POSSIBLE DETACHMENT FAULT PRESERVED IN THE NEOPROTEROZOIC TRINITY OPHIOLITE, KLAMATH MOUNTAINS, CALIFORNIA

A Precambrian ophiolite is exposed along the northwestern side of the Trinity Terrane. This unit, termed the Trinity Ophiolite (Lindsley-Griffin, 1991), is distinct from the well-studied composite Ordovician-Devonian mantle and crustal assemblage exposed to the southeast. It consists of volcanics of basalt and basalt-andesite composition, a gabbro- and plagiogranite-hosted dike complex (~565 Ma), and may include a foliated amphibolitic gabbro unit. All rocks bear evidence of sub-seafloor hydrothermal alteration.

The volcanic section has been interpreted to have formed at an island arc, immature arc, or remnant arc, based mainly on high LREE/HREE, high LILEs relative to HFSE, low K₂O, and low TiO₂. However, low LREE measured locally in the volcanics have been interpreted as oceanic tholeiites consistent with derivation at an oceanic spreading center.

Field relationships suggest another possibility to explain the relationship between rocks present at this locality. A mapped contact between the intrusive plagiogranite- and gabbroic-hosted dike complex, both of which crystallized at depth, and extrusive pillow basalts suggests the presence of a low-angle fault. If the extrusive section section is coeval with the underlying plagiogranite and gabbros, this fault may be a low-angle normal fault that has attenuated section. We test this possibility with new U-Pb dates and geochemistry from the volcanics, dike, and gabbro units.

Further observations are consistent with the existence of this fault. Rocks toward the base of the volcanic section are brittly deformed, while plagiogranite and gabbroic rocks in the conjectured footwall are ductily deformed and appear mylonitic. Gabbros have been altered to amphibolite facies during an early, pre-orogenic event, consistent with the presence of hydraulically conductive fault zones.

Such low-angle faults have been reported from oceanic core complexes formed at slow-spreading ridges, both at mid-ocean ridges and supra-subduction zone settings. In this case, the apparent island arc chemistry in the Trinity Ophiolite volcanics may be due to spreading occurring in a Precambrian supra-subduction zone environment, such as a back arc.