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Geological control of physiography in southeast Queensland: a multi-scale analysis using GIS

Jane Helen Hodgkinson

Bachelor of Science (Hons), Geology (Birkbeck University of London, UK)

School of Natural Resource Sciences

A thesis submitted for the degree of Doctor of Philosophy Queensland University of Technology 2009

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STATEMENT OF ORIGINAL AUTHORSHIP The work contained in this thesis has not been previously submitted for a degree or diploma at any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Signed…………………………………

Jane Helen Hodgkinson

Date……………………………

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ABSTRACT The study reported here, constitutes a full review of the major geological events that have influenced the morphological development of the southeast Queensland region. Most importantly, it provides evidence that the region’s physiography continues to be geologically ‘active’ and although earthquakes are presently few and of low magnitude, many past events and tectonic regimes continue to be strongly influential over drainage, morphology and topography. Southeast Queensland is typified by highland terrain of metasedimentary and igneous rocks that are parallel and close to younger, lowland coastal terrain. The region is currently situated in a passive margin tectonic setting that is now under compressive stress, although in the past, the region was subject to alternating extensional and compressive regimes. As part of the investigation, the effects of many past geological events upon landscape morphology have been assessed at multiple scales using features such as the location and orientation of drainage channels, topography, faults, fractures, scarps, cleavage, volcanic centres and deposits, and recent earthquake activity. A number of hypotheses for local geological evolution are proposed and discussed. This study has also utilised a geographic information system (GIS) approach that successfully amalgamates the various types and scales of datasets used. A new method of stream ordination has been developed and is used to compare the orientation of channels of similar orders with rock fabric, in a topologically controlled approach that other ordering systems are unable to achieve. Stream pattern analysis has been performed and the results provide evidence that many drainage systems in southeast Queensland are controlled by known geological structures and by past geological events. The results conclude that drainage at a fine scale is controlled by cleavage, joints and faults, and at a broader scale, large river valleys, such as those of the Brisbane River and North Pine River, closely follow the location of faults. These rivers appear to have become entrenched by differential weathering along these planes of weakness. Significantly, stream pattern analysis has also identified some ‘anomalous’ drainage that suggests the orientations of these watercourses are geologically controlled, but by unknown causes. To the north of Brisbane, a ‘coastal drainage divide’ has been recognized and is described here. The divide crosses several lithological units of different age, continues parallel to the

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coast and prevents drainage from the highlands flowing directly to the coast for its entire length. Diversion of low order streams away from the divide may be evidence that a more recent process may be the driving force. Although there is no conclusive evidence for this at present, it is postulated that the divide may have been generated by uplift or doming associated with mid-Cenozoic volcanism or a blind thrust at depth. Also north of Brisbane, on the D’Aguilar Range, an elevated valley (the ‘Kilcoy Gap’) has been identified that may have once drained towards the coast and now displays reversed drainage that may have resulted from uplift along the coastal drainage divide and of the D’Aguilar blocks. An assessment of the distribution and intensity of recent earthquakes in the region indicates that activity may be associated with ancient faults. However, recent movement on these faults during these events would have been unlikely, given that earthquakes in the region are characteristically of low magnitude. There is, however, evidence that compressive stress is building and being released periodically and ancient faults may be a likely place for this stress to be released. The relationship between ancient fault systems and the Tweed Shield Volcano has also been discussed and it is suggested here that the volcanic activity was associated with renewed faulting on the Great Moreton Fault System during the Cenozoic. The geomorphology and drainage patterns of southeast Queensland have been compared with expected morphological characteristics found at passive and other tectonic settings, both in Australia and globally. Of note are the comparisons with the East Brazilian Highlands, the Gulf of Mexico and the Blue Ridge Escarpment, for example. In conclusion, the results of the study clearly show that, although the region is described as a passive margin, its complex, past geological history and present compressive stress regime provide a more intricate and varied landscape than would be expected along typical passive continental margins.

The literature review provides background to the subject and discusses previous work and methods, whilst the findings are presented in three peer-reviewed, published papers. The methods, hypotheses, suggestions and evidence are discussed at length in the final chapter.

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Keywords Geomorphology, GIS, Drainage patterns, Stream-ordering, southeast Queensland, Passive margin, Earthquake distribution

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LIST OF PUBLICATIONS REFEREED INTERNATIONAL JOURNAL PAPERS PAPER 1 Title: The influence of geological fabric and scale on drainage pattern analysis in a catchment of metamorphic terrain: Laceys Creek, southeast Queensland, Australia Authors: Jane Helen Hodgkinson, Stephen McLoughlin, Malcolm Cox Status: Published November 2006 (available on-line from July 2006) Journal: Geomorphology, 81 394-407

PAPER 3 Title: Drainage patterns in southeast Queensland: the key to concealed geological structures? Authors: Jane Helen Hodgkinson, Stephen McLoughlin, Malcolm Cox Status: Published December 2007 Journal: Australian Journal of Earth Sciences, 54 1137-1150

REFEREED CONFERENCE PAPER PAPER 2 Title: The correlation between physiography and neotectonism in southeast Queensland Authors: Jane Helen Hodgkinson. Stephen McLoughlin, Malcolm Cox Status: Reviewed for DEST purposes, published in conference proceedings, presented with poster (Appendix 1) Conference: Australian Earthquake Engineering Society Conference, Canberra, ACT November 2006

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ACKNOWLEDGEMENTS I wish to acknowledge the support and encouragement provided by my principle supervisor, Dr Stephen McLoughlin who gave invaluable advice and comments, who gave his time generously and whose ability and standards I will always strive to emulate. My associate supervisor, Associate Professor Malcolm Cox also provided constructive support, held many useful discussions, and contributed his valuable time for the review of manuscripts, for which I am very grateful.

I am indebted to Dr Andrew Hammond for his help, support and valuable advice, and for our countless beneficial talks and debates. I am grateful to Dr Micaela Preda for sharing her wealth of GIS knowledge with me and for our constructive discussions with regard to local geology and geomorphological analysis. I also acknowledge the helpful and educational discussions and field trips with Mr Bill Ward whose interest in the subject of geomorphology is valuable and encouraging. Further, I wish to express my gratitude to Mr Nate Peterson who provided valuable assistance with GIS methodology.

