Bushmanland Plateau Namaqualand Highlands Bushmanland Plateau Namaqualand Highlands (margin) − 8 − 8 − 8 − 7 − 7 − 7 − 7 − 7 − 7 − 6 − 6 − 6 − 6 − 6 − 6 − 6 −5 −5 − 5 − 5 − 5 − 5 − 5 − 5 − 5 −4 −4 − 4 − 4 − 4 − 4 − 4 − 4 − 4 − 4 −3 − 3 − 3 − 3 − 3 − 3 −3 − 3 − 3 − 3 − 3 − 2 − 2 −2 −2 − 2 − 2 − 2 − 2 − 2 − 1 − 1 −1 − 1 − 1 − 1 − 1 − 1 − 1 − 1 − 1 − 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 16˚ 20˚ 24˚ − 36˚ − 32˚ − 28˚ 0 200 400 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Ma 60 80 100 120 140 160 Orange Basin A A' A A A B A A A A' A B A B' 0 6 12 18 24 Km Intracontinental deformation of the interior plateau and Atlantic continental margin of South Africa: Insights from combined AFT and AHe analysis Mark Wildman 1 , Roderick Brown 1 , Cristina Persano 1 , Fin Stuart 2 and Romain Beucher 1, 3 m.wildman.1@research.gla.ac.uk 1: School of Geographical and Earth Sciences, University of Glasgow, UK 2: Scottish Universities Environmental Research Centre, East Kilbride, UK 3: Department of Earth Science, University of Bergen,Norway Distance from margin Original Age Age of rifting Fission Track Age 0 A B C Accummulated Volume (x10 14 m 3 ) 50 75 100 125 150 0 25 0 6 12 Kimberlite Frequency (5Ma Bins) 20 0 10 Cenozoic Cretaceous J Maas Camp Sant Con Tur Cen Alb Apt Bar Haut Val Berr Tith Opening of South Atlantic Major offshore unconformity and margin uplift (South Africa) Compression (North and Central Africa) Rifting (North, Central and South Africa) Compression (North Africa and Damara Belt) Okawango Delta Depression Neotectonics in Namaqualand Major Cooling episodes identified Tectonic events Distal Margin Thermal Relaxation Sedimentation (loading) Erosion (Unloading) Flexural Isostasy Mantle Convection Underplating Dynamic Uplift Buckling In-plane stress Upwelling Plume Offshore Basin Coastal Plain Escarpment Interior Plateau Craton/Terrane boundary Fault bounded Highlands Uplift of fault blocks Crustal Thinning Removal of mantle lithosphere Fig 5: Major episodes of proposed denudation driven cooling (this study), record of regional tectonic events [10], offshore sediment accumulation in the orange river basin [11] and kimberlite intrusive activity across Southern Africa [12] since the onset of South Atlantic rifting (c. 150 Ma). Fig 4: Left hand side: Thermal history models for one sample highlighting the effects of joint inversion of AFT and AHe data. Single solid lines - expected thermal history; background colours - uncertainties on the expected model. Right hand side: Plots of predicted data vs. observed data for AFT and AHe data. The measured AHe age is represented by a point symbol for the mean age and a shaded circle for the St. Dev. of the mean. ? A C A C' A C A C' 0 25 50 75 100Km 83.7 ± 2.7 13.5 ± 0.2 79.7 ± 6.3 14.4 ± 0.2 156 ± 7 13.3 ± 0.2 98.7 ± 3 14 ± 0.1 96 ± 2.6 13.6 ± 0.1 88.7 ± 16.8 (13) 126.8 ± 14.2 106.6 ± 3.8 13.6 ± 0.2 86 ± 15.8 (13) 121 ± 17.5 78 ± 17.6 (14) 102.8 ± 24.5 110.4 ± 6 13.5 ± 0.2 102.8 ± 50 (8) 133.3 ± 65.9 84 ± 4.1 13.6 ± 0.1 104.7 ± 24.5 (9) 138.0 ± 29.6 73.2 ± 3.3 14.2 ± 0.2 97.8 ± 25.3 (14) 120.7 ± 28.2 75.1 ± 8.3 10.9 ± 0.2 85.4 ± 3.4 13.4 ± 0.1 110 ± 7 13.5 ± 0.3 85 ± 4.8 13.4 ± 0.3 91.4 ± 6.3 13.9 ± 0.2 77.2 ± 15.3 (10) 101.1 ± 20.5 74.4 ± 3 13.8 ± 0.1 89.3 ± 19.5 (12) 112.3 ± 24.4 64.5 ± 4.1 13.5 ± 0.3 74.7 ± 69.3 (2) 93.3 ± 83.8 99 ± 5 13.8 ± 0.2 93.4 ± 5.5 13.3 ± 0.2 87.6 ± 33.5 (15) 104.7 ± 36.8 85.7 ± 4.4 14.3 ± 0.1 74.2 ± 17.3 (15) 94.4 ± 22.6 85.4 ± 3.8 13.8 ± 0.1 89 ± 10 14 ± 0.5 112.5 ± 5.9 14 ± 0.2 89.1 ± 43 (5) 128.6 ± 55 104 ± 9 14 ± 0.4 92 ± 19.4 (9) 120.6 ± 25.3 97.7 ± 6.5 13.4 ± 0.1 83.9 ± 6.7 13.4 ± 0.5 67.6 ± 19.6 (3) 88.1 ± 18.6 122.8 ± 7.5 12.9 ± 0.3 173.7 ± 95.9 (7) 221.7 ± 120.9 90.2 ± 2.4 12.5 ± 0.1 98.3 ± 15.3 (8) 129.4 ± 21.4 62.1 ± 40.5 (12) 80.3 ± 51.4 119 ± 10 13.7 ± 0.2 102.1 ± 8.7 13.3 ± 0.3 62.7 ± 23.9 (6) 84.7 ± 34.7 58.9 ± 5.9 13.3 ± 0.5 25.5 ± 8 (5) 35 ± 12.2 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 300 250 200 150 100 50 0 Time (Ma) 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 200 