Jocelyn K. Campbell

Visualization of active faults using geometric attributes of 3D GPR data: An example from the Alpine Fault Zone, New Zealand

Three-dimensional ground-penetrating radar GPR data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these dataareinvariablybasedonsubjectiveanalysesofreflectionpatterns. Such analyses are heavily dependent on interpreter expertiseandexperience.UsingdataacquiredacrossgravelunitsoverlyingtheAlpineFaultZoneinNewZealand,wedemonstratethe utilityofvariousgeometricattributesinreducingthesubjectivity of3DGPRdataanalysis.Weuseacoherence-basedtechniqueto compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrixGLCMmethod to compute the texture-basedenergy,entropy,homogeneity,andcontrastattributes. A selection of the GPR attribute volumes allows us to highlight key aspects of the fault zone and observe important features not apparent in the standard images. This selection also provides information that improves our understanding of gravel deposition andtectonicstructuresatthestudysite.Anewdepositional/structuralmodellargelybasedontheresultsofouranalysisofGPRattributes includes four distinct gravel units deposited in three phases and a well-defined fault trace. This fault trace coincides with a zone of stratal disruption and shearing bound on one side by upward-tilted to synclinally folded stratified gravels and on the other side by moderately dipping stratified alluvial-fan gravelsthatcouldhavebeenaffectedbylateralfaultdrag.Whenused in tandem, the coherence- and texture-based attribute volumes can significantly improve the efficiency and quality of 3D GPR interpretation, especially for complex data collected across activefaultzones.

Holocene paleoearthquakes on the strike‐slip Porters Pass Fault, Canterbury, New Zealand

Abstract The Porters Pass Fault comprises a series of discontinuous Holocene active traces which extend for c. 40 km between the Rakaia and Waimakariri Rivers in the foothills of the Southern Alps. There have been no historical earthquakes on the Porters Pass Fault (i.e., within the last 150 yr), and the purpose of this paper is to establish the timing and magnitudes of displacements on the fault at the ground surface during Holocene paleoearthquakes. Displaced geomorphic features (e.g., relict streams, stream channels, and ridge crests), measured using either tape measure (n = 20) or surveying equipment (n = 5), range from 5.5 to 33 m right lateral strike slip and are consistent with six earthquakes characterised by slip per event of c. 5–7 m. The timing of these earthquakes is constrained by radiocarbon dates from four trenches excavated across the fault and two auger sites from within swamps produced by ponding of drainage along the fault scarp. These data indicate markedly different Holocene earthquake histories along the fault length separated by a behavioural segment boundary near Lake Coleridge. On the eastern segment at least six Holocene earthquakes were identified at 8400–9000, 5700–6700, 4500–6000, 2300–2500, 800–1100, and 500–600 yr BP, producing an average recurrence interval of c. 1500 yr. On the western segment of the fault in the Rakaia River valley, a single surface‐rupturing earthquake displaced Acheron Advance glacial deposits (c. 10 000–14 000 yr in age) and may represent the southward continuation of the 2300–2500 yr event identified on the eastern segment. These data suggest Holocene slip rates of 3.2–4.1 mm/yr and 0.3–0.9 mm/yr on the eastern and western sections of the fault, respectively. Displacement and timing data suggest that earthquakes ruptured the western segment of the fault in no more than one‐sixth of cases and that for a sample period of 10 000 yr the recurrence intervals were not characteristic.

Triassic tube fossils from Tuapeka Rocks, Akatore, South Otago

Abstract Agglutinated tube fossils, Titahia corrugata Webby and Torlessia sp. are preserved in separate zones within a Tuapeka Group sequence of greywacke, argillite, spilitechert at Akatore, South Otago. The genera are recorded from Torlesse Supergroup sequences of similar facies in Canterbury and Wellington where their presence has been taken to indicate Triassic, probably Balfour Series, age. A Triassic age for part of the Tuapeka Group is indicated and, for east Otago at least, the generally accepted simple progression of age from Murihiku Supergroup sediments to Haast Schist is precluded.

Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault

Abstract Prior to the 2010–2011 earthquake sequence, several fault and fold structures were mapped beneath the Canterbury Plains using seismic reflection surveys and surface observations and depicted on the Christchurch and Aoraki 1:250,000 scale geological maps. Localised grabens associated with east-southeast-striking normal faults formed largely during the Late Cretaceous. South of Rakaia River, some graben-bounding faults show minor normal offset extending into the late Cenozoic. Near Ashley River, proximity to a Late Cretaceous–Paleogene graben suggests that the active, predominantly contractional, east-striking Ashley Fault is at least in part a rejuvenated pre-existing normal fault. The easterly strike of the previously unknown Greendale Fault implies that it too may be a reactivated Late Cretaceous fault. Northeast-striking, southeast-facing reverse faults and fault-propagation folds beneath the western and northern parts of the plains are primarily late Cenozoic features. Variation in the distributions of Miocene sedimentary strata strongly suggests that contractional faulting was initiated as early as the Miocene. The overall late Cenozoic tectonic pattern is extension beneath the southern Canterbury Plains and contraction farther north.

The tectonic and structural setting of the 4 September 2010 Darfield (Canterbury) earthquake sequence, New Zealand

Abstract Plate boundary deformation creates a south-easterly advancing, repetitive structural pattern in Canterbury dominated by the propagation of northeast-striking thrust assemblages. This pattern is regularly segmented by east-striking faults inherited from reactivated Cretaceous normal faults. The more evolved and deeply exposed structures in the foothills of north Canterbury provide insights into the tectonic processes of the blind structures now emerging from under the northern and eastern Canterbury Plains, where thrust and strike-slip fault activity are closely linked. The east-striking faults separate relative motion between thrust segments and accommodate oblique transpressive shear. Early stages of thrust emergence are dominated by anticlinal growth and blind, or partially buried, thrusts and backthrusts. The east-striking transecting faults therefore record timing of coseismic episodes of uplift and shortening with variable horizontal to vertical ratios and displacement rates on the hidden adjacent thrusts. The Greendale and blind Port Hills faults, with their associated aftershock patterns, are compatible with this style.

Structural evolution and landscape development of a collapsed transpressive duplex on the Hope Fault, North Canterbury, New Zealand

Abstract This study examined the transpressional Conway segment of the Hope Fault in North Canterbury, the fastest moving fault of the Marlborough Fault Zone in northern South Island of New Zealand, in an attempt to reconstruct, via air photograph interpretation, detailed field mapping, and theoretical constraints, the styles of structural geometry of the late Quaternary deformation. We relate the evolving landscape to the development and modification of this fault in an active tectonic setting. The section of the fault zone studied is a 13 km long, 1.3 km wide, asymmetric transpressional reverse fault duplex, bounded to the southwest by the Lottery River and to the northeast by the Mason River, which tapers down to a single fault trace to the northeast. Between the bounding faults of the duplex are approximately 100 subsidiary fault scarps that initially formed an imbricate set of footwall propagating reverse/thrust faults. These faults became inactive and topographically unsupported when younger footwall propagating reverse faults migrated northeast along the main trace of the Hope Fault as the duplex also migrated to the northeast. The unsupported duplex structurally collapsed back into the dilating fault zone, causing reversal of slip on the imbricate reverse faults so that they became normal faults. As the duplex collapsed, the adjacent hanging wall was uplifted triggering landsliding, rapid incision by streams, formation of large alluvial fans, and minor normal fault gravity collapse structures. The footwall block outside the duplex became rapidly incised by streams and experienced widespread topographic slumping and associated structurally controlled, shallow‐level ridge renting. Stream dissection of flights of late Pleistocene aggradation/degradation surfaces also occurred at this time, leaving remnant flat‐topped hills between the dissected and slumped valleys. Our analysis of the interrelationships between the structural geology and the landscape over time is a new approach for the Hope Fault and reconfirms the necessity of integrating detailed structural geology with geomorphology in areas of active tectonism.

Recognition of active reverse faults and folds in North Canterbury, New Zealand, using structural mapping and geomorphic analysis

Abstract In areas of limited geological exposure and discontinuous marker beds, many faults or parts of faults are difficult to detect and their displacement and strike length, for example, hard to quantify. Here, we use structure contouring of a partly exhumed basement unconformity surface together with stream gradient and sinuosity indices to help resolve moderate sized faults (<100 m throws) and folds that were either not detected or only poorly delineated by either structural or geomorphic mapping. Data are from active reverse faults and associated folds in North Canterbury, at the outer edge of the Australia‐Pacific plate boundary collision zone in New Zealand. Structural mapping defines three major, parallel, ENE‐striking reverse fault systems, with associated folds and vertical displacements of 100–950 m. In addition to showing greater trace lengths and connectivity of the main faults, structure contours and stream analyses highlight six faults with throws of <100 m, which were previously unrecognised. These faults accounted for <15% of the total vertical displacement across the region of study. Conventional mapping, therefore, permits the identification of the principal faults, while geomorphic analysis provides a more complete understanding of the locations and displacements of faults in the system. In addition, geomorphic analysis provides a means of identifying low slip‐rate structures (e.g., <0.2 mm/yr) which have experienced Quaternary activity.