This research project would not have been possible without the datasets provided by various sources. Special thanks go to Dr Dion Weatherly and Mr Col Lynam at the Earth Systems Science Computational Centre (ESSCC), University of Queensland, for their enormous encouragement with my project, for their collaborative discussions and for providing their valuable earthquake database. Thanks also goes to Geoscience Australia for further earthquake data, and to Pine Rivers Shire Council and the Geological Survey of Queensland at the Department of Mines and Energy (Queensland Government) for providing geological, topographical and drainage data, which were required for GIS analysis. Datasets were also obtained from USGS/NASA via their on-line service, without which, the first paper could not have been written.

I would like to thank all the staff and students in the School of Natural Resource Sciences at QUT, whose help and encouragement have been a great benefit to me in the course of this study. I also thank the staff at the School of Earth Sciences, Birkbeck University of London, among other things, for inspiring me in the

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wonderful subject of geology. I also thank all my friends and family both in Australia and the UK, who believed I could do this. I especially thank Jean and David Hodgkinson, Madonna O’Brien, Ben Henderson, Jennifer Baker, Caroline Cole, Jeanette Fleming, John Hodge and everyone from the ‘Whitton School Class of 1982’, whose humour and friendship has been invaluable. A particularly special thank you goes to Jonathan Hodgkinson, my husband, friend, field assistant, fellow student and room-mate at QUT, whose help and encouragement is beyond measure. I also express my gratitude to my late parents to whom I dedicate this work. One of the most important things they taught me was the value of enquiry, without which, science would go nowhere.

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TABLE OF CONTENTS STATEMENT OF ORIGINAL AUTHORSHIP ABSTRACT Keywords LIST OF PUBLICATIONS Refereed International Journal Papers Refereed Conference Paper ACKNOWLEDGEMENTS INTRODUCTION Setting Scope of study Methods and summary of results LITERATURE REVIEW INTRODUCTION INTRODUCTION TO GEOMORPHOLOGY Surface processes: Weathering and erosion Climate Palaeoclimate and present-day climate in southeast Queensland Regolith and ground cover Effects of sea-level change Anthropogenic influence The role of lithology and rock fabric in geomorphology The role of tectonics and major geological events in geomorphology Neotectonism in Australia Post-Mesozoic tectonism in southeast Queensland GEOMORPHOLOGICAL ANALYSIS Drainage patterns Palaeosurfaces, palaeodrainage and current drainage patterns in southeast Queensland Stream ordering Data analysis Erosion analysis SOUTHEAST QUEENSLAND Introduction to the study area Reasons for selecting the study region Geological history Faulting Sea level influences Terraces Incision Geology of Pine Rivers and Laceys Creek: a fine-scale case study Previous geomorphological studies of southeast Queensland ANALYSIS METHODS USED IN THIS STUDY Digital elevation models (DEMs) Geological data Earthquake data Geographic Information Systems (GIS) and choice of GIS products Remote sensing Spatial analysis Methods of channel analysis Stream ordering SUMMARY References PAPER 1

iii v vii viii viii viii ix 1 4 5 6 10 12 14 14 20 22 23 24 26 30 38 48 54 59 59 62 63 65 66 70 70 70 74 85 86 86 87 93 95 102 102 103 103 103 106 106 107 108 111 114 135

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Introduction to Paper 2 PAPER 2 PAPER 3 CONCLUSIONS Summary of results and major findings Hypothesis 1 Hypothesis 2 Hypothesis 3 Additional findings Implications for future research Other observations and general discussion Implications for evaluating the evolution of the landscape Evidence of past geological events in the present landscape National and international significance Evolutionary model Précis of main findings Future work References APPENDICES Appendix 1 - AEES2006 Poster Appendix 2 - A GIS and map-analysis deficiency Appendix 3 - Other software products used Appendix 4 - Statistical analysis of planar features in Laceys Creek

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163 165 183 215 216 216 217 218 219 222 223 230 230 234 238 238 239 240 243 245 247 249 251

INTRODUCTION

1

2

INTRODUCTION Geological processes are a primary control upon landscape physiography, as internal modifications of the Earth often lead to distortion and contortion of the surface. These internal modifications of Earth’s crust are also responsible for the location of hydrocarbon reservoirs, aquifers, mineral accumulations and geological hazards such as active faults. Geological controls on physiography range from large scale processes such as plate movements, folding, faulting and jointing to the finer scale, where variations in the mineralogy of rocks, micro-fractures and porosity can lead to differing susceptibility to weathering. The derived physiography is, therefore, a result of both endogenic processes and exogenic modification by agents of weathering and erosion. These interactive processes may also generate geological hazards, such as unstable slopes or the superposition of valleys on faults (of importance for dam location). Various methods of physiographic analysis can be used to interpret the evolution of the landscape, from which we may deduce the underpinning lithological and structural influences. This type of analysis can greatly enhance understanding of sub-surficial geology and is an effective tool for geohazard identification and resource exploration.

The main objective of this study is to improve understanding of the controls on landscape evolution in southeast Queensland, as this may be an important consideration for future land use. An improved understanding of the influences on local physiographic evolution can then be used to interpret landscape development in similar tectonic settings globally. Key to this study was identification of the extent to which geological processes influence the existing landscape morphology of southeast Queensland. This was undertaken by analysis and interpretation of the genesis of various physiographic features including fine-scale streams, major drainage systems, scarps, valleys and plains at varying scales. Geomorphological analysis was undertaken at both regional and catchment scales to assess the influences of different-magnitude geological features on southeast Queensland’s physiography.

Although landscapes develop from the interaction of both internal and external processes, one of these processes typically dominates the evolution of the system. In particular, this research seeks to determine whether geological features (structure and 3

lithology) sufficiently correlate with physiographic features to be deemed the dominant influence on the evolution of the landscape in southeast Queensland. An individual index is not sufficient to determine the main control of the landscape, so this study evaluates multiple criteria at various scales, to identify whether geological processes are (or were) dominant in the development of southeast Queensland’s landscape. To establish whether more than one major influence exists, the research uses a sequential approach and the results are presented in three peer-reviewed, published papers.

