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46A
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River
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15
0 105Meters
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
PegmatitePeg
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 contour312
Amphibolite
Pseudotachylyte
Namibia
South Africa
Atlantic Ocean
Botswana
Field Area
Lüderitz
Location
Orange
NamibiaRiver
Zone
20 km
Pofadder
Shear
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. Melosh1, Christie D. Rowe1,3, Christopher Gerbi2, Louis Smit3
1McGill University, Montréal, Canada2University of Maine, Orono, USA
3University of Cape Town, South AfricaResults • 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
Gri�th, 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 �ow 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
M in :0 . 34
Max :2. 69
M in :0 . 51
Max :1. 65
M in :0 . 64
Type 1 cross girdles, single girldes, and Y maxima suggest temperatures of deformation between ~300 - 600 ºC (Passchier and Trouw, 2005)
0001
1-1001-2100-110
Max :7. 65
0001
M in :0. 24
M ax :2. 38
1-2 10
M in :0. 31
C and A axes quartz LPO
PS196PS181 PS182 PS030C axes quartz LPOC axes quartz LPOC 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
Pseudotachylyte
Pegmatite
Pseudotachylyte
Pseudotachylyte
Pink Mylonite
Folds and Lineations
• Pseudotachylyte injection viens.
• Isoclinally folded pseudotachylyte.
• Pseudotachylyte in CL.
• Dynamic breccia fracture networks.
Breccia
72 mm
slip surfaceUltram
ylonite
Breccia
10 20
30
3060
90
120
150
180
0
Slip Surface
propagation directionR
upture
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 areaLower 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 LineationsFold Axes Lineations
There are open to tight folds in the shear zone core.
Fold Axis
Lineation
Cw
Cp
Bs
Bs
BrBr
Cw
View toward the SE
Overprinted
Pseudotachylyte
N N= Pole to foliation
= Lineation
Fold Axes
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