Mark A. Pearce

Quantification of fold curvature and fracturing using terrestrial laser scanning

Terrestrial laser scanning is used to capture the geometry of three single folded bedding surfaces. The resulting light detection and ranging (LIDAR) point clouds are filtered and smoothed to enable meshing and calculation of principal curvatures. Fracture traces, picked from the LIDAR data, are used to calculate fracture densities. The rich data sets produced by this method provide statistically robust estimates of spatial variations in fracture density across the fold surface. The digital nature of the data also allows resampling to derive fracture parameters that are more traditionally measured manually from outcrops (e.g., one-dimensional line transects of fracture spacing). The fracture statistics derived from the LIDAR data are compared with the calculated principal and Gaussian curvatures of the surface to assess whether areas of extreme curvature correlate with high-fracture density. For the folds studied, all the fracture spacing distributions showed an exponential distribution, and no significant correlation between fracture density and surface curvature was observed. This questions the validity of using curvature as a proxy for high brittle strains and highlights the need for a complete understanding of fold and fracture mechanics that include considerations of other factors including lithology, strain rate, and confining pressure, not just finite strain. The three case studies also illustrate how terrestrial laser scanning can be used to gather detailed quantitative data sets on fracture and fold distributions from outcrop analogs.

The microstructural character and mechanical significance of fault rocks associated with a continental low-angle normal fault: the Zuccale Fault, Elba Island, Italy

Abstract The core of the Zuccale low-angle normal fault contains a distinctive fault-rock zonation that developed during exhumation, composed of a diversity of fault rocks derived from lithologically heterogeneous wall rocks. Field and microstructural analyses indicate that deformation mechanisms active within the fault core, including brittle fracture, dissolution–precipitation creep and crystal-plasticity, were active broadly contemporaneously. Initially, deformation was accommodated within frictionally weak and inherently stable talc-phyllonites. Although the talc-phyllonites can account for slip at low angles, grain-scale weakening effects were limited by changes over time to the structure of the fault core, resulting from interactions with subsidiary footwall faults. Ultimately, the talc-phyllonites were dismembered into a series of isolated lenses incapable of transmitting grain-scale weakening up to the fault scale. Following this, deformation was accommodated within well-connected units of dolomite-, quartz- and calcite-bearing cataclasite, fault breccia, and foliated fault gouge. Deformation progressively migrated through this latter sequence as a result of precipitation-hardening due to the widespread growth of dolomite. The complexity of fault-zone structure, combined with changes to fault-rock distribution over time, may have resulted in fundamental changes in fault-slip behaviour, an important point to consider given the recent spectrum of slip mechanisms identified along many tectonic faults.

Grain growth and the lifetime of diffusion creep deformation

Abstract Extreme grain-size reduction due to cataclasis, neocrystallization or phase change results in a switch to diffusion creep and dramatic weakening in deforming rocks. Grain growth increases strength until dislocation creep becomes a significant deformation mechanism. We quantify the ‘lifetime’ of diffusion creep by substituting the normal grain growth law into the diffusion creep flow law to calculate the time taken for dislocation creep to become significant. Stress-temperature and strain-rate-temperature space is outlined where diffusion creep may accommodate significant strain: these regions have an upper temperature limit beyond which grain growth is fast enough to move the rock quickly into the dislocation creep field. For plagioclase the limit lies in the amphibolite facies. Rocks in a mantle upwelling experience grain-size reduction during phase changes. Pressure-dependent grain growth limits the deformation that can be accommodated by diffusion creep. This time limit and associated strain limit is independent of starting grain size with a small dependence on upwelling rate and plume width. In both these tectonic environments, second phases are likely to play a role in the maximum achievable grain size due to grain-boundary pinning. Hence we predict the minimum lifetimes of diffusion-creep-dominated deformation following extreme grain-size reduction.

Lithospheric-Scale Transtension between Vertical, Non-parallel, Planar Zone Boundaries

Strain distribution in crustal-scale transtensional zones with non-parallel zone boundaries is heterogeneous, with higher strain gradients in the narrower parts of the zone. Kinematic modeling of such zones shows that, for a constant displacement vector and finite displacement of the zone boundary, the finite strain achieved at a point is dependent on its initial position within the zone. These heterogeneities are governed only by the geometry of the deforming zone; rheologic heterogeneity adds to the departure from homogeneous finite and instantaneous strain. The finite strain distribution can be deduced by determining the instantaneous strain and how it changes both spatially and temporally. In zone geometries where there is a prolate (bouncing) point, marking a change from a horizontal principal finite shortening direction (and therefore vertical foliation) to a vertical principal finite shortening direction (horizontal foliation), it separates areas of wrench- and extension-dominated transtension. This causes spatial partitioning of the strain without any necessity for inherited fabric anisotropies. Migration of this point results in polyphase deformation and, if initial foliation development results in sufficient mechanical anisotropy, overprinting of fabric orientation. Whereas the formation of non-parallel-walled shear zones is promoted by existing structures such as conjugate basement faults and shear zones, the complications predicted in this model result from considering only kinematic arguments and are made more complex by existing heterogeneities found in real rocks.

Structural Influences on Facies Distribution in Silurian Medina Group of Northwestern New York: ABSTRACT

The gas-bearing sandstones of the Medina Group in western New York are the basal Whirlpool Sandstone and the Grimsby Sandstone. Toward the west, these are separated by the Cabot Head Shale and the Manitoulin Limestone. Farther east, the Grimsby overlies and interfingers with the Whirlpool. The reservoir is underlain by the Ordovician Queenston Shale and is capped by shales of the Silurian Clinton Group. The depositional sequence of the Medina Group may be summarized as a marine transgression toward the east that yielded the Whirlpool, Cabot Head, and Manitoulin formations, followed by westward progradation of the deltaic Grimsby formation. Aspects of internal stratigraphy noted in subsurface studies may be correlated with anomalies in units much higher in the Silurian section and with disturbances on the Silurian-Devonian unconformity. Repetitive adjustments of large-scale structural features are inferred. These changes mark the transition from the westward-facing depositional front of the Ordovician to development of the isolated basins that dominated the Silurian Period. End_of_Article - Last_Page 1926------------