Setting Around 370-220 Ma, the easternmost part of Australia was accreted onto the older cratonic part to the west. Subsequently, the easternmost part has been modified by epicratonic basin development, rifting and putative hotspot volcanism, which has contributed to a vast array of natural resource deposits in the region and has presumably imposed strong influences on the land’s topography. The area for this study is situated on the eastern margin of the Australian continent (approximately 151° 53', 26° 11'S to 153° 31', 28° 30'S), and covers approximately 41,000 km2. Although the topography of the region is generally subdued by world standards, some steep and mountainous areas also exist. Outcrop availability and accessibility is generally moderate to poor and commonly limited to road cuttings, due to deep soil and extensive vegetation cover. Furthermore, the region is currently undergoing extensive land-use change particularly due to rapid urbanisation and development; the area represents one of the fastest population growth centres in Australia (Australian Bureau of Statistics, 2006).

The region’s geomorphology has been generally well studied, albeit on piecemeal basis (e.g. Marks, 1933; Watkins, 1967; Arnett, 1969; 1971; Donchak, 1976; Beckmann and Stevens, 1978; Lucas, 1987; Cuthbertson, 1990; Childs, 1991). In summary, the western, southern and northern margins of the study area are fringed by highlands (mainly plateaux over 300 m. a.s.l.), and a dissected highland area (the D’Aguilar Ranges) occurs central to the region. Foot hills and coastal plains occur across the remainder of the area, principally in the east, and escarpments are also fairly common across the region. Eastern Australian rivers may be considered to be 4

shorter than others by global standards, although runoff is generally higher and more variable. This is due to the climate variations dictated by the region’s mid-latitude position and the competing influences of the western Pacific tropical monsoon and temperate frontal systems that impose a Mediterranean climate on the southern half of the continent (Finlayson and McMahon, 1988). By Australian standards, rivers in southeast Queensland are moderate in size with moderate to high discharge that varies seasonally. Many streams are ephemeral and flow mainly in response to heavy summer rainfall events. The main drainage systems of the region display strong northwest-southeast and northeast-southwest trends similar to those of some large faults in the region. Similarly, a strong northwest-southeast trend is evident in the distribution and foliation of rock units located in southeast Queensland, this being related to late Palaeozoic – early Mesozoic convergent margin deformation. Earthquake data over the past 130 years records only two earthquakes of >5 magnitude (Richter scale) in the region and more than 50 that were >2 magnitude (ESSCC, 2006). Although the database provides discontinuous evidence of earthquake activity spatially and temporally, the foci of many earthquakes are clearly positioned in shallow clusters and even a casual examination reveals alignment of many epicentres within discrete corridors.

Scope of study Southeast Queensland has been selected as the study area primarily due to its complex geological history and varied physiography. Despite many investigations of the region, detailed information on the genesis of landscape features at a regional and local scale is deficient. As the human population in the region is rapidly increasing concomitant with residential, commercial and industrial development, a better understanding of the driving forces behind geomorphological change is critical for future landscape management. Understanding the evolution of other regions with a similar climatic setting and complex mix of both convergent- and passive-margin geological histories may also benefit from the methods and results presented in this study.

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Methods and summary of results Remote sensing is a valuable tool for studying landscapes that may be inaccessible and/or very large, such as the southeast Queensland region. Remote sensing has been used here to analyse the alignment, position and form of physiographic features including mountains and highlands, valleys, drainage channels, scarps and lowlands. Other datasets were also integrated into the analysis including rock types and fabric, earthquake locations and geological structures. The method further lent itself to analysis at multiple scales from regional to sub-catchment settings.

In the following section a review of the literature provides an introduction to some of the principles of geomorphology. This is followed by a review of the methodologies employed for geomorphological analysis, their advantages, problems and applicability to the southeast Queensland region. The geology of southeast Queensland is then summarized and previous studies of the region’s geomorphology are examined. The literature survey concludes with a review of the specific analytical methods employed in this study, the limitations of the data and an outline of the need for a new methodology for stream ordering in drainage network analysis.

The first part of the results (presented as paper one) analysed whether drainage orientation is affected by its underlying rock fabric, including cleavage, joints, fractures and faults. The hypothesis tested in paper one was: “Complex geological fabric of metamorphic rocks of southeast Queensland has control over orientation of streams at the sub-catchment scale”

This was tested in a sub-catchment where the meta-sedimentary rock types retain some bedding features but are complicated by multiple orientations of cleavage, joints and faults. The aim was to assess the orientation of streams within a catchment developed on two juxtaposed metamorphic rock types and identify the extent to which streams show alignment with the underlying rock-fabric. Having introduced a new stream-ordering method for this study, as other methods were not suitable, a positive result for some stream-orders suggested that there is some geological control on drainage architecture at this scale. The results showed that higher order streams in the sub-catchment had a similar orientation and close spatial relationship with

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fractures and faults. Analysis of other faults and drainage channels outside of the sub-catchment also revealed a similar correlation which required further exploration.

A second investigation of geological control on the landscape incorporated a spatial investigation of recent shallow earthquakes across the whole of southeast Queensland, to assess whether the streams and valleys that showed a correlation with ancient faults might be influenced by continuing earth movements along these structures. The hypothesis tested in paper two was: “The location of recent earthquakes in southeast Queensland aligns with geomorphological features such as scarps, mountain ranges and valleys”

The aim of paper two was to identify, firstly, whether earthquake epicentres show spatial

alignment

and,

secondly,

whether

any

ancient

geological

and

geomorphological features (including faults, scarps, river valleys, highland lineaments) align with trends of earthquake epicentres. From this information the study discusses whether neotectonics plays an ongoing role in the evolution of the landscape. The results showed an alignment of low magnitude, shallow earthquakes with some strong geomorphological trends such as large river systems and areas of highlands. This suggests that tectonics may be playing an active albeit minor role in present-day landscape modification. Although the results were positive, the earthquake database and earthquake monitoring is fairly sparse and may not represent a thorough picture of neotectonism in the region. Therefore, the results are not sufficiently comprehensive to reflect the full scale of neotectonic influence on the landscape. Further data is clearly needed to clarify the role of this process.

