protomylonite

A protomylonite is a fault rock formed where intense shear stress has begun to crush and deform the original mineral grains, but the rock has not been pulverized enough to lose its structure entirely. You are looking at something between fresh host rock and true mylonite: the grains are flattened, fractured, and sometimes partially recrystallized, yet they remain large enough and numerous enough that you can still identify the original texture of the parent rock under magnification.

The defining feature is the proportion of intact matrix to broken debris. In a protomylonite, typically 10 to 50 percent of the rock volume consists of fine-grained paste or matrix created by grain comminution, while the remainder preserves original mineral phases and crystal boundaries. This distinguishes it sharply from true mylonite, where matrix dominates and the original fabric is obliterated, and from cataclasite, where mechanical fracturing happens without the plastic flow and recrystallization that characterizes mylonitization.

Protomylonites form in fault zones and shear zones where temperatures and pressures sit in a middle range, typically in the range of 300 to 500 degrees Celsius depending on rock type and strain rate. At these conditions, minerals like feldspar and quartz are hard enough to fracture under stress, yet warm enough that some strain energy is relieved through crystal plasticity and recovery. The result is a rock that shows both brittle fracture features (visible cracks in porphyroclasts) and ductile flow features (reorientation of minerals, thinning of layers).

Mining geologists and structural engineers encounter protomylonites most often in deep open pit operations, underground mine development, and in tunneling through active or ancient fault zones. The presence of protomylonite signals that ground has been significantly weakened by shear deformation but has not been reduced to low-friction gouge. This affects both stability and dewatering: protomylonite zones conduct groundwater preferentially along foliation and former grain boundaries, yet retain enough interlocking to avoid sudden collapse if properly supported.

The name follows the standard fault-rock taxonomy introduced by Higgins in 1971. The prefix proto- signifies "early" or "incomplete" mylonitization, reflecting the transitional state of the rock. Field identification relies on visual examination of porphyroclasts (larger relict grains) within a finer matrix under hand lens, coupled with thin-section microscopy to confirm the ratio of matrix to intact grains and to detect strain-induced twinning and undulose extinction in quartz and feldspar.