DRIVERS OF CENOZOIC ACTIVE TECTONISM IN THE KLAMATH MOUNTAIN PROVINCE: FROM TERRANE ACCRETION TO TRIPLE JUNCTION ARRIVAL

The Klamath Mountain Province (KMP), a Cretaceous and older metamorphic/plutonic terrane in the Cascadia forearc, records widespread Cenozoic tectonics driven by a diverse set of regional plate tectonic processes. In addition to Cascadia subduction, Cenozoic plate tectonic events include early Eocene accretion of the Siletz terrane immediately north of the KMP and Pliocene and younger deformation associated with the arrival of the Mendocino Triple Junction (MTJ) to the south.

Integrative analysis of geological and stratigraphic data with targeted thermo-chronologic dating and forward thermal modeling reveals a sequence of tectonic responses within the KMP following accretion of the Siletz terrane. An initial stage of Siletzia accretion driven ~N-S directed extension and crustal thinning is recorded both by detachment faulting (extensional unroofing) in the central KMP and abrupt, localized basin subsidence in the northeastern KMP. This period of extension rapidly transitions into an interval of localized rapid uplift and unroofing at widespread locations within the KMP in the early Miocene. After this interval of early Miocene unroofing, the region was generally quiescent. However, middle Miocene sediments preserved within small grabens in the south-central KMP record both local extension and topographic relief sufficient to isolate graben-filling sediments from sources outside the KMP. Combined high-resolution seismic tomography, topographic observations, and geodetic data depict the most recent period of KMP tectonics. This stage of deformation is observed primarily in the western KMP. Here, ephemeral crustal thickening and surface uplift, which record the approach of the MTJ and the Mendocino Crustal Conveyor tectonic driver, are expressed in rugged landscape relief and rapid erosion (>500 m/Myr). We interpret regional differences in the geomorphology of the range as recording the emplacement of over-thickened Franciscan terrane crust within the middle-crust of the KMP.