Strong relationships have been proven to exist between ‘non-random’ drainage patterns and structures underpinning the landscape, especially where tectonic features such as uplifted or down-thrown blocks, and faults and folds are of strong amplitude. Combining the geology-drainage relationships similar to those found in paper one, with the scale and relationships in paper two, a third measure of geological control on the landscape was introduced into the research (paper three). This involved drainage pattern analysis at the regional scale. Drainage orientation and patterns and physiography were compared to the distribution of geological

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structures, lithostratigraphic packages of various ages, igneous intrusions and earthquake corridors, to assess the degree to which geological features of various ages and origins influence the modern physiography. The hypothesis tested in paper three was: “Drainage networks across southeast Queensland show repeating, aligned and anomalous patterns that are controlled by a range of geological structures of varying age”

The history of southeast Queensland is tectonically complex and although the affects of some ancient tectonic events on the landscape might still be evident, it is equally possible that over the intervening time, other processes may have obscured some of the original influences on the physiography. However, the results of paper one show a strong relationship exists between fine-scale streams and underlying rock structure and fabric that date back to at least the Late Carboniferous, even though these rocks have been influenced by a range of subsequent geological processes (nearby convergent margin pluton intrusion, regional uplift, passive margin development, and local emplacement of volcanic plugs). The results of paper two revealed common alignment between recent shallow earthquakes and large ancient valleys that also correspond to a Permian-Triassic trend of strike-slip faulting and the orientation of major tectono-stratigraphic unit boundaries.

The results of paper three show a

variety of associations between geological features and physiography that include volcanoes, rock types, faults, block emplacements and tectonic tilting that occurred over various stages of southeast Queensland’s geological history. Integration of the datasets within a Geographic Information System (GIS) provided a suitable platform for this analysis and led to new insights into the extent to which the physiography of the region is controlled by geological structures such as faulting and rock fabric. Additionally, the results suggest that previously unknown geological structures may control some important physiographic features and that some parts of the region may have been subject to neotectonic modifications. The results ultimately lead to the conclusion that there is a strong geological control over existing morphology providing sufficient evidence that surface or exogenic controls are not the dominant factor influencing the present landscape.

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“The greatest obstacle to discovering the shape of the earth, the continents, and the oceans was not ignorance but the illusion of knowledge.” Daniel J. Boorstin 1914 - 2004

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LITERATURE REVIEW

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INTRODUCTION Tectonism must be acknowledged as a primary control on the structure and physiography of land masses. It is responsible for the position of landmasses, and within them, the location of geological units, faults, joints and other planes of weakness. It also involves uplift of broad regions and basin subsidence. Weathering and erosion further modify the landscape and, as they are acting upon a terrain that has been ‘processed’ by a primary force, are described as secondary processes. The use of the terms ‘primary’ and ‘secondary’ describes the order in which the processes occur and not the relative importance of the process. Although these processes may, and often do, occur concurrently, weathering and erosion are exogenic, acting upon the endogenically processed landscape, hence the term ‘secondary process’ is applied. Primary processes can directly generate landscape features, such as hills or mountains and scarps, resulting from folds, uplift and faults. However, the additional action of secondary processes upon planes of weakness, such as faults, joints and folds, or the differential weathering of juxtaposed lithologies of different durability, can also lead to further landscape alteration. Geological structure and tectonics are intrinsically linked to a range of geomorphological features so, in reverse, it is possible to analyse the landscape to establish how it has been influenced by geological features and tectonic events (e.g. Ellis et al., 1999; Burrato et al., 2003; Vannoli et al., 2004; Delcaillau et al., 2006). Primary controls can have an influence at multiple scales: on entire landmasses, terrane emplacement, fault systems, folding and even microstructures. Tectonics may down-throw rocks, positioning them for deep burial and metamorphism; uplift them, exposing them to weathering and erosion; fracture and weaken them for further preferential weathering; move and deform them one or many times causing complex, multiple-scale changes to a rock’s strength, form and character. These changes may produce landscape features of varying magnitudes either as a direct result of the primary process, or a result of secondary processes acting upon the primary landforms. The product of primary controls at multiple-scales across a complex landscape is the primary focus of this research. The aim is to determine the extent to which geological control over the landscape is reflected in its morphology, by performing analyses of integrated multiple-scale geomorphological, geophysical and

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geological datasets using a Geographic Information System (GIS). In particular the work will establish whether underpinning structure or lithology correlate with the positions and orientations of drainage channels as opposed to stream placement being dominantly influenced by secondary processes. Scales analysed vary according to the types of features being investigated. At the finer scale, analysis has been performed on terrains developed on low-grade metamorphic rock that was originally turbiditic sediments, but which was later buried, deformed, uplifted and deformed further. On a broader scale, analysis was performed on the regional structure of an area that has been affected by both convergent and passive margin tectonism and which has been fractured, uplifted, in places down-thrown, and is composed of sedimentary, metamorphic and igneous rocks. The study also focuses on a variety of spatial scales from sub-catchment to multi-catchment regions. The drainage pattern analysis seeks to reveal drainage patterns that cannot be explained by presently defined geological structures but which, by the nature of their patterns, suggest a geological control. The research, therefore, may reveal the locations of previously unknown, unmapped or deep-seated geological structures that may have implications for planning infrastructural developments in the region. The literature review explains how primary and secondary processes lead to geomorphological change, and considers the relevance of geological control over geomorphology. The regional features of southeast Queensland and the finer-scale case study area of Laceys Creek are presented, together with a review of previous work in this branch of the geosciences both within this study area and globally. The review also discusses the methods involved in the current study in the context of methodologies applied elsewhere and of limitations of the available data. The review then considers the importance of identifying geological control where landscape alteration is being evaluated in relation to anthropogenic activity.

