312

310

314

312

313

315

317 318

319

320

311

316

310

311

9

24

44

6

238

27

61

42

2884

5

40

50

19

51

26

32

7976

30

10

1650 40

2519 45

301

46A

Q

Q

Q

Q

A

Q

A

Q

A

QCp

Cp

Cp

Um

Cp

Um

Um

Cp

Um

Bs

Bs

UmCp

Um

Bs

Br BrCw

Bs

CpBr

Cp

Cp

BsBs

Cp

Cp

Cp

UmUm

Cp

UmCp

Cp

Cp

Cp Cp

Bs

Um

Um

Bs

Br

Bs

Bs

Bs

Bs

Bs

Bs

Cw

Cp

Cp

BrBs

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

UmCp

PegPeg

Br

Bs

Gum

Bs

Peg

Pum

83

7578

6457

64

75

52

76

76

78

85

75

7650

7676

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

8676

86

78

87

85

67

52

59

8267

6884 73

8470

69

85

89

75

89 70

66

60

65

84

81

89

82

89

5640

7689

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

8488

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