PALEONANOQUAKES THROUGH PALEOMEGAQUAKES—FRICTION MELTING FEATURES WITHIN FAULT ZONES ACROSS FIVE ORDERS OF MAGNITUDE IN THICKNESS

It is commonly accepted that the presence of bona fide pseudotachylyte along a normal (non-superfault) crustal fault zone is evidence for ancient coseismic slip along the fault, and that the friction melt or pseudotachylyte represents a petrologic °¥fossil earthquake'. To what extent are the characteristic structures associated with such slip developed along single-slip-event fault zones, considering the thickness (and presumably the displacement) can range over many orders of magnitude? Three examples will be presented representing fault-zone thicknesses ranging over approximately five orders of magnitude, and implications for scaling relationships discussed.

At the small-displacement extreme, ~1 Ga micro-pseudotachylytes exist within fault rocks from the Grenville Front, Ontario, Canada. They developed within ultramylonites, where microfaults formed parallel to Ĕ10 micron-thick alternating felsic and mafic laminae developed during plastic strain. Pseudotachylyte fault veins commonly thin to less than 10 microns thick, and are commonly less than 50 microns thick. Despite their minute size, they display many of the characteristics of mesoscale pseudotachylytes, including 1) lithic clasts and crystal fragments, typically <20 microns across; 2) flow banding including concentric flow patterns; 3) bifurcation of veins around rigid layers; 4) paired shears with inwardly-projecting injection veins; 5) dilatant zones of anomalously thick pseudotachylyte; 6) highly variable fault vein thickness. Although many diagnostic features may be lacking at this scale, clearly they can be preserved, and considering fault scaling relationships, ought to be common, suggesting recognition and possibly preservation is a problem.

Some of the most familiar coseismic structures are evident at the mesoscale, and examples will be presented from several locations.

At the other extreme, a 5 m-thick pseudotachylyte zone occurs near Loch Scadabhagh on North Uist, Scotland. Although this zone is clast-rich, there do not appear to be any discontinuities, and magnetic susceptibility measurements on the pseudotachylytes broadly agree throughout the zone. Clasts, spherulites, and other textures are consistent with large scale displacement.