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INTRODUCTION TO GEOMORPHOLOGY Geomorphology, from the Greek ge, meaning earth and morfé meaning form, is the study of landforms including their evolution and origin. Geomorphological analysis provides an understanding of past surface-shaping processes and events and helps predict future landscape changes. Additionally, geomorphological studies are increasingly becoming a valuable supplementary method for identifying recent and current tectonic processes.

It has become a useful tool for a wide variety of

applications including land management (e.g. Gupta and Ahmad, 1999; Andreas and Allan, 2007), engineering of roads and dams (e.g. Seppala, 1999; Graf, 2005), location and management of water resources (e.g Thoms and Sheldon, 2002; Ghayoumian et al., 2007), geohazard analysis including evaluation of slope stability (e.g. Dominguez-Cuesta et al., 2007; Schulz, 2007; Kirby et al., 2008), and it has even

been

used

as

an

indicator

of

global

warming

(Goudie,

2006).

Geomorphological features such as mountains, hills, slopes, valleys, gullies, plateaux and drainage channels of all scales, typically form from the interaction of both endogenic and exogenic processes. The following section discusses these processes and their inter-relationships.

Surface processes: Weathering and erosion From an exogenic perspective, weathering and erosion (‘surface processes’) act upon uplifted, extruded, exposed or emplaced rock units. Alabyan and Chalov (1998) stated that channel development strongly relies upon water discharge and river slope and emphasised that the greater the stream power, the stronger the branching tendency of a river system. Nevertheless, there are other controls that may be equally important whether from an exogenic or endogenic perspective. Endogenically, for example, lithology and induration controls a unit’s erodibility. Weaknesses and differences in rock strength within a rock unit or between abutting or juxtaposed units, may lead to differential weathering causing physical surficial features to form at varying scales, such as gullies, channels and basins. A plane or line of weakness in a rock may be exploited by surface processes such as fluid flow. Then, as a preferential course of drainage, it may eventually erode to form

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a channel. Such channels may become widened or incised, retaining the original orientation, even though it may no longer rely on the original plane of weakness to be the preferred location of flow. Furthermore, the physical feature that originally caused preferential flow may no longer exist although the orientation and drainage pattern that it initiated may persist.

Channels, directly following geological

weaknesses or following ‘ghost’ features (structures now completely eroded), both represent manifestations of geological control of the landscape. The form and pattern of river channels is affected by a multitude of processes such as: changes in climate that may, for example, lead to increase or decrease in precipitation and vegetation cover; anthropogenically forced changes to ground cover; sea-level changes; and uplift and down-warp of the landscape. These factors are discussed below and although they are typically described as discrete processes, they frequently occur simultaneously. Early workers such as Gilbert (1917) recognized that a river channel may adjust its character following many types of disturbance to the land such as uplift, down-throw, tilting or warping, hence these factors have been the focus of fluvial geomorphological studies for many years. Alteration of these parameters may lead to adjustment of the stream’s longitudinal profile but may equally induce stream widening and downstream aggradation (Doyle and Harbor, 2003). Further incision of the landscape and backfilling of the upper reaches of channels may also ensue (Woolfe et al., 2000). Although erosion may occur over long periods of time, it may also occur suddenly and the causes of abrupt erosion events have been explored widely. For example, Thornes and Alcantara-Ayala (1998) investigated the main cause of mass hill-slope failure that occurs on metamorphic rocks in the mountains of southeast Spain. Anthropogenic activities had been suggested as the cause, although this was later discounted as the events were ‘unpredictable’. They concluded that mass failures depended on the slope material properties, topography, climate and hydrological interactions: where there appeared to be relatively poor resistance and impermeability in the metamorphic rocks (such as phyllites), mass movement was enhanced, both at shallow depths, within the regolith and deep-seated within the bedrock. However, in northern Spain, Calcaterra et al. (1998) noted that natural slopes failed less frequently than man-made slopes of similar angle in low-grade, weathered, metamorphic rocks, suggesting that the natural slopes were more likely to

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be closer to equilibrium and better able to withstand variations in weather patterns. In the French Riviera, where high-relief steep slopes with a large variety of rock types occur, shallow, first time landslips (i.e. those that are not repeated) were linked to long periods of heavy rain, and deeper-seated landslips were the result of longerperiod gravitational and tectonic effects coupled with weathering (Julian and Anthony, 1996). Two rain thresholds were identified as being necessary to destabilise slopes in the Himalayas during monsoonal rain – the rain thresholds being values of the seasonal accumulation plus the daily total (Gabet et al., 2004). Additionally, it was found that the slope angle controls the amount of daily total rainfall required to destabilise the slope; water storage determines the amount of seasonal rainfall required to destabilise and trigger slope failure. They concluded that thinner regolith on steeper slopes will fail faster than thicker regolith on gentler slopes. In southeast Queensland, Granger and Hayne (2000) reported that the most common trigger for slope failure in the region is an episode of intense rainfall and that antecedent rainfall may be of critical importance. Hoffmann et al. (1976) stated that this is particularly the cause for landslides on the steepest slopes. Granger and Hayne (2000) further stated that long, antecedent rainfall events are most relevant to deep-seated, slow moving landslides and that short, antecedent rainfall periods are relevant to shallow slips and shallow debris flows. Where natural forest cover has been removed, groundwater levels may rise significantly due to a reduction in transpiration. With the increase in groundwater levels, the pore pressure is also amplified leading to a reduction in shear strength of surface materials. In these circumstances, even minor rainfall events may be sufficient to cause failure of rock and/or the overlying soil horizon by raising pore pressure above critical levels. This has been found to occur on the cleared slopes of the Tertiary basalt plateaux and ranges of southeast Queensland where landslides are particularly common (Willmott, 1987). On the Maleny-Mapleton plateau for example, Willmott identified several types of slides including debris slides or flows on scarps and very steep slopes; small rotational slides or slumps on the moderate slopes; and also complex multiple rotational slides that affect broad areas up to 1 km in width. Although the latter type is slow moving, they are typically reactivated in extreme wet seasons. Willmott (1987) also described the basalt terrain as geologically sensitive, resulting from

