From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, October 7, 2005
Expanding the Tectonic History of the Highlands: a Strain Study Utilizing Fractures from Cambrian Eriboll Group Sandstones
Discovery of previously unrecognized, pervasive, cement-filled microfracture arrays in Cambrian Eriboll Group Sandstones provides new information on the tectonic history of these otherwise slightly deformed rocks. The populations of microfractures filled with cement form a structural fabric in many rocks deformed at upper crustal conditions. They are partly visible in transmitted light as fluid-inclusion planes but become obvious using SEM-based cathodoluminescence (CL), which delineates their shapes, sizes, and crosscutting relationships. A method for collecting mosaics of high-resolution CL imagery allows for an accurate and repeatable measurement of strain within these sandstones.
Ranging in length from microns to millimeters, but sharing size distributions with larger fractures, microfractures are divisible into three sets having the same orientations as large fractures. According to crosscutting relations, the sets strike from oldest to youngest: N to NNE, WNW, and WNW to ENE. These may reflect deformation before, during, and after emplacement of the Moine Thrust Zone. Cumulative apertures along a line of observation record strains of as much as 4.9 percent for the oldest, N to NNE-striking fracture sets. Both small and large fractures are inclined normal to ESE-dipping beds. These fracture orientations with respect to tilted beds and local overprint of opening fracture sets by faults associated with the MTZ indicate that N- to NNE-striking fractures most likely formed prior to tilting of the Cambro-Ordovician before emplacement of the MTZ. Judging from increased abundance near the fault zone and kinematically compatibility, some WNW-striking fractures result from WNW emplacement of the MTZ. Some WNW- to ENE-striking fractures, which are youngest according to crosscutting relations, are locally bridged by quartz containing crack-seal texture but otherwise retaining porosity in fractures having apertures >0.1 mm. Residual porosity in fractures implies that after they formed, fractures cooled to less than a quartz accumulation threshold of about 80ºC. Preliminary diagenetic analysis of these patterns suggests that they formed in the Mesozoic or later, and some of these open fractures are localized near probable Mesozoic or Tertiary WNW- to ENE-striking normal faults.