Structural collapse of a transpressive hanging-wall fault wedge, Charwell region of the Hope Fault, South Island, New Zealand

Abstract The northeast‐trending dextral‐reverse oblique slip Hope Fault is one of the major structures of the Marlborough Fault System and the Australia‐Pacific plate boundary zone in the South Island of New Zealand. This study presents an analysis of the structural and tectonic geomorphic development of the Hope Fault Zone in the vicinity of the Charwell River to understand the near‐surface temporal and spatial structural style of deformation and fault zone kinematics along a 10 km section of the fault. Significant fault‐related landscape units include: (1) flights of aggradation‐degradation terraces in the footwall forming an extensive piedmont; (2) fault‐dissected, sloping topography in the hanging wall containing 95% of all the faults; and (3) eroded subhorizontal piedmont terrace remnants that indicate the range front has repeatedly propagated to the southeast into the footwall block. We recognise four distinct types of fault scarps: (1) the main range‐front trace of the Hope Fault defines a releasing bend geometry, with a projected step‐over width of c. 1000 m; (2) at the foot of the range front, two thrust faults ramp over the aggradational surfaces in the footwall block; (3) in the toe of the hanging‐wall block, c. 20 normal fault scarps are mapped near‐parallel to the main Hope Fault; and (4) more than 100 late normal faults are oblique to the main Hope Fault and cut obliquely across all other faults. The overall fault pattern outlines an initial fault wedge between the thrust and early normal faults that is 5 km in length and 1 km at its widest point. A secondary wedge defined by the late normal faults is 7 km in length, 2 km at its widest, and it overprints the initial wedge. The structural/geomorphic interactions between the initial and secondary fault wedges developed in a series of at least four successive stages. These spatial and temporal changes in the structural geometry and style of deformation of the Hope Fault Zone at Charwell are interpreted to reflect the role of topographic loading in adjusting the fault zone break‐out along the range front, and is recorded by the tectonic geomorphic landforms there.

The impact of episodic fault‐related folding on late Holocene degradation terraces along Waipara River, New Zealand

Abstract The Waipara River flows eastwards through growing folds in the tectonically active foothills of New Zealands Southern Alps. In the middle Waipara region, flights of degradation terraces are widespread and rise to 55 m above river channels. Ages of terrace surfaces and paleoearthquakes on four faults are constrained by radiocarbon samples and weathering‐rind dates from surface cobbles of Torl esse Group sandstone. Terrace ages indicate rapid incision (c. 30–100 mm/yr) of Waipara River and three tributaries during the late Holocene. Cumulative‐incision curves suggest a 15–25 m lowering of regional base level over the last thousand years and an additional 20–25 m of local incision 200–600 yr BP along Waipara River where it crosses Doctors Anticline. Rapid river incision was strongly influenced by rock uplift on the anticline associated with fault rupture during an earthquake 300–400 yr BP. From incision data we infer that the earthquake was preceded and followed by aseismic fold growth. Tectonic uplift during folding was probably, at most, one‐third of local river incision; this discrepancy may relate to the short sample period and to locally elevated stream erosive power due in part to a reduction in floodplain width.

Late Cenozoic thrust tectonics, Picton, New Zealand

Abstract In the Marlborough Sounds area near Picton. multiple deformation during the Miocene produced early (D1 north-northeast-trending folds, and north- and east-striking thrusts. Later folding (D2 about east-west axes led to the formation of basin and dome interference structures. During D1, Mesozoic rocks of the Marlborough Schist and Permian-Triassic Pelorus Group were thrust over an Oligocene sedimentary sequence. which is now exposed as erosional inliers. Thrust transport from west to east is inferred from fibre striations. Striation-derived M-axis data, calcite grain fabric studies, and the maximwn shortening direction of north-northeast-trending folds suggest that thrusting developed in association with approximate west-east compression. Thrust motion resulted in progressive late Cenozoic stacking of the basement sequence in this area, with considerable shortening and thickening of the Marlborough Sounds region.