of perovskites from kimberlites in southern Renne, P.R., Glen, J.M., Milner, S.C., and Duncan, A.R., 1996, Age of Etendeka flood volcanism Africa and Somerset Island, Canada: Lithos, v. 115, p. 205–222, https://​doi​.org​/10​.1016​/j​ and associated intrusions in southwestern Africa: Geology, v. 24, p. 659–662, https://doi​ ​ .lithos​.2009​.12​.010. .org​/10​.1130​/0091​-7613​(1996)024​<0659:​AOEFVA>2​.3​.CO;2. Richardson, J.C., Hodgson, D.M., Paton, D., Craven, B., Rawcliffe, A., and Lang, A., 2017, Where MANUSCRIPT RECEIVED 3 JULY 2019 is my sink? Reconstruction of landscape development in southwestern Africa since the REVISED MANUSCRIPT RECEIVED 2 DECEMBER 2019 Late Jurassic: Gondwana Research, v. 45, p. 43–64, https://doi​ .org​ /10​ .1016​ /j​ .gr​ .2017​ .01​ .004.​ MANUSCRIPT ACCEPTED 7 JANUARY 2020

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