Structure and Tectonic Geomorphology of the Lowry Peaks Range-Waikari Valley District
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Structure and Tectonic Geomorphology of the Lowry Peaks Range-Waikari Valley District, North Canterbury A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science in Geology in the University of Canterbury by Nicola Jane Litchfield ~, University of Canterbury 1995 Frontispeice An oblique aerial view towards the west, of the \Vaikari Flat area. In the foreground are the plunging Mt Alexander and Waikari Anticline structures. In the centre is the Waikari Flat region, bound to the west by Hawarden Anticline (traversing the photo) and the north by the Trig C Structures. The Southern Alps in the background are bound on their eastern side by N-S and NW SE trending structures of the Mt Mason to Island Hills Region. Q 637.5" . C::3 . Lr7b I qq-c; Structures of the Lowry Peaks Range - Waikari Valley district are complex. The majority comprise three members of a predominantly WSW -ENE striking major northwards-directed, leading edge imbricate thrust system, with associated angular, asymmetric fault-propagation folds. This system forms anomalously within a large NE SW trending belt of structures characterising the entire east coast of north Canterbury, both onshore and offshore and terminates westwards against N-S striking, east facing fold-fault zone. The objectives of this study address the origin, geometry and kinematics of the interaction between these diversely trending systems. Stratigraphy and small-scale structures denote three periods of deformation, namely: i) Middle Cretaceous deformation of the basement rocks, ii) weak Middle Oligocene deformation associated with the inception of the plate boundary through the South Island, and iii) major Pliocene - Recent deformation that formed the majority of the above-mentioned structures. Stress tensor analyses within competent basement and limestone cover rocks suggest two sets of sub-horizontal compression, NE-SW and NW-SE, the former likely to relate to a localised earlier period of deformation, now overprinted by the latter. NW-SE oriented sub-horizontal compression correlates well with results from other parts of north Canterbury. The result of NW-SE compression on the W-E to WSW-ENE striking structures is a large component of oblique motion, which is manifest in four ways: i) movement on two, differently oriented splays rather than a single fault strand, ii) the development of a sinuous trace for a number of the major folds, whereby the ends are oriented normal to the compression direction, the centres parallel to the strike of the faults, iii) the development of a number of cross-folds, striking NNE-SSW and iv) the apparently recent development of a strike-slip component on at least one of the major thrust faults. The origin of the W-E, or WSW-ENE striking structures may be reactivation of Late Cretaceous faults, stra,.tigraphic evidence for the existence of a "structural high" (the Hurunui High) over the majority of the area in the Late Cretaceous to Early Eocene times suggests the formation of a W-E trending horst structure, with a corresponding asymmetric graben to the south. The junction of WSW-ENE trending structures with N-S trending structures to the west centres on an alluvial-filled depression, Waikari Flat, into which the structures of the WSW-ENE trending imbricate thrust system plunge, locally curling to the SWat their ends to link with N-S trending structures to the south. Roof thrusting on two orientations, W-E andN-S, towards to SE is currently occurring above these structures. Currently the area is not highly seismically active, although a magnitude ---6.4 Ms earthquake in historic times has been recorded. The effects of tectonics on the drainage of the area does suggest that the majority of the systems, are still potentially active, albeit moving at a comparatively slow rate. The majority of the recent motion appears to be concentrated on the roof-thrusting occurring in Waikari Flat, and uplift along the Lowry Peaks Fault System. Increasing amounts of secondary movement on back-thrusts and cross fractures is also implied for western ends of the major imbricate thrust system. In contrast, the southern-most fault system appears to be largely sustaining dextral strike-slip motion, with some local folding in central portions. TITLE PAGE........................................................................................................ .i FRONTISPIECE .......... ......................................................................................... ii ABSTRACT ........................................................................................................... iii TABLE OF CONTENTS ...................................................................................... iv LIST OF FIGURES .............................................................................................. ix LIST OF TABLES ................................................................................................. x CONTENTS OF VOLUME TWO........................................................................ xi chapter one: INTRODUCTION............................................................................. 1 1.1 BACKGROUND ....................................................................................... 1 1.2 REGIONAL SETTING ............................................................................. 1 1.3 THESIS AIMS ........................................................................................... 7 1.4 THESIS ORGANISATION ....................................................................... 8 1.5 DESCRIPTION OF THE STUDY AREA ................................................ 8 1.6 PREVIOUS WORK................................................................................... 10 chapter two: STRATIGRAPHY ............................................................................. 14 2.1 INTRODUCTION .................................................................................... 14 2.2 BASEMENT ROCKS ................................................................................ 16 2.2.1 Torlesse Supergroup ...... .............................................................. 16 2.3 COVER ROCKS........................................................................................ 17 2.3.1 Eyre-Group ........ .......................................................................... 17 2.3.1.1 Conway Formation .......................................................... 17 2.3.1.2 Waipara Greensand Formation ........................................ 18 2.3.1.3 Ashley Mudstone Formation ............................................ 19 2.3.1.4 Homebush Sandstone Formation ..................................... 20 2.3.2 Amuri Limestone ......................................................................... 21 2.3.3 Motunau Group ........................................................................... 23 2.3.3.1 Omihi Formation ............................................................. 23 Weka Pass Stone Member .......................................... 23 Gorries Creek Greensand Member ............................ 26 2.3.3.2 Waikari Formation .......................................................... 27 2.3.3.3 Mt Brown Formation ...................................................... 28 2.3.3.4 Greta Formation .... .......................................................... 29 2.3.3.5 Kowai Formation . ..................................................... 29 2.3.4 Late Quaternary deposits ... ......................................................... 30 2.4 DISCUSSION AND SUMMARy ........ ...................................................... 31 2.4.1 Basement-cover rock unconformity ............................................ 31 2.4.1.1 The Hurunui High ........................................................... 31 2.4.1.2 Lateral extent. ................................................................. 32 2.4.1.3 Structural significance ..................................................... 32 2.4.2 Deformation recorded within the cover rocks ............................ 34 2.4.2.1 Middle to Late Oligocene deformation ............................ 34 v 2.4.2.2 Deformation events synchronous with Motunau Group deposition ........................................................................ 35 2.4.2.3 Motunau Group - Late Quaternary deposits unconformity ................................................................... 35 chapter three: MACROSCOPIC DEFORMATION .............................................. 37 3.1 INTRODUCTION ...................................................................................... 37 3.2 THE KAIWARA FAULT SYSTEM AND ASSOCIATED STRUCTURES......................................................................................... 37 3.2.1 Major range-bounding faults ..................................................... .42 3.2.2 Major folding ............................................................................... 43 3.2.2.1 Waikari Anticline ............................................................. 44 3.2.2.2 Moores Hill Anticline ...................................................... 45 3.2.2.3 Moores Hill Syncline ....................................................... 47 3.2.2.4 Folding in Waikari Valley ................................................ 47 3.2.2.5 Folding associated with Greta Peaks Fault.. ..................... 47 3.2.3 Summary and Conclusions .......................................................... 49 3.3 THE MT ALEXANDER-MAXWELTON FAULT