No abstract

[1] Deformed fluvial terraces preserved over active thrust-related folds record the kinematics of folding as fault slip accumulates on the underlying thrust. In the Mackenzie Basin of southern New Zealand, the kinematics revealed by folded fluvial terraces along the active Ostler and Irishman Creek fault zones are inconsistent with traditional models for thrust-related folding in which spatially uniform rock uplift typically occurs over planar fault ramps. Instead, warped and tilted terraces in the Mackenzie are characterized by broad, continuous backlimbs and abrupt forelimbs and suggest folding through progressive limb rotation. By relating this pattern of surface deformation to the underlying thrust with a newly developed, simple geometric and kinematic model, we interpret both faults as listric thrusts rooted at depth into gently dipping planar fault ramps. Constraints on the model from detailed topographic surveying of deformed terraces, ground-penetrating radar over active fault scarps, and luminescence dating of terrace surfaces suggest slip rates for the Ostler and Irishman Creek faults of $1.1-1.7 mm/yr and $0.5-0.7 mm/yr, respectively. The predicted depth of listric faulting for the Ostler fault (0.7 À0.2 +0.1 km) and the Irishman Creek fault (1.3 À0.5 +0.1 km) generally agrees with geophysical estimates of basin depth in the Mackenzie and suggests control of preexisting basin architecture on the geometry of active thrusting. Despite the potential effects of changes in fault curvature and hanging wall internal deformation, the methodology presented here provides a simple tool for approximating the kinematics of surface deformation associated with slip along listric, or curviplanar, thrust faults.

Geophysical surveys and chemical analyses on cores were carried out in three Ontario peatlands, from which we have gained a better understanding of the peat properties that control the geophysical responses. The electrical conductivity depends linearly on the concentration of total dissolved solids in the peat pore waters and the pore waters in turn bear the ionic signatures of the underlying mineral sediments. The ionic concentration, and thus the electrical conductivity, increase linearly from the surface to basement. The average bulk electrical conductivity of peatlands at Ellice Marsh, near Stratford, and at Wally Creek Area Forest Drainage Project, near Cochrane, are of the order of 25 mS/m. The Mer Bleue peatland, near Ottawa, has extremely high electrical conductivity, reaching levels of up to 380 mS/m near the base of the peat. The Mer Bleue peatland water has correspondingly high values of total dissolved solids, which originate from the underlying Champlain Sea glaciomarine clays. The dielectric permittivity in peats is largely controlled by the bulk water content. Ground penetrating radar can detect changes in water content greater than 3%, occurring within a depth interval less than 15 cm. The principal peatland interfaces detected are the near-surface aerobic to anaerobic transition and the peat to mineral basement contact. The potential for the successful detection of the basement contact using the radar can be predicted using the radar instrument specifications, estimates of the peatland depth, and either the bulk peat or the peat pore water electrical conductivities. Predicted depths of penetration of up to 10 m for Ellice Marsh and Wally Creek exceed the observed depths of 1 to 2 m. At Mer Bleue, on the other hand, we observe that, as predicted, a 100 MHz signal will penetrate to the base of a 2 m thick peat but a 200 MHz signal will not. In trod uct ionA variety of geophysical methods have been used on pe penetrating radar (GPR), resistivity, and electrom veying. The full potential of these techniques remains because our understanding of the physical factors that control the instrument response in peatlands is incomplete. The water table is normally at, or slightly below, the ground surface; the peat thickness can range from 40 cm to 10 m (e.g. Remotec 1982; Welsby 1988). In bulk, peat is mostly water, with a supporting matrix made of purely organic materials. The organic materials are t decomposed remains of plants, which can take millennia to accumulate of peat. Internal stratigraphic features and thicknesses of peatlands can vary dramatically with little predictability on the basis of surface vegetation.We discuss the use of geophysics, alone and as a supplement to conventional coring programs. We have investigated three peatland sites (Figs 1 4 , and have carried out comparisons of the electrical property variations and of the haat p h p ical properties that may be detected by geophysical method$, from sit$ rn iiim within individual pearlands. We also suggest a systematic protocol for ths ge...

Three-dimensional ground-penetrating radar ͑GPR͒ data are routinely acquired for diverse geologic, hydrogeologic, archeological, and civil engineering purposes. Interpretations of these data are invariably based on subjective analyses of reflection patterns. Such analyses are heavily dependent on interpreter expertise and experience. Using data acquired across gravel units overlying the Alpine Fault Zone in New Zealand, we demonstrate the utility of various geometric attributes in reducing the subjectivity of 3D GPR data analysis. We use a coherence-based technique to compute the coherency, azimuth, and dip attributes and a graylevel co-occurrence matrix ͑GLCM͒ method to compute the texture-based energy, entropy, homogeneity, and contrast attributes. 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 and tectonic structures at the study site.Anew depositional/structural model largely based on the results of our analysis of GPR attributes 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 gravels that could have been affected by lateral fault drag. When used 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 active fault zones.