ALBERT

Description: This study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long-lived subduction wedge. We provide a synthesis of the geology of the South Island of New Zealand, with an emphasis on a wedge tectonics perspective that contrasts with a traditional view that interprets New Zealand geology in the context of terrane collision and accretion. We argue that the Otago subduction wedge evolved in a steady fashion throughout its 290 to 105 Ma history in response to accretion of trench-fill and abyssal-plain sediments, and slow erosion of a subaerially-exposed forearc high. Maximum temperatures for rocks in the flanks of the forearc high were no greater than 150 to 300 {degrees}C, with solution mass-transfer active as the dominant ductile mechanism. The 54 studied samples provide information about the absolute ductile strains acquired all along their flow-path, from the site of accretion to exhumation in the forearc high. We use tensor-averages to estimate strain at a regional scale. These show plane-strain uniaxial flattening, given that the tensor-averages for Sy and Sx are close to one. On average, Sz is approximately 0.77, and this is balanced by a mass loss of about 23 percent. The average Z direction is sub-horizontal in the prowedge and moderately plunging in the retrowedge, a difference attributed to spatial variations in the mode of accretion. We infer that rocks presently in the pro-side of the Otago high were sourced by frontal accretion, and those in the retro-side were underplated. This result highlights the important role of accretion in determining the style of within-wedge deformation, and also demonstrates the benefit of using a tensor-averaging approach to examine regional strain.

Description: We investigate the influence of stress conditions during fracture formation on the geometry and roughness of fracture surfaces. Rough fracture surfaces have been generated in numerical simulations of triaxial deformation experiments using the discrete element method and in a small number of laboratory experiments on limestone and sandstone samples. Digital surface models of the rock samples fractured in the laboratory experiments were produced using high-resolution photogrammetry. The roughness of the surfaces was analyzed in terms of absolute roughness measures such as an estimated joint roughness coefficient (JRC) and in terms of its scaling properties. The results show that all analyzed surfaces are self-affine but with different Hurst exponents between the numerical models and the real rock samples. Results from numerical simulations using a wide range of stress conditions to generate the fracture surfaces show a weak decrease of the Hurst exponents with increasing confining stress and a larger absolute roughness for transversely isotropic stress conditions compared to true triaxial conditions. Other than that, our results suggest that stress conditions have little influence on the surface roughness of newly formed fractures.

Description: Geology as a science has an important visual component and the knowledge of any geologist is deeply linked to visual experience of rock outcrops, thin sections and analytical images. One of the shortcomings of most geological images such as maps, cross sections and outcrop photographs is that they are 2D, while processes geologists are interested in are typically occurring in 3D space. The 3D geometry of faults, fractures and joints is crucial to quantify geological processes related to fracture mechanics, such as hydrothermal mineralization and ground water flow, but also geotechnical problems such as rock mass stability. A number of studies have shown that some geological structures can be described with a scale invariant, fractal distribution. So far these observations on which these findings are based were restricted to one and two dimensions and has been difficult to obtain a full spatial geometric picture of fracture sets from rock outcrops, because much of the rock is not directly accessible. However, without taking into account the spatial distribution of geological structures the true geometry of joint patterns cannot be fully described and scaling laws, fractal or not, cannot be derived. We present images of joint patterns based on datasets acquired by digital photographs which are processed to three dimensional images using the photogrammetry software Siro3D. This technique allows to obtain a highly accurate 3D picture of the visible outcrop. The spatial pattern of joints in nature is investigated using the software SiroJoint. For the analysis of joint systems a large data set was collected from the Heavitree Quarzite at Ormiston Gorge, near Alice Springs. The Heavitree Quartzite is fragmented by a spectacularly regular three-dimensional joint pattern, which is repeated at different scales and therefore represents a perfect laboratory for our investigations (Hobbs 1993). Siro3D generates a spatially fully referenced 3D image from overlapping digital images, such that each pixel of the image is assigned spatial coordinates. The software SiroJoint routinely constructs planes from the intersection of the rock-face with the linear trace of planar features (Poropat 2001). It provides stereographic plots of structural elements and additionally measures joint persistence, area, and joint spacing. Our measurements allow to analyse geometrical scaling relationships of joint sets with high accuracy and will help explore the character of their 3D complexity. Several hundred joint planes were defined with SiroJoint in an Ormiston Gorge outcrop. Three different joint sets can be distinguished. Joint set one and two are characterized by steeply inclined planes with joint spacings ranging between 2 cm to 40 cm and 2 cm to 10m respectively. Both joints sets depict a power law distribution in joint spacing/frequency plots. The third set is defined by a subhorizontal orientation. It shows a very regular spacing in the meter scale and lacks an exponential distribution. We intend to use the results as a basis to compare observed fracture pattern with those generated by computational methods like Iterated Function Systems. This might help to understand how physical rock properties influence the spatial complexity of fracture systems and develop constitutive scaling relationships for certain rock types.

