312 310 314 312 313 315 317 318 319 320 311 316 310 311 9 24 44 6 2 38 27 61 42 28 84 5 40 50 19 51 26 32 79 76 30 10 16 50 40 25 19 45 30 1 46 A Q Q Q Q A Q A Q A Q Cp Cp Cp Um Cp Um Um Cp Um Bs Bs Um Cp Um Bs Br Br Cw Bs Cp Br Cp Cp Bs Bs Cp Cp Cp Um Um Cp Um Cp Cp Cp Cp Cp Bs Um Um Bs Br Bs Bs Bs Bs Bs Bs Cw Cp Cp Br Bs Pum Pum Peg Peg Peg Peg Peg Peg Peg Gum Wum Wum Peg Pum Wum G Cp G Pum Orange River Peg Cp Cp Pum Bs Cw Um Cp Peg Peg Br Bs Gum Bs Peg Pum 83 75 78 64 57 64 75 52 76 76 78 85 75 76 50 76 76 74 75 60 80 84 52 75 75 66 75 71 89 71 90 75 70 85 84 86 65 89 80 66 75 89 66 44 86 76 86 78 87 85 67 52 59 82 67 68 84 73 84 70 69 85 89 75 89 70 66 60 65 84 81 89 82 89 56 40 76 89 81 57 80 66 68 84 36 82 58 79 85 83 87 89 79 88 80 85 88 66 78 65 86 59 86 82 81 83 89 86 87 84 88 15 0 10 5 Meters Rock Type A Br Bs Biotite Schist Cp Coarse pink mylonite G Act/chl rich mylonite Quaternary cover Green ultramylonite White ultramylonite Ultramylonite The Orange River Pegmatite Peg Cw Coarse white mylonite Pink ultramylonite Gum Pum Q Um Wum 42 76 Strike and dip of foliation Lineation Syncline Anticline Overturned syncline Overturned anticline Topographic contour 312 Amphibolite Pseudotachylyte Namibia South Africa A t l a n t i c O c e a n Botswana Field Area Lüderitz Location O r a n g e Namibia R i v e r Z o n e 20 km P o f a d d e r S h e a r Cape Town South Africa Belly of the Beast: detailed mapping in the deformation core of a quartz-plastic transitional zone fault, implications for deep fault seismicity on major strike-slip faults Ben L. Melosh 1 , Christie D. Rowe 1,3 , Christopher Gerbi 2 , Louis Smit 3 1 McGill University, Montréal, Canada 2 University of Maine, Orono, USA 3 University of Cape Town, South Africa Results • The Pofadder Shear Zone is a mid-crustal, continental-scale, dextral shear zone, lo- cated in Namibia and South Africa. In respect to rock type, scale and kinematics this is an ancient analog to the modern San Andreas Fault. We present a 1:10 scale map of the de- formation core of this shear zone. • Temperatures of defromation estimated from quartz LPOs are ~300 - 600 °C, opening angles suggest a temperature of 512 ± 50 ºC. • Brittle faults occur parallel to the mylonitic foliation and host tensile fracture networks suggestive of dynamic earthquake rupture. Some breccias are plastically healed. • Pseudotachlytes are isoclinally folded and boudinaged, suggestive of the quartz-plastic transitional zone. • We find a distribution of earthquake markers across the width of the 30 m wide shear zone. Temperature of deformation Earthquake Markers References Research Questions • Earthquakes on deep (~15 - 20 km) strike slip faults host earthquakes across a wide zone rather than a single fault. • Ultramylonites, pseudotachlytes and dynamic breccias occur near competency contrasts toward the shear zone boundaries. This suggests that rock strength plays a dominant roll in earthquake distributions along deep strike slip structures. • Where do earthquakes occur in transitional zone strike slip faults? • What is the relationship between total strain, strain rate and shear zone geometry? • What was the temperature during deformation in the Pofadder Shear Zone? Implications Griffith, W.A., Rosakis, A., Pollard, D.D., Ko, C.W., 2009. Dynamic rupture experiments elucidate ten- sile crack development during propagating earthquake ruptures, Geology, Vol 37, No 9. Krul, J.H., 1998, Reply: Prism- and basal-plane parallel subgrain boundaries in quartz: a microstruc- tural geothermobarometer, Journal of Metamorphic Geology, 16, 142-146. Law, R.D. and Searle, M.P., Simpson, R.L., 2004. Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet, Journal of the Geological Soci- ety, Vol 161, No 2. Melosh, B.L., Rowe, C.D., Smit, L., Groenewald, C., Lambert, C.W., Macey, P., (in review at EPSL). Snap, Crackle, Pop: dilational fault breccias record seismic slip below the brittle-plastic transition. Passchier, C.W. and Trouw, R.A.J., 2005, Microtectonics, Springer, Berlin. Rosakis, A.J., 2002, Intersonic shear cracks and fault ruptures, Advances in Physics, Vol 51, No 4. Max: 2.91 Min: 0.34 Max: 2.69 Min: 0.51 Max: 1.65 Min: 0.64 Type 1 cross girdles, single girldes, and Y maxima suggest temperatures of deformation between ~300 - 600 ºC (Passchier and Trouw, 2005) 0001 1-100 1-210 0-110 Max: 7.65 0001 Min: 0.24 Max: 2.38 1-210 Min: 0.31 C and A axes quartz LPO PS196 PS181 PS182 PS030 C axes quartz LPO C axes quartz LPO C axes quartz LPO PS181 has opening angle of ~65º corresponding to a temperature of 512 ± 50 ºC (Kruhl, 1998, Law et al., 2004) Aligned hornblende on foliation surface Actinolite schist boudin Actinolite-chlorite schist boudins C’ C’ Amphibolite Greenschist A A Ultramylonite Biotite Schist Ultramylonite View toward the SE View toward the SE Ultrahigh Strain Zones Area Mapped Below View toward the SE Orange River View toward the SE Pseudotachylyte Dynamic Breccia 52 mm P se u d o t a c hy l y te P eg m a ti t e P se u d o t a c hy l y te P se u d o t a c hy l y te Pink Mylonite Folds and Lineations • Pseudotachylyte injection viens. • Isoclinally folded pseudotachylyte. • Pseudotachylyte in CL. • Dynamic breccia fracture networks. Breccia 72 mm slip surface Ultramylonite Breccia 10 20 30 30 60 90 120 150 180 0 Slip Surface propagation direction Rupture See Rosakis,2002, Griffith et al., 2009, and Melosh et al., (in review). View toward the SE Fold Axis Quartz Stretching Lineation Pole to Foliation Equal area Lower hemisphere Small circle apex Small circle apex is not aligned with fold axes, suggesting transpressional folding rather than two deformation events... ...at higher strain the lineations tend to approach the fold axes. Fold Axes Lineations Fold Axes Lineations There are open to tight folds in the shear zone core. Fold Axis Lineation Cw Cp Bs Bs Br Br Cw View toward the SE Overprinted Pseudotachylyte N N = Pole to foliation = Lineation Fold Axes