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 200 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 0 50 100 150 Temperature (C) 0 50 100 150 A B' 1000 800 600 400 200 Elevation (m) 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Distance (km) 0 0 1200 1000 800 600 400 200 0 Elevation (m) Distance (km) 0 2 4 6 8 10 12 14 16 18 40 80 20 100 120 140 160 180 200 220 240 260 280 300 60 Distance (km) 1100 900 700 1300 Elevation (m) 0 Fig 1: Top left: Predicted trend in AFT ages across a passive continental margin for parallel scarp retreat (A); downwearing (B); or pinned drainage divide (C) conceptual models. Main centre: Cartoon representation of the thermal, structural and geomorphic mechanisms influencing uplift at continental margins Fig 2: Right hand image : Interpolation map of AFT ages across southwestern Africa including point symbols for individual sample locations from this study (white circles) and previous AFT studies (grey circles; see References [1-5] for details). Bathymetry data from [6]. Star shows location of sample modelled in section 4. Left hand image : Satellite images (Landsat ETM+ RGB: 321) showing the location of transects from the Bushmanland Plateau (upper) and Namaqualand sector of the continental margin (lower). Major structural lineaments have been digitised and data points coloured according to the colour map used for the interpolation map. Fig 3: Topographic profiles across the Namaqualand sector of the southwest African margin and Bushmanland plateau. Outcrop samples within 10km (for margin transects) and 25 km (for plateau transect) either side of the line of section were projected at 90° onto the profile. Modelling was performed using QTQt [7] and run for a minimum of 2x10 5 iterations. The expected model represents an average of all models tested weighted for their posterior probability. Where both AFT and AHe data were available data were jointly inverted. AFT data was modelled using multi-kinetic annealing model of [8] while AHe data was modelled accounting for radiation damage following [9]. Cooling event 1: 150-130 Ma; 2: 110-90 Ma; 3: 80-60 Ma. [1] Raab, M. J., Brown, R. W., Gallagher, K., Carter, A., and Weber., 2002, Tectonophysics, v. 349, p. 75-92. [2] Tinker, J., de Wit, M., and Brown, R., 2008, Tectonophysics, v. 455, 1, p. 77-93. [3] Kounov, A., Viola, G., de Wit, M., and Andreoli, M. A. G., 2009, Geol. Soc. London, SP. v. 324, 1, p. 287-306. [5] de Wit, M. C. J., 1988, In Dardi, G. F., and Moon, B. P., (eds), Geomorphological Studies in Southern Africa, Balkema, Rotterdam, p. 57-69. [4] Kounov, A., Viola, G., Dunkl, I., & Frimmel, H. E., 2013, Tectonophysics, 601, 177-191. [6] Rouby, D., Bonnet, S., Guillocheau, F., Gallagher, K., Robin, C., Biancotto, F., Dauteuil, O., & Braun, J. (2009), Marine and Petroleum Geology, 26(6), 782-794. [7] Gallagher, K., 2012, Journal of Geophysical Research: Solid Earth, v. 117 (B2) [8] Ketcham, R. A., Carter, A., Donelick, R. A., Barbarand, J., and Hurford, A. J., 2007, American Mineralogist, 92(5-6), 799-810. [9] Gautheron, C., Tassan-Got, L., Barbarand, J., and Pagel, M., 2009, Chemical Geology, v. 266, 3, p. 157-170 [10] Viola, G., Kounov, A., Andreoli, M. A. G. and Mattila, J., 2012, Tectonophysics, v. 514-517, p. 93-114 [11] Guillocheau, F., Rouby, D., Robin, C., Helm, C., Rolland, N., Le Carlier de Veslud, C. and Braun, J., 2012, Basin Research, v. 24, 1, p. 3-30 [12] Jelsma, H., Barnett, W., Richards, S., and Lister, G., 2009, Lithos, v. 112, p. 155 - 165. Observations Interpretation Approach Joint inversion of AFT and apatite (U-Th)/He (AHe) data to constrain the thermal history of outcrop samples cooling through c.120 - 40°C. Both AFT and AHe ages are early-late Cretaceous. Cor. AHe ages are often > AFT ages and are dispersed. Thermal histories show 3 major cooling episodes. Distinct cooling histories separated by major faults. 150 - 130 Ma cooling: Erosion of initial rift topography. 110 - 90 Ma cooling: Regional tectonic inversion of South Africa. 