Subduction-driven, long-duration, margin-parallel extension in the Klamath Province, California and Oregon, in mid-Tertiary time

The Klamath Mountains Province is an anomalous element in the Cascadia margin; these mountains have the highest average topography, the oldest rocks, and the only identified example of late Cenozoic detachment faulting in the coastal mountains of the Cascadia forearc. Low-temperature thermochronology (apatite fission track, apatite (U-Th)/He analyses) from the central and southern Klamath Province records two domains: a southward-migrating locus of rapid cooling/exhumation in the middle Tertiary, bordered by regions with relatively uniform early Miocene (U-Th)/He ages, independent of north-south position. We infer that this pattern reflects two distinct processes of exhumation: (1) migrating localized fault exhumation and (2) regional surface erosion. At the southern limit of this region of rapid cooling, slickenside striations on the exposed La Grange Fault surface record southward displacement of the upper plate along a shallowly dipping (~ 20°) detachment surface. Thermochronologic data constrain the overall average dip of the detachment fault to a few degrees, upper plate thickness to <~ 4km, and fault slip rate to <2mm/yr for a duration of 30 m.y. (~ 45 Ma to 15 Ma). The fault dip is unusually low compared to that of typical detachment faults, the duration of this extensional event is unusually long compared to other detachment faults, and the timing of extensional faulting does not correlate with recognized tectonic events in the Cascadia region. Immediately north of the Klamath Province, Early Eocene (ca. 55 Ma) accretion of the Siletz terrane initiated a westward jump of active subduction, and was accompanied by tearing of the subducting Farallon slab at the southern margin of the Siletz terrane (~ the northern margin of the Klamath Mountains Province) allowing continuous subduction south of the Siletz terrane [Schmandt and Humphreys, 2011]. One possible consequence of this disruption of the subducting Farallon slab is that following subduction zone reorganization, the northward component of motion of the northern edge of the Farallon slab beneath the Klamath Mountains Province could have driven both a northward displacement of the Siletz terrane and northward extension within the Klamath Province.