Active Faulting and Folding in and Around Lake Tekapo, Mackenzie Basin, New Zealand

Phaedra Upton

University of Otago, now at University of Maine

Poster presented at New Zealand Geological Society Meeting, November 2001, Hamilton

Introduction

The outboard fold and thrust belt of the Southern Alps is dominated by N-S trending mountain ranges oblique to the main backbone of the Alps, and NE-SW trending intermontane basins, cut by N-S oriented glacial valleys and lakes. Uplift of the N-S trending ranges was initiated along N to NNE striking thrust faults followed by movement along NE-SW striking faults, which run oblique to the trend of the ranges. Movement along these two sets of active faults is currently responsible for the continuing uplift of these basement ranges at rates of 1-1.5 mma-1 (James, 1998, Upton et al., submitted). Faulting occurs on the flanks of these ranges while folding dominates within the basins. This study was aimed at relating active structures across Lake Tekapo and determining their relationship, if any, to pre-existing Mesozoic structures.

Active structures within the northeastern Mackenzie Basin include the Irishman Creek Fault west of Lake Tekapo, and the Coal River Fault east of the lake, both of which strike NE-SW. Evidence of a Mesozoic fault zone, dipping to the SE, was found on both shores at the south end of the Lake (Figure 1). It projects into the Irishman Creek Fault to the west. On the western shore the fault has been reactivated in the Cenozoic with a zone, at least 40 m wide, of cataclasite and fault gouge.

Figure 1: Map of Lake Tekapo

Figure 2: Seismic sections

Figure 3: Cross-sections across Lake Tekapo

Points to note

• Measured modern day sedimentation rates vary from 2 to 20 mma-1 implying 100 m of sediment represents 5000 to 50000 years of sedimentation. The limit of the last glaciation at 14000 BP suggests that the higher sedimentation rates are more likely, especially immediately following retreat.
• Lake bottom offsets do not always mimic basement topography suggesting these are tectonic features rather than a result of depositional processes
  Offsets in the lake surface project onto faults mapped onshore
• The Irishman Creek Fault shows evidence of Cenozoic movement where exposed on the lake shore, however, it is not currently active as there is no offset of the lake sediments which represent the last 5000 - 14000 years.
• A shift in the locus of contractional movement is occurs beneath Lake Tekapo. To the west, the active faults are the Irishman Creek Fault and possibly a structure beneath the Gammack Range. East of the lake, the faults of the Coal River Fault system are the major structures.
• These observations suggest that within the lake basin, a poorly seismically imaged transfer structure exists which relays movement along the lake.
  

Acknowledgements

Erich Osterberg, boomer operator, and Brian Grant, boat driver, are both thanked for their invaluable assistance collecting the seismic data. Belinda Mellish provided able field assistance. Dave Craw, Peter Koons, Chuck Landis and David Barrell are thanked for many helpful discussions. FRST and the Active Earth Processes Emerging Theme of the University of Otago are thanked for financial assistance.

References

James, Z. 1998: Geology, Quaternary Structure, Fault Rocks and Fluid Flow, Fox Peak Range, Eastern Southern Alps. Unpublished MSc Thesis. Geology, University of Otago

Upton, P., Craw, D., James, Z. and Koons, P. O. submitted: Structure and neotectonics of the Southern Two Thumb Range, mid-Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics.