TECTONIC SIGNIFICANCE OF AN INVERTED METAMORPHIC GRADIENT IN THE CENTRAL METAMORPHIC TERRANE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA: NEW INSIGHTS FROM 40AR/39AR THERMOCHRONOLOGY

Metabasites of the Central Metamorphic terrane (CMT) lie in the footwall of the Trinity thrust fault and record an inverted metamorphic field gradient (IMFG) decreasing from amphibolite facies (~600°C) at the fault to greenschist facies its base (Davis et al. 1965; Peacock and Norris 1989). Longstanding models view the CMT as MORB crust accreted to the base of the Trinity ophiolite (Trinity thrust hanging wall) at a Devonian convergent margin. The prevailing hypothesis (Peacock, 1987) is that the IMFG in the CMT records a true geothermal inversion formed by thrusting hot plate (Trinity ophiolite) over cold plate (CMT). New ⁴⁰Ar/³⁹Ar hornblende cooling ages were measured on samples from both the footwall and hanging wall to test this hypothesis.

Hornblende from two samples of CMT amphibolite near the Trinity fault yielded similar Early Permian ⁴⁰Ar/³⁹Ar ages of 274 +/-2 Ma, considerably younger than the previously reported Devonian age (380 Ma Rb-Sr isochron ages for CMT metasediment, Lanphere et al. 1968). Hornblende from a Trinity gabbro sample collected adjacent to the Trinity fault yielded a Late Silurian ⁴⁰Ar/³⁹Ar age of 422 +/-5 Ma, broadly concordant with reported U-Pb zircon crystallization ages for Trinity gabbroic rocks (431-398 Ma, Wallin and Metcalf 1998). These data suggest (1) that CMT was above 500°C as recently as Early Permian, (2) metamorphism that produced the CMT IMFG may have been Early Permian rather than Devonian as previously thought, and (3) the Trinity ophiolite was too cool to serve as the heat source for Early Permian inverted metamorphism in the CMT.

These data necessitate rejection of the hot-plate-over-cold-plate thermal model as a mechanism for producing the CMT IMFG and a re-evaluation of the convergent margin model for the Eastern Klamath Mountains. An alternative hypothesis is a modification of the longstanding convergent margin model. The younger than expected CMT cooling ages could be explained by a reactivation of the Trinity fault as a normal fault subsequent to Devonian subduction-related thrusting. In this model the CMT reached a greater depth than its present position relative to the Trinity subterrane but was exhumed during Early Permian extension. We are currently testing whether the Trinity fault was later reactivated as an extensional structure.