Description: conference

Description: The orogenic wedge model (Davis et al. 1983; Platt 1986) marks a conceptual breakthrough in understanding the growth and long-term evolution of accretionary wedges. The characteristic rheology of subduction-related accretionary wedges is thought to change from Coulomb to viscous when the wedge becomes thicker than ca. 15 km, a transition that may influence the stability and dynamics of these wedges. Platt (1986) proposed that viscous flow may trigger extensional faulting in the upper rear part of the wedge and Wallis et al. (1993) argued that viscous flow may cause vertical ductile thinning of the rear part of the wedge. Material fluxes control the geometric shape of an accretionary wedge (Brandon et al. 1998; Platt 1986). Frontal accretion and erosion both tend to drive the wedge into a subcritical condition as the taper angle of the wedge is progressively reduced. This leads to horizontal shortening across the wedge. If underplating is dominantly controlling the flow field in the wedge and frontal accretion or erosion at the rear of the wedge are small, the wedge is supercritically tapered and leading horizontal extension. Horizontal extension leads to a subhorizontal foliation and may eventually lead to normal faulting in the rear-part of the wedge. Despite the importance of these issues, there remains a paucity of detailed information about ductile deformation and how viscous flow influences the stability of subduction-related accretionary wedges. Strain measurements are an instrument to address whether viscous flow strongly influences the deformation in accretionary wedges. They provide direct information about the kinematics of ancient orogenic belts. Additionally, they allow understanding important tectonic processes in subduction wedges such as the pattern of flow within the wedge. We focus on deformation analysis on a suite of samples from the Otago wedge exposed in the South Island of New Zealand. The Otago accretionary wedge offers a unique opportunity to study the tectonic evolution of a typical subduction-related accretionary complex. Its across-strike length of ca. 600 km makes it one of the largest exposed ancient accretionary wedges on Earth. Pressure and temperature estimates indicate that our samples are representative of deformation conditions to depths as great as ca. 35 km. This is similar to maximum depths observed for subducting slabs beneath modern forearc highs. The deformation measurements show that the strain magnitude is generally small in the Otago wedge. The oct values, a measure of the distortion a sample experienced (independent from the strain geometry), range from 0.34– 3.87 for the Rf /? strains, 1.01–4.28 for XTG strains across the whole suite of the Otago rock pile, and 0.08–0.70 for the absolute strains obtained from low metamorphic grade rocks. The Otago samples are characterized by considerable volume strain that increases from the lower textural zones towards the high-grade interior of the wedge. Our strain results are inconsistent with the models which advocate supercritically tapering of accretionary wedges and that supercritical tapering eventually triggers normal faulting. Taking averages of our strain measurements, a residence time in the wedge of 35 Myr, burial depths of 30 km, coaxial deformation and a depth-dependent rate for ductile deformation, we calculate vertically-averaged strain rates. Because the principal strain axes of the tensor average are all inclined, the vertical averaging changes the principal stretches. The horizontal principal stretch parallel to the 160°-striking Otago wedge becomes 0.79, that for across strike 0.88 and for vertical strain 0.44. Averaged strain rates are −1.44−16 s−1 for parallel-strike horizontal strain, −6.2−17 s−1 for across-strike horizontal strain, and −8.02−16 s−1 for vertical strain. The strain rates are related to volume loss and to the efficiency with which dissolved chemicals are advected away. The rates are similar to the ones calculated by Bolhar & Ring (2001) and Ring & Richter (2004) for the Franciscan wedge. These strain rates are orders of magnitude smaller than the 1−14 s−1 strain rates assumed by Platt (1986). Thus, our data imply that the Otago wedge could not shorten horizontally fast, and hence could not have steepened up its surface slope. The fact that shortening was accompanied by volume loss has another important and interesting consequence. Even if a case was envisioned in which horizontal shortening was fast enough to steepen up the surface slope of the wedge, the volume loss would not necessarily change the wedge geometry into a supercritical configuration triggering normal faulting. As a consequence of the slow strain rates and the high volume loss, viscous flow probably was not fast enough to significantly influence the stability of the wedge and to form a supercritically tapered wedge.

Description: conference