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accumulations of unconsolidated debris that may be easily mobilised; alternating horizons of porous and massive basalts that direct groundwater flow outwards to the slopes; swelling clays within the soil and colluvium that lose strength on saturation; and the presence of soft sediments underlying the basalt, which themselves may fail. From his work, it is clear that surface or endogenic processes are responsible for shaping the landscape, and although the slopes are less stable due to previous deforestation or major rainfall events, the ultimate control of the landslides is typically geological. Rotational slides, both semi-circular and back-tilted, are evident in the Mount Mee area and sites underlain by the Neranleigh-Fernvale Beds through the Brisbane region. Some rotational slides in the area were found to have occurred on slopes as low as 11° (Granger and Hayne, 2000). Soil slumps were also observed on grassed slopes of 11 to 17° in areas underlain by greenschist. They concluded that there may be an increased susceptibility to landslips in the greenschist-derived soils and also colluvial soils derived from banded chert. Willmott and Surwitadiredja (2003) also confirmed that landslides in the Mount Mee area were primarily caused by groundwater seepage and the removal of forest cover on the deep soils that developed on the basalts and greenschists. A debris flow is evident in the western part of Pine Rivers where loose material has been mobilised by torrential rainfall on a steep mountain side (Granger and Hayne, 2000). Such events are fairly common on slopes of greater than 25° and particularly common on slopes that formerly supported rainforests on the Neranleigh-Fernvale Beds. Small landslips have also occurred on the bank of the South Pine River in Cenozoic sediments of the Petrie Formation. On the Bunya Phyllite, rockslides have been observed on the steep banks of the Brisbane River (Granger and Hayne, 2000). A large debris flow occurred in the Laceys Creek catchment, southeast Queensland, after heavy rains in January, 1974, a rainfall event that caused widespread flooding, ground saturation and many other events of masswasting in southeast Queensland. The debris flow in Laceys Creek consisted of completely weathered Bunya Phyllite and highly weathered Neranleigh-Fernvale Beds; although it was triggered by saturation, it was aided by ‘an intersecting system of a vertical faults and joints’ leaving a head scarp of 10 m height corresponding to the orientation of the fault and joints (Hofmann et al., 1976). Other landslides that were observed at this time, for example at Mt Nebo, Mt Mee and near Woodford

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were on varying slopes, but all were caused by ground saturation. However, the example at Laceys Creek demonstrates that lithological fabric and structure of the underlying rocks may strongly control mass-wasting episodes. Examples of some erosional features that have been identified in the Pine Rivers area of southeast Queensland are highlighted in Table 1. Angle

Description of erosion

Example in SEQ

of slope >25°

Small debris slides, rotational slumps, Neranleigh debris flows

Beds,

Fernvale

Brisbane

River

region, 17-25°

Rotational slumps in soil and colluvium on Neranleigh

Fernvale

concave slopes around gully heads (slumps Beds; Bunya Phyllite can also occur from 11° on greenschist derived or red colluvial soil) 20-25°g Small debris slides, rotational slumps or Rocksberg

Greenstone,

debris flows in deep pockets of soil or high country such as colluvium 11-25°

6000 year history, its current state suggests human activities surrounding the catchment have caused no adverse affects (Frank and Fielding, 2004). A review of major catchments of southeast Queensland, including the Bremer, Lockyer and Wivenhoe subcatchments (Caitcheon et al., 2005), identified the major sources of sediment to Moreton Bay and the lower Brisbane River. Using the SedNet modelling package, coupled with erosion process tracing, the results identified gully and stream bank erosion, and also hillslope erosion from both grazing or cultivated lands. From their previous analysis, they identified that soil from forests was deemed to be similar to grazing soil. The majority of sediment that reached Moreton Bay, originated from the Lockyer catchment and a smaller but substantial amount of sediment came from the Bremer catchment. Little sediment that originated north of Wivenhoe and Somerset dams reached the mid-Brisbane

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River, probably due to the sediment being captured by the lakes. In the Wivenhoe catchment, sediment was mostly sourced from the lower to middle reaches of Kilcoy Creek primarily as a result of stream bank and gully erosion, although hillslope erosion was predicted as the dominant sediment source in western tributaries of Kilcoy Creek. In the upper Brisbane River, sediment was mainly sourced from grazed hillslopes, whereas downstream of the Lockyer, in the lower Brisbane River, sediment was primarily derived from channel erosion. The results in the Lockyer catchment were less conclusive although they suggested that upper Lockyer sources were mainly eroded grazing land, and the lower Lockyer was a combination of channel erosion and cultivated land erosion. Results from the lower Bremer suggested the dominant sediment source is channel erosion with smaller amounts of sediment derived from cultivated soils. Gully formation, [defined by Caitcheon et al. (2005) as ‘incision of valley floor alluvium since European settlement’ p8] and gully alteration over the past 150 years, were also analysed. Caitcheon et al. concluded that most gully erosion occurs at a slower rate now than when the gully networks originally developed, having potentially reached an approximate state of equilibrium. Gully erosion, as with riverbank erosion, continue to add to bed-load and suspended sediment supply, whereas hillslope sources only supply sediment to the suspended load budget although some new gullies develop in these areas, particularly during times of flood and increased overland flow rates.

The role of lithology and rock fabric in geomorphology Crustal deformation may be instantaneous or may take place over millions of years. Where the crust is weakest, deformation may occur due to compressional or extensional stress often caused by the movement of tectonic plates (Figs. 1,2 and 3). Nevertheless, whether movement is gradual or sudden, each motion can contribute to the generation of various surface features such as gently undulating hills or sheer scarps, although not all tectonic movements produce surficial expressions. The theory of plate tectonics has been dominant since the 1960’s and its processes can explain diverse styles of deformation and movement of Earth’s crust (e.g. Dietz, 1961; Hess, 1962). Through plate motion and the associated crustal stress, landscape features such as fault systems, orogenic belts, hills and valleys may form, which are 30

