Frictional melting in fluid-rich faults (Bolfin Fault Zone, Chile

Abstract

Pseudotachylytes (solidified friction melts produced during seismic slip) are currently considered one of the veryfew geological markers of seismic faulting in exhumed faults. Pseudotachylytes are thought to be rare in thegeological record because they are typical of particular seismogenic environments characterized by water-deficientcohesive rocks and possibly associated with particular earthquakes with exceptionally large static stress drops.However, experimental evidence suggests that frictional melting may easily occur in the presence of pressurizedliquid water. This possibility is supported by (though rare) occurrence of vesiculated and amygdules-richpseudotachylytes. But even if produced during seismic slip, the delicate pseudotachylyte matrix may alter whenpermeated by post-seismic fluids and the pseudotachylyte lost from the geological record.Here we discuss the occurrence of poorly to strongly altered pseudotachylytes hosted in a fluid-rich ex-humed fault strand of the Atacama Fault System (Northern Chile). The Bolfin Fault Zone (BFZ) is > 30 km longand cuts amphibolites and diorites of the Coastal Cordillera. The BFZ records a series of deformation and veiningevents lasting from the Jurassic (under granulitic facies) to the Pliocene (T 5 km of strike-slip displacement at 6-8 km depth and 280-350◦C ambient temperature. Thefault core is bounded by an up to 50 m thick damage zone characterized by intense hydrothermal sub-greenschistto greenschist facies alteration. The pseudotachylytes include black to brownish in color cm-thick fault andinjection veins, with spectacular flow structures. The pseudotachylyte consists of suspended clasts of saussiritizedfeldspar, albite and minor quartz immersed (locally) in a poorly altered and homogenous (glassy-like) feldspathicin composition matrix with tabular microlites of feldspar and (more often) in a strongly altered matrix made oftens of micrometer in size albite, chlorite, and epidote crystals. The matrix hosts rounded to ellipsoidal concentricfeatures up to∼1 mm in size with an inner core of chlorite, epidote or calcite and an external rim of quartz. Theselatter features are interpreted as vesicles filled by post-seismic sub-greenschist facies minerals precipitated fromthe percolating hydrothermal fluids (i.e. amygdules).The identification of pseudotachylytes, the first so far to our knowledge in South America, and its associa-tion with intense pre- and post-seismic alteration challenges the common belief that these fault rocks are rare.Consistent with the experimental evidence, pseudotachylytes (1) could be a common coseismic fault product atintermediate crustal depths, (2) may easily be produced in fluid-rich hydrothermal environments as well as fluidabsent conditions but, (3) are easily lost from the geological record because they are prone to alteration.