80 - 60 Ma cooling: Local fault block reactivation. Modelling AFT OR AHe data independently: models are poorly constrained and contradictory. Modelling AFT AND AHe data together: models are better constrained models and more robust. Temperature (C) 200 150 100 50 0 Time (Ma) 350 300 250 200 150 100 50 0 Temperature (C) 200 150 100 50 0 A variety of different mechanisms are capable of generating uplift at continental margins. AFT data across the SW African margin does not readily agree with traditional models of scarp evolution (see Fig. 1). Do AFT data across continental margins always conform to predictions of scarp evolution models? Reactivation of major structures during the post-rift phase may disrupt expected spatial trends. Abrupt variations in AFT age appear to be structurally controlled. Resolving the timing and magnitude of major denudation across the margin and interior will help to resolve the driving mechanism behind post-rift margin development. Expected T-t History Fault 95% Credible Intervals Constraint box General prior range Partial Annealing Zone Major cooling events Sample Raw AHe Age ± 1std Ma (# Xtals) Corr AHe Age ± 1std Ma AFT Age ± 1se Ma MTL ± 1se μm 1 2 3 Proposed episodes of crustal cooling coincide with regional tectonic events. Resolving the exact temporal relationship between Mid-Late Cretaceous erosional cooling and enhanced offshore accumulation remains challenging. Offshore sedimentary record supports extensive erosion shortly after rifting; and in the Mid-Late Cretaceous but limited erosion/accumuation during the Cenozoic. Incorporating radiation damage - improves the data fit of single grain AHe ages. Mid-Late Cretaceous: Discrete tectonic inversion on major normal faults promoting differential erosion across the margin. Early Cretaceous: Widespread erosion driven by rift flank uplift and the establishment of a lower geomorphic base level. Combining both AFT and AHe data provide more robust thermal history models than either one on their own. AFT Only AHe Only (No RD) AHe Only (w/ RD) AFT & AHe (No RD) AFT & AHe (w/RD) Cenozoic: Limited (<1km) of regional denudation across the margin however spatial and temporal variations remain uncertain. Thermal histories support a more complex and regionally extensive post-rift denudational history than traditional models of 'passive' margin evolution. The denudation history across the SW African margin can be summarised as follows: I would like to thank Prof. A. Carter, Dr. J. Schwanthenal and Dr. M. Rittner for assistance in (U-Th)/He analysis at UCL. I would also like to thank Dr. Marco Andreoli and staff at NECSA for assistance during sample collection; Prof. Kerry Gallagher for his time and assistance in improving the quality of thermal history models; and Dr. O. Dauteuil, Prof. F. Guillocheau, Dr. T. Redfield, Dr. D. Rouby, Dr. G. Viola, for helpful discussions throughout the course of this project. References and Acknowledgements Continental margins: An uncomfortable geological setting 1 2 Southwest African continental margin 3 Data Conclusion 6 5 Discussion 4 Modelling of AFT and AHe data Obs. Length (um) Obs. Age (Ma) AFT Only Pred. Length (um) Pred. Age (Ma) 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 45 55 65 75 85 95 105 115 125 135 145 45 65 85 105 125 145 AHe Only (no RD) Obs. Age (Ma) Pred. Age (Ma) Pred. Length (um) 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 45 55 65 75 85 95 105 115 125 135 145 45 65 85 105 125 145 Obs. Length (um) AHe Only (w/ RD) Obs. Age (Ma) Pred. Length (um) Pred. Age (Ma) Obs. Length (um) 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 45 55 65 75 85 95 105 115 125 135 145 45 65 85 105 125 145 AFT & AHe (no RD) Pred. Age (Ma) Pred. Length (um) Obs. Age (Ma) Obs. Length (um) 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 45 55 65 75 85 95 105 115 125 135 145 45 55 65 75 85 95 105 115 125 135 145 AFT & AHe (with RD) Pred. Length (um) Pred. Age (Ma) Obs. Age (Ma) Obs. Length (um) 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 45 55 65 75 85 95 105 115 125 135 145 45 65 85 105 125 145 AFT Age MTL Dpar MTL StDev Resampled AHe Age Measured AHe Age