The Bonggil Pseudotachylyte, SE Korea: Its occurrence and characteristics

Abstract

Pseudotachylytes, i.e., solidified friction-induced melts, are strong evidence of seismic slip on faults. Here we report pseudotachylyte (PT)-bearing faults studied in outcrops of granodiorite (SHRIMP U-Pb zircon age, 75.0 +/- 1.5 Ma) and biotite granite (SHRIMP U-Pb zircon age, 58.4 +/- 1.1 Ma) on the coast of Bonggil-ri, Yangbuk-myeon, Gyeongju, SE Korea. Three types of pseudotachylyte are identified on the basis of thickness and morphology: Single dyke-, fault vein-, and injection vein-type PT. The single dyke-type PT shows a variation of thickness from 15 similar to 40 centimeters along its strike and has an average thickness of 21 cm, which is the largest in the world, as far as we know. The PT is dark gray and neighbors with several tens meters-thick cataclasite zone. At a first glance it looks like a mafic dike, but it has a chemical composition almost identical to the wall rock of granodiorite. Also, it has many subrounded clasts which consist predominantly of quartz and feldspar and newly crystallized tiny grains (submicrometers to several micrometers in size) such as plagioclase, K-feldspar, quartz, biotite, and Fe-oxides. The feldspars and biotite are euhedral and some plagioclase grains show zoning. Flow structures and embayed clasts are also observed. A number of fault vein-type PTs occur as thin (as thick as 2 cm) layers generated on the fault plane, and striations, such as slickensides indicating slip direction, develop along the fault planes and formed during co-seismic slip at the interface between the wall rock and frictional melt. Smaller injection vein-type PTs are found along the single dyke-type PT and the fault vein-type PTs, and appear in a variety of shapes (bleb, lens, sigmoid, network, and breccia) based on field occurrence and vein geometry. All of these observations indicate the PT formed due to frictional melting of the wall rock minerals during fault slip. We propose to call the single dyke type-PT to "Bonggil Pseudotachylyte" considering the thickness of the PT and the locality of the study area. The Bonggil PT-bearing fault strikes N54 degrees W, dips to NE with an angle of 65 similar to 72 degrees, shows sinistral-reverse oblique-slip sense, and can be traced continuously over similar to 110 m. Since Ar-40/Ar-39 whole rock age of the Bonggil PT is 47.3 +/- 1.4 Ma, the age of seismic faulting which is responsible for the formation of the Bonggil PT should be younger than the Middle Eocene. Further work will be conducted to understand the mechanical aspect of the PT formation.