16 July 2004 S-wave velocity structure beneath the Kaapvaal Craton from surface-wave inversions compared with estimates from mantle xenoliths Angela Marie Larson M.S. defense 16 July 2004

Acknowledgments I would like to thank my advisor, Arthur Snoke, as well as my committee members, David James and Martin Chapman Thanks also go to the VT Geosciences Dept for their support as well as the Aubrey E. Orange Award I think an eternal debt is owed to Connie and Richard for always making things go a little more smoothly Thanks to all of my friends and associates in the dept A special thank you goes to Brian, Laura O, and my animal family for your support (I know I can be hard to deal with…)

16 July 2004 This presentation compares two methods for calculating the S-wave velocity structure of the Kaapvaal Craton Where and Who? Study area and previous work What? Conclusions How? Xenoliths, surface waves, inversions Why? The purpose behind the study Oh my!

16 July 2004 The purpose of this research is to: Calculate the S-wave velocity structure of the Kaapvaal Craton using surface waves This slide is for Maddy! Compare xenolith analysis and surface-wave analysis for the upper-mantle velocity structure Look for the existence of a low-velocity zone beneath the Archean craton using synthetics

16 July 2004 The region contains almost 1 Ga of Archean history and has remained largely unaltered since its formation Political boundariesGeologic provinces

16 July 2004 The Southern African Seismic Experiment collected seismic data in a dense (1º spacing) array There were 79 stations deployed for a minimum of 1 year

16 July 2004 The Kaapvaal Project researchers also performed geochemical/petrologic studies on local xenoliths Approximately 100 on-craton samples with ages ~90 Ma were analyzed geothermobarometrically

16 July 2004 Kimberlite pipes are for more than just diamonds; some can be analyzed for their records of mantle properties The procedure of analyzing mantle xenoliths for upper mantle seismic structure has not been utilized much…yet

16 July 2004 The Kaapvaal xenoliths provide a snapshot of seismic velocities and density for the craton at ~90 Ma The xenoliths studied were emplaced in the southern half of the craton

16 July 2004 The xenolith results indicate a slight decreasing trend in S-wave velocities for the applicable depth range The xenoliths reflect seismic velocities and density for a depth range of 50 to 180 km.

16 July 2004 The surface-wave data is selected by choosing well- recorded earthquakes The earthquake must be large enough to record in the array with a reasonable signal-to-noise ratio Favorable earthquakes had epicenters with great-circle paths passing through the NE and SW quadrants of the array These earthquakes must cross continent-ocean boundaries and mid-ocean ridges (if applicable) at near-normal incidence

16 July 2004 Five events were used for study There were four events near the coast of central Chile and one in Iran. Their great-circle paths to the center of the array are shown

16 July 2004 Frequency-time analysis helps with the selection of a range of useful periods and frequencies A range for this event would be km/s and about seconds

16 July 2004 Station pairs have their own selection criteria No more than 3º difference in backazimuths At least 200 km between stations Well-recorded at both stations

16 July 2004 NO Bushveld in the interstation path

16 July 2004 The Bushveld Complex appears to affect the velocity structures of the Kaapvaal Craton To work with “pure” craton, I excluded all station pairs with interstation paths within the Bushveld

16 July 2004 I found 16 paths from the five events using the selection criteria Several events use the same station pair and some events use collinear paths

16 July 2004 “The velocity with which an observable, individual wave or wave crest is propagated through a medium” The phase velocities are calculated for many (or all) of the periods over a range to make a dispersion curve

16 July 2004 A dispersion curve was calculated for each of the 16 different event/paths The solid line on the left is the dispersion curve calculated from the xenolith-based model

16 July 2004 The 16 sets were combined into a single composite curve and compared with the xenolith-based curve The curve fits the data very well, and this is a pre-inversion result

16 July 2004 The Neighbourhood Algorithm (NA) is used to find an ensemble of models that fit a chosen misfit criterion The NA is a non-linear direct search method. The search results are not limited to a single maximum

16 July 2004 The misfit criterion was essentially a standard deviation with the potential for a penalty to be assessed The penalty is designed to cause physically-unlikely models to be thrown out

16 July 2004 There were eight non-independent parameters arranged over the depth range for the model All solution models were constrained to match the reference model values for velocities greater then 400 km

16 July 2004 After 10,000 model iterations, 1,788 models fit within the misfit cutoff of This selected misfit allowed for enough potential models that their dispersion curves visually filled the space of the error bars

16 July 2004 Between the “best” model and an “average” model, I chose the average model The best model had the smallest overall misfit, but it is a physically less likely model

16 July 2004 To test for uniqueness, I compared the xenolith-based results with those from different reference models These global models are not plausible for continental shields. The dispersion curves produced from the unaltered models don’t fit

16 July 2004 After inversions performed by the NA, models are found that have reasonable fits to the original seismic data The gentle reverse “s”-curves are a result of the fewer parameters of NA and from using reference models that were so unlikely

16 July 2004 The results are that the xenolith-derived results match the seismic-derived S-wave velocity model The region of most interest is from 50 to 180 km depth

16 July 2004 I ran synthetic tests to check for evidence of a low- velocity zone (LVZ) The SYNTH model was made by perturbing the XNLTH model; a LVZ was put in starting at 180 km depth.

16 July 2004 An NA inversion run produced acceptable models that fit the SYNTH generated dispersion velocities There is no LVZ starting at 180 km depth --- the models are significantly different

16 July 2004 In summary The xenolith analysis and surface-wave analysis agree for the upper-mantle velocity structure This means that the geotherm of the Kaapvaal Craton is almost the same today as it was 90 Ma

16 July 2004 And There is no LVZ starting at 180 km depth or less beneath the Kaapvaal craton Therefore, either there is a significant tectonic root and the LVZ is deeper than 180 km or low velocities and low viscosities are not closely correlated

16 July 2004 There is no LVZ starting at 180 km depth or less beneath the Kaapvaal craton The xenolith analysis and surface-wave analysis agree for the upper-mantle velocity structure and Conclusions: Questions? Mischief managed!