23432011 AGU Fall Meeting

SHALLOW TRANSPRESSIONAL SEGMENTATION & PARTITIONING REVEALED BY LiDAR DATA: CENTRAL ALPINE FAULT, NEW ZEALAND

Nicolas C. Barth1, Virginia G. Toy1, Robert M. Langridge2, Richard J. Norris1

1Dept of Geology, U. of Otago, Dunedin, NZ2GNS Science, Lower Hutt, NZ

nic@geology.co.nz

2 km

UN

IVE

RSITY OF O TA G O NE W

ZE

AL

AN

D

GEOLOGY

We combine recently acquired air-borne LiDAR data with aerial photo

interpretation and geologic map-ping to define a structural hierarchy based on along-strike observations on the transpressional plate bound-

ary Alpine Fault from <106-100 m scales and examine the controls on

the fault’s structure that dominate at different scales.

1. INTRODUCTION

5. LiDAR OVERVIEW

6. SECOND ORDER (LiDAR)

PACIFIC PLATE

AUSTRALIAN PLATE

ALPINE FAULT

300 km

45°S

170°E

2. SETTING

100m

? ? ?

Western Province

fluvial/glacial/marinesediments

fluvioglacialsediments

ultramylonitemylonite

cataclasite

B

A

weakest

weakstrong

strong

damage zone

damage zone

C

7. THIRD ORDER (103-100 m)

500m

McCulloughsCreek

500m ArthurCreek

500m

DochertyCreek

500m

MatainuiCreek

thrust/reverse faultnormal faultstrike-slip faultanticlinal ridge

Key

9. REFERENCES CITED1 Norris, R.J., Koons, P.O., and Cooper, A.F., 1990, The obliquely-convergent plate boundary in the South Island of New Zealand: Implications for ancient collision zones: J. of Structural Geology, 12, n. 5/6, p. 715-725.2 Sutherland, R., Eberhart-Phillips, D., Harris, R.A., Stern, T., Beavan, J., Ellis, S., Henrys, S., Cox, S., Norris, R.J., Berryman, K.R., Townend, J., Bannister, S., Pettinga, J., Leitner, B., Wallace, L., Little, T.A., Cooper, A.F., Yetton, M., and Stirling, M., 2007, Do great earthquakes occur on the Alpine fault in central South Island, New Zealand?, in Okaya, D., et al. (eds), A Continental Plate Boundary: Tectonics at South Island, New Zealand: Washington, D.C., American Geophysical Union, Geophysical Monograph 175, p. 235-251.3 Sutherland, R., Davey, F.J., and Beavan, J., 2000, Plate boundary deformation in South Island, New Zealand, is related to inherited lithospheric structure: Earth and Planetary Science Letters, 177, p. 141-151.4 Koons, P.O., Norris, R.J., Craw, D., and Cooper, A.F., 2003, Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand: Geology, 31, n. 1, p. 3-6.5 Norris, R.J., and Cooper, A.F., 1995, Origin of small-scale segmentation and transpressional thrusting along the Alpine fault, New Zealand, Geol. Soc Am. Bull., 107, 231-240.6 Norris, R.J., and Cooper, A.F., 1997, Erosional control on the structural evolution of a transpressional thrust complex on the Alpine Fault, New Zealand, J. of Structural Geology, 19, 1323-1342.7 Wells, A., Yetton, M.D., Duncan, R.P., and Stewart, G.H., 1999, Prehistoric dates of the most recent Alpine fault earthquakes, New Zealand: Geology, 27, p. 995–998.8 Barrell, D.J.A, 2011, Quaternary Glaciers of New Zealand, in Ehlers, J., et al., eds., Quaternary Glaciations- Extent and Chronology, ch. 75, p. 1047-1064. 9 DeMets, C., Gordon, R.G., and Argus, D.F., 2010, Geologically Current Plate Motions: Geophys. J. Int., 81, p. 1-80 10 Norris, R. J., and Cooper, A. F., 2007, The Alpine Fault, New Zealand: Surface geology and field relationships. In Okaya, D., Stern, T. & Davey, F. (eds) A Continental Plate Boundary: Tectonics at South Island, New Zealand. AGU Geophysical Monograph 175, American Geophysical Union, Washington D. C., pp 157-175.

