TIMESCALES OF OROGENY - JURASSIC CONSTRUCTION OF THE KLAMATH MOUNTAINS

Classical interpretations of orogeny were based on relatively imprecis e biostratigraphic and isotopic age determinations that necessitated g rouping apparently related features that may in reality have been grea tly diachronous. Isotopic age techniques now have the precision requir ed to resolve the timing of orogenic events on a scale much smaller th an that of entire mountain belts. Forty-five new Ar-40/Ar-39 ages from the Klamath Mountains illuminate the deformation, metamorphism, magma tism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserve d orogenic features relate to ancient plate tectonic processes. The ne w geochronologic relationships show that many Jurassic units of the Kl amath Mountains had 200 Ma or older volcanoplutonic basement. Subseque nt formation of a large similar to 170 Ma are was followed by contract ional collapse of the are. Collision with a spreading ridge may have l ed to large-scale NW-SE extension in the central and northern Klamaths from 167 to similar to 155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large re gion of the central Klamath Mountains to below similar to 350 degrees C at similar to 150 Ma may have occurred as the igneous belt was extin guished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains-and perhaps other si milar orogens-were constructed during areally and temporally variant e pisodes of contraction, extension, and magmatism that do not fit class ical definitions of orogeny.