Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

dynamic metamorphism

Dynamic metamorphism results from frictional heat and confining pressures associated with major fault planes.

The metamorphic changes are localised adjacent to the fault plane, and cataclasis is the crushing and grinding of rocks into angular fragments (cataclastic texture) in dynamic metamorphic zones.

Textures of dynamic metamorphic zones depend on the depth of formation where confining pressures determine the predominant deformation mechanisms. At depths of less than 5km, confining pressures are to low to produce dynamic metamorphism, and a zone of breccia or cataclasite is formed instead, with the rock milled and broken into a mélange of random fragments. At greater depths, angular breccias transform into ductile shear textures and mylonite zones.

Pseudotachylites form at depths from 5-10 km, where confining pressures are focused into discrete fault planes and are sufficient to prevent brecciation and milling. The frictional heating at these depths can melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, can result in growth of new mineral assemblages.

Within the depth range of 10-20km, ductile deformation conditions prevail and frictional heating is dispersed throughout shear zones, resulting in distributed deformation and a weaker thermal imprint. Here, deformation forms mylonite, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

Overthrusting forces may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, often producing metamorphism in the hotter blocks. These metamorphic assemblages are diagnostic of the depth and temperature and the throw of the fault and can also be dated to determine an age of the thrusting.

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