P–T–TECTONIC PATH IN METAMORPHIC ASSEMBLAGES ALONG A SUTURE IN THE NORTHERN KLAMATH MOUNTAINS, OREGON

A regional thrust running the length of the Klamath Mountains province separates rocks of the lower plate Western Jurassic (WJ) belt from the upper plate western Paleozoic and Triassic (WTrPz) belt. In the northern Klamath Mountains rocks of the western Klamath terrane (WKt) and Rattlesnake Creek plus May Creek terranes (RCt/MCt) form the lower and upper plates, respectively. Field and petrologic data indicate that the WKt and RCt/MCt were metamorphosed and deformed together during and after their intrusion by the White Rock (~156 Ma) and Wimer (~160 Ma) plutons. However, regional data such as deformational styles and metamorphic assemblages in WKt and RCt/MCt rocks point to different tectonic settings and thermal regimes in the two plates over time (Helper, 1986; Donato, 1987). For example, prehnite–pumpellyite (PP) and low greenschist facies (GS) metamorphism older than ~150 Ma and probably younger than ~ 157 Ma (Gray, 2006; Yule et al. 2006; Harper et al. 1994) is widespread regionally in the WKt. In the older part of the WJ belt, e.g., the Condrey Mountain terrane (CMt) (protolith age >~170 Ma, U/Pb, Saleeby et al. 1984), assemblages are transitional blueschist–greenschist facies (GS–BS) with average temperatures of ~ 400oC and pressures in the range of 5 to 7 kbar. Metamorphism in all these WJ belt rocks plot along a geothermal path of ~15oC/km or less. However, in the RCt and equivalent WTrPz terranes, metamorphic assemblages are commonly amphibolite facies that is moderate– to low–pressure andalusite–sillimanite zone with P<~4 kbars and metamorphic cooling ages in the range ~145–150 Ma (Hacker et al., 1995). Assemblages in the area of this study in the dynamic aureole of both plates along the suture, possibly as young ~130 Ma (Kays et al. 1977), have pressures in the range ~2.6–3.1 kbar and temperatures in the range of 525oC–650oC. Thus, these WKt and RCt/MCt assemblages plot along a geotherm of ~50oC–62oC/km. An attractive tectonic model for PP through GS–BS facies WJ and WKt rocks requires a geotherm that formed during burial/underthrusting outboard of the North American (NAM) continental margin in the Middle to Late Jurassic. The steeper geotherm that fits RCt/MCt and later WKt metamorphic assemblages of this study requires uplift and heating close to the continental margin and associated arc after suturing of WJ and WTrPz rocks.