altered further by surficial processes including weathering and erosion, concurrently acting upon them. The mineralogical composition and organization of rocks dictates their susceptibility to weathering and erosion; marl or clay-rich beds may erode faster than indurated quartzose sandstone, for example, and if juxtaposed, this will be reflected in physiographic differences in the landscape (Fig. 4). Most major mountain ranges such as the Pyrenees in Spain (Fig. 4), show examples of slope variations that have been caused by the differential weathering of distinct sediment types. Sedimentary rocks typically contain many beds of different lithology caused by the sorting of sediment type and size during deposition. This can lead to dissimilarity in rock strength and chemical stability between beds. When exposed or close to the surface, the variable competence layers will be subjected to differential weathering. Regions with breached folds in sedimentary rock typically show strongly surficial expression of differential weathering of lithologies from whole landscape to outcrop scale (Figure 5). Igneous rocks may also vary in strength depending on their mineralogy. Often intruded into rocks of a different strength, they may weather more slowly than the surrounding rock. Excellent examples of this are expressed in southeast Queensland where the outer flanks of mid-Cenozoic felsic volcanoes have eroded, and the more resistant volcanic necks now remain as the Glass House Mountains (Figure 6) and plugs of the Mt Alford region. Alternatively, the intruded igneous body may erode more quickly than the surrounding rock, forming a basin in the landscape. An example of this type is the bowl-shaped Samford Valley in southeast Queensland, which is formed on the strongly weathered material of the Samford Granodiorite surrounded by hills developed on a more resistant thermally metamorphosed aureole in the Neranleigh-Fernvale Beds (Figure 7). Minerals within a rock may alter over time if heat, temperature and pressure are changed after its initial crystallisation or deposition. Where temperature and/or pressure are increased, mineral alteration may lead to a preferred orientation of minerals, such as mica, and the development of foliation. Foliation is best developed in regionally metamorphosed rocks; this is a planar fabric caused by the parallel alignment of crystals leading to a slatey, phyllitic, schistose or gneissose texture or cleavage. Stress and associated strain may also lead to physical alteration in the structure of the rock causing folds and discontinuities such as faults, shear zones and joints, which

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may occur in association with one another even at the microscale. In folded rocks, for example, weaknesses may occur due to tension within the rock-fabric, allowing weathering to exploit these zones. This will present natural planes of weakness that, if eroded, will expose other rocks that may weather differentially. The fractures, cleavage, joints, preferred mineral alignment or fault gouge displayed by a rock are caused by a specific orientation of stress or folding. This may cause repeated and parallel planes of weakness in rocks. Bedding and other sedimentary features may also result in repeated and aligned fabrics. Such bedding and structural features are commonly exploited by differential erosion to produce step- and ridge-like patterns in the landscape (Figure 8). These rock patterns can occur in areas ranging from outcrop to continental scale. Where streams incise to bedrock, they commonly follow bedding and fracture patterns and become deflected along these discontinuities to generate ‘anomalous’ drainage patterns (for example Holbrook and Schumm, 1999). Such patterns are typically expressed by parallel, conjugate, radial concentric or other geometric arrangements of streams, gullies, hill slopes and scarps. Recognition of anomalous drainage patterns is typically a strong index of geological control rather than exogenic or regolith control of stream flow. Surface processes take advantage of the geology to form the landscape’s morphology, but the geology itself is potentially a product of numerous events and tectonic settings. The morphology of some regions may display patterns that were controlled by a previous terrain or rock unit that has since been eroded or altered. In such a case, the remnant morphology still shows there was an original geological control, but to ascertain whether control is recent or ancient, further examination of the geomorphology is required. This involves study of the morphology at varying scales, such as that being undertaken in this research, in order to identify the longterm geological history of a region and the degree and extent to which drainage is being geologically controlled. Stream orientation and control can be used to identify the timing of events. For example, large, aligned streams that both meander and cut across lineaments may provide evidence that the stream orientation was caused by an ancient control no longer present, and that surface processes have since taken over its morphological development. Alternatively, where many low-order streams are found to change direction, this might suggest fairly recent uplift. Drainage patterns will respond to

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changes both in topography and base level imposed by uplift and subsidence (Burbank et al., 1996). Changes may include adjustments to channel gradient and width, sinuosity, bed load grainsize and extent of alluvial cover, and bed morphology and roughness (Whipple, 2004). A stream’s orientation may be an indicator that changes have been constrained by an endogenic factor as outlined above. Where 1st order or other low order streams are aligned with others of similar and higher orders, or aligned with visible and underlying rock fabric, it generally indicates an existing or ongoing geological control. Therefore, study of these features at multiple scales is important to understand whether the control is recent or ancient. Depending on the exposed surface and angle of dip of the planes of weakness, the resulting eroded surface may have broad scale implications: for example, the River Torrens in the Mt Lofty Ranges, east of Adelaide is strongly controlled by phyllitic and gneissic cleavage (Twidale, 2004). Planes of weakness resulting from faulting and jointing can be exploited by precipitation seepage. Fluid flow within and over this type of weakened rock may lead to erosion and widening of such planes, providing preferential conduits for further fluid flow. The courses of the Rhône and Rhine rivers in the Alps for example, are trapped by faults, the planes of which display lower erosional resistance than adjacent lithologies. This, combined with enhanced discharge of the rivers and low erosional resistance of their bedrocks probably increased surface erosion relative to neighbouring areas (Schlunegger and Hinderer, 2001). Geological features such as faults, fractures and lithological differences are rarely visible in a continuous manner across large regions due to regolith cover and in cases where groundcover is dense, such as in southeast Queensland, inference of underlying control must be made based on what can be seen and measured. If geological fabric is known in only part of a region, and alignment of that fabric with morphological features is evident, the implication may be that the same geological control exists across the broader region containing that pattern of morphological alignment or repetition.