• Remarkably linear strike (055°) over hundreds of kilometers• Motion is oblique & unpartitioned1-2• Inherited from an Eocene passive margin3• Localization on a single planar shear zone at depth due to seismic loading and thermal weakening by advection and exhumation 4 • Orientation is inherited above the brittle-viscous transition due to the anisotropic fabric developed at depth

• 34 km by 1.5 km swath collected• 2m DEM produced allowing topographic examination at 104-100 m scales• 75% dense temperate rainforest 25% active river floodplains (AD 1717 earthquake7 surface rupture erased) • Extensive glacial deposits indicate the Alpine Fault was completely covered by glaciers in this area during the Last Glacial Maximum (LGM) ~18 000 yrs ago8 - all fault traces are presumed to be post-LGM• Is it a fault trace or lineament? - factors: definitive offsets, adjacency to anticlinal ridges, scarps on millennially-inactive glacial or fluvial surfaces, particularly well-defined range-front lineaments, signs of disequilibrium erosion, and otherwise atypical geomorphology best explained by the presence of a fault - incorporation of field checking and geological data

• Oblique-thrust partitions display third order fault wedge geometry as discussed in (7)• Strike-slip partitions tend to have 2 parallel fault traces with dextral offsets• Surface deformation is most diffuse at outward serial partition transitions (inward transitions erased by rivers)

• Rangefront dextral-thrust fault traces typicially have curvilinear poorly-preserved NW-facing scarps 20-50 m in height - field observations frequently show these faults to have Pacific Plate fault rocks thrust over Quaternary sediments at low to moderate dips10 • Dextral-normal traces are ubiquitous within 100-1000 m of the rangefront as linear or curvilinear SE/S-facing scarps 1-10 m in height - normal motion arises from extrusion of fault wedge, not rangefront collapse • Geomorphology requires rangefront dextral- thrust faults to accomodate most movement; field observations agree• Parallel-partitioned fault wedges have widths of ~300 m

(A) Oblique three-dimensional model based on geometry near Franz Josef. Arrows show movements relative to a stationary Australian Plate. (B) Fault-perpendicular cross section through front of model in (A) showing fault wedge structure. (C) Cross section from (B) simplified to show relative rock mass strengths of materials. Note the depth to basement in the footwall controls the basal fault dip and the width of the fault damage zone controls the width of the fault wedge.

100 kmSTUDY AREA

U

D

STUDY AREA

N

0 2 4 km

8. SUMMARY

(1) First order structure is inherited(2) Second order structure is due to topography and erosion(3) Third order structure is influenced by the presence of weak and less anisotropic materials in the near-surface (thickness of footwall sediments, width of damage zone) -most slip is still accomodated on the basal dextral-thrust contiguous with the Alpine Fault at depth

055 082

FAULT W

EDGE

047

047047

strike-slip partition

oblique thrust / strike-slip transitions

2m LiDARorthophoto

3. FIRST ORDER (<106-104 m)

after Norris & Cooper (1997) after Norris & Cooper(2007)

*

*

*

*

*

*

*

DarnleyCreek

500m

fault tracesn = 2685° binning

055: average strike of Alpine

Fault

oblique-thrustpartitions

strike-slippartitions

weighted by lengthtotal length: 68.9 km

N

39.6 ± 0.7 mm/yr245.4 ± 0.9°

Australian-Pacific Plate Motion Vector9

DochertyCreek

WaihoRiver

FRANZJOSEF

Waitangi-taonaRiver

Darnley Creek

Gaunt Creek

Matainui CreekArthur Creek

WhataroaRiver

McCulloughsCreek

N

2m LiDAR-derived hillshade on 25m Land InformationNew Zealand (LINZ) hillshade

SERIAL PARTITIONING

after Norris & Cooper (2007)

after Norris & Cooper (1997)

1st 2nd 3rdFRANZJOSEF

500mN

100m

AUSTRALIANPLATE

PACIFICPLATE

dextral fault

thrust fault

• 1-10 km sequenced (“serial”) partitions along 100 km of the central Alpine Fault5-6 • Norris & Cooper (1995) demonstrated that partitions arise from topography-induced stress perturbations and erosion due to major rivers • Compare map at right to LiDAR imagery

4. SECOND ORDER (104-103 m)

fault wedge

oblique-thrustpartition

average strike of boundary

average strike of boundary

direction of

plate motion

strike-sli

p fault

thrust zone

strike-slippartition

angle of obliquity

PARALLEL PARTITIONING

25m LINZ