Cobble of metamorphic rock with pseudotachylite (fossil earthquake)

The medium gray part of the specimen is the original host rock, i.e., the Otaga Schist. The black layer and the ‘dikelets’ of black shiny material is pseudotachylite. Pseudotachylite represents frictional melting of rock in the vicinity of the hypocenter of an earthquake. The rocks at such depths are already hot. The addition of sudden frictional heat causes local melting, and the melt ‘squirts’ into fractures. The black material is indeed glass, somewhat devitrified. South Island affords the optimum conditions: a major crust-busting fault capable of creating melts during earthquakes, and rapid uplift of the South Island permitting the fossil earthquake record to become exposed at the surface.

I am indebted to Rick Sibson, expert on pseudotachylite and fault mechanics, who inspired me to tcollect a piece or two of pseudotachylite.  I first met Rick when he was teaching at University of California at Santa Barbara.  He showed me the first examples I had ever seen of pseudotachylite, and I immediately got a bad case of ‘pseudotachylite envy.’  Finally I took care of this condition in 2010, traveling for the first time in New Zealand.  In advance of the trip, Rick told me about several (largely) dry river beds where I might find a pebble or cobble of pseudotachylite.  I was lucky enough to find 2 pieces among the thousands and thousands of cobbles in Hare Mare River in Westland.  This was a rewarding experience!!

Age & Formation
Pseudotachylite (fossil earthquake) from Alpine fault zone, New Zealand. The black glassy material was generated during the last 10 Ma.