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Figure 1 Simple schematic cartoon to illustrate some structural geology features that may control landforms in a compressional regime

Figure 2 Faulting that may occur in an extensional regime

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Figure 3 Folded sandstones in a sheer cliff face, Sandgate, Queensland

Figure 4 Differential weathering causing slope variations, Pyrenees, Spain

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Figure 5 Faulted and folded sedimentary rocks displaying differential weathering, Shorncliffe, southeast Queensland

Figure 6 The Glasshouse Mountains, southeast Queensland. The outer part of the volcanoes and the surrounding rocks have been eroded leaving the volcanic plugs protruding from the landscape. (Photograph courtesy of David Hodgkinson)

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Figure 7 Aerial view of Samford Valley, southeast Queensland. Samford is situated on an eroded granitic batholith that intruded the regionally metamorphosed Neranleigh-Fernvale Beds. The Samford Granodiorite has eroded preferentially, and has formed a basin surrounded by a thermally metamorphosed aureole developed within the Neranleigh-Fernvale Beds. (Photograph – Google Earth)

Figure 8 Folded strata under compressive stress may form microstructures within the fabric. Extension within the crests of antiformal folds may cause joints and fractures to form which could increase weathering in these zones

Bedrock lithology may influence stream behaviour such as the orientation of flow, degree of meandering and anastamosing, and the magnitude of down cutting. Fine-scale structures such as phyllitic cleavage have been identified as controlling large streams (for example Holbrook and Schumm, 1999; Twidale, 2004) where the rock type is ‘soft’ or weakened and more susceptible to erosion allowing incision, or

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where the rock types are more resistant and force the direction of flow. One aim of this research is to identify whether such a control is exerted over streams at multiplescales, from first order through to major channels.

The role of tectonics and major geological events in geomorphology The influence of structural control over surface features including stream patterns has been recognised as an important asset for better understanding of the geology and structure of large areas (Hills, 1960). Tectonics is a widely accepted cause of geomorphological change, and acts as a primary control upon the shape of the landscape (e.g. Ollier, 1995; Burrato et al., 2003; Vannoli et al., 2004; Delcaillau et al., 2006). The geomorphological approach has been used to analyse and better understand the effects of recent tectonism in several areas (for example Oguchi et al., 2003; Palyvos et al., 2006). In Greece, for example, geomorphology and drainage patterns have been used to identify active normal fault evolution (Goldsworthy and Jackson, 2000). Schlunegger and Hinderer (2001) examined the potential geological controls upon the anomalous drainage patterns in the Alps and concluded that enhanced rates of crustal uplift, associated with frequent small earthquake events were responsible. Drainage pattern anomalies have been used in the Turkana Rift, north Kenya, as ‘key-markers’ to establish the location of large-scale transverse fault zones (Vétel et al., 2004). Barbed drainage such as that seen in the Cowan River, Western Australia (Clarke, 1994) and the Clarence River, New South Wales (Haworth and Ollier, 1992) are distinctive evidence of warping or uplift of the landscape. Drainage is known to commonly follow the orientation and plane of faults that provide a natural channel as outlined above. Faults caused by high magnitude earthquake events may cause large-scale surface ruptures providing a natural location for preferred fluid flow. However, small faults and fractures can also be the site of preferred overland flow. Joints and faults resulting from even minor (low magnitude) earthquake events, may occur in repeating alignment, as stress orientation is typically relatively stable for long periods of time. Small, aligned faults and fractures may eventually merge where fault tips slowly migrate, lengthening the faults and changing the stress dynamics (as discussed in more detail below). Coalescence of faults in this way may lead to the appearance of large faults which may simply 38

represent a composite system of smaller faults. Therefore, even very small fractures caused by low magnitude earthquake events can, over time, provide a preferred position for overland flow. Attraction of further stream flow along the plane will eventually lead to deepening and widening of a channel. Cowie and Roberts (2001) presented a conceptual model showing multiple stages in fault-growth. Initially, an array of small faults forms across a region and slowly extends in length and throw. As this continues, the faults interact and the overall fault-array profile changes. The displacement-to-length ratios increase over time and when multiple faults have joined with others, central portions of the fault will eventually have greater throw than the distal portions. The temporal and spatial variations of movement along faults and fault arrays may be explained using the noncharacteristic earthquake model, first proposed by Roberts (1996b), which provides a model that not only accounts for spatial variations in cumulative throw but also for ruptures that are shorter than the host fault segment. The model also implies that recurrence intervals vary temporally for an individual locality. Roberts concluded that palaeoseismological evidence from one site along a fault segment should not be used to imply earthquake recurrence at another on the same fault. Roberts and Minchetti (2004) and Roberts et al. (2004), working in the Lazio-Abruzzo Appennines, central Italy, further showed that interactions of multiple fractures along a fault segment are complex, but knowledge of scaling relationships between the fault throws and lengths may assist prediction of throw-rates and with it seismic hazard. The ratio of maximum displacement to length on a fault is an important characteristic for assessing slip rates and for future earthquake prediction (Cowie and Roberts, 2001). The fact that many faults grow by the connection of smaller fault segments (Peacock and Sanderson, 1991; Roberts, 1996a) is an important consideration in a region where low magnitude earthquakes are most typical; numerous small scale earthquakes over time may not directly be a geohazard, but could play a large part in the evolution of landscape morphology. Some earth movements may be slow and steady, leading to potentially large displacements over time. Slow earthquakes have occurred in many regions. They are defined in the literature as discontinuous events that release energy over long periods of up to several months, unlike typical earthquakes that may release energy in just seconds or minutes (e.g. Kanamori and Stewart, 1979; Linde et al., 1996). These events may be

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accompanied by earth tremor and may be detected at some very low frequencies. They may provide a link between shallow and deep crustal events. Aseismic creep and slow earthquakes may lead to earth movements equivalent to moderate or large earthquake magnitudes and on such faults, seismicity may account for as little as 2% of total moment release (Amelung and King, 1997; Scholz, 2002). However, in order to resolve aseismic creep, an accurate background deformation rate must be measured to identify regions of anomalously high deformation rates (e.g. Linde et al., 1996; Kitagawa et al., 2006) and background deformation rates have not yet been calculated for southeast Queensland. Earthquakes of > M 5 (M 5.6 for example) are known to cause ground surface displacement by nearly 10 m (for example, Fort Sage Mountains, CA., USA, 1950 cited in Wells and Coppersmith 1994, p.976) and earthquakes of M 7 or greater have caused surface displacements of 10’s or hundreds of kilometres in length (for example Luzon, Phillipines, M 7.8, 1990, surface rupture length 120 km, Wells and Coppersmith, 1994, p.981). It is generally considered that low magnitude events (M

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