MICROSTRUCTURES AND CALCITE TEXTURES IN THE ZERO FAULT CARBONATE MYLONITE ZONE, FRANKLIN, NJ

The high angle, normal Zero Fault juxtaposes mylonitic Mesoproterozoic Franklin marble against the Cambro-Ordovician Allentown dolomite. Nine oriented samples were collected from the footwall mylonite over a 25 m transect normal to the fault trace. The mineral assemblage is dominantly calcite with dolomite, quartz, graphite, muscovite, pyrite and Fe-Ti oxides. Relatively undeformed Franklin Marble consists of twinned calcite and dolomite grains 1-8 cm in diameter and irregular quartz grains up to 2 cm in length. Micas are typically 2-14 mm in length. Mylonitic Franklin marble shows centimeter-scale cataclastic zones within dolomite-rich layers. Calcite-rich zones are dominated by crystal plasticity, grain boundary rotation and recrystallization migration and dynamic recrystallization. Recrystallization increases and relict cm-scale calcite porphyroclasts decrease with proximity to the fault contact. Dynamically recrystallized matrix grains are about 2-20 µm in diameter and generally untwinned. Type III and IV calcite twins in porphyroclasts suggest temperatures of deformation > 250-300 °C.

Shear sense indicators include oblique foliation, C/S fabrics, calcite fibers, asymmetric porphyroclasts and pressure shadows, offset microfolds, and mica fish. Seven of the nine samples record top-SE, normal displacement, consistent with the stratigraphic separation on the fault. One sample, located farthest away from the fault trace shows reverse, top-NW shear sense. Preliminary EBSD c-axis measurements yield maxima that are back rotated ~25˚ to the foliation normal, consistent with macroscopic shear indicators in the same specimen.

A spectacularly folded calcite porphyroclast reveals valuable insights into the mechanical behavior of the mylonite. Mechanical twins within the crystal are tightly folded at an amplitude of 0.5 mm and wavelength of 1.5 mm. One set of mechanical twins undergoes a maximum of 142˚ of rotation while another set of twins creates a 44˚convergent fan around fold hinges. This geometry precludes a passive shear folding mechanism within the crystal. Instead, the geometry reflects an active folding process involving flexural folding followed by flattening in the plane of the mylonitic foliation.