Timescales of orogeny: Jurassic construction of the Klamath Mountains

Abstract

Classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous. Isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts. Forty-five new 40Ar/39Ar ages from the Klamath Mountains illuminate the deformation, metamorphism, magmatism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes. The new geochronologic relationships show that many Jurassic units of the Klamath Mountains had 200 Ma or older volcanoplutonic basement. Subsequent formation of a large -170 Ma arc was followed by contractional collapse of the arc. Collision with a spreading ridge may have led to large-scale NW-SE extension in the central and northern Klamaths from 167 to -155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large region of thecentral Klamath Mountains to below -350øC at -150 Ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains•and perhaps other similar orogens•were constructed during areally and temporally variant episodes of contraction, extension, and magmatism that do not fit classical definitions of orogeny. modern orogens, there has been a tendency to simplify descriptions and evaluations of orogenies. In large part, this simplification has been necessary because isotopic and fossil ages have been imprecise and/or scarce. Isotopic ages are becoming ever more precise and abundant, permitting reevaluation of classical ideas regarding orogenesis. This paper reconsiders the Jurassic orogenies that built the Klamath Mountains and northern Sierra Nevada of California and Oregon in light of forty five new 40Ar/39Ar ages. Three fundamental issues about orogeny in general, and the Klamath Mountains in particular, are addressed: (1) What do orogenic features and their ages tell us about the relations among magmatism, deformation, sedimentation, and metamorphism? (2) What evidence do orogenic features provide for deciphering plate interactions? (3) Are current concepts of orogeny appropriate for resolving the plate tectonic history of convergence zones? The growing abundance and precision of isotopic ages and the detail of geologic mapping make the Klamath Mountains an excellent place to conduct such an analysis, which may serve as a model of mountain-building processes in other orogens built from predominantly oceanic material. The classic interpretation of Klamath-Sierran mountain building is that the Jurassic Siskiyou and Nevadan orogenies were relatively short events with differentiable deformation, magmatism, sedimentation, and metamorphism. On the contrary, we sho...

Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

“…The intrusion of $170 Ma granitoid bodies in the westernmost Klamaths and restricted occurrences of younger plutons to progressively more easterly belts [Hacker et al, 1995;Irwin and Wooden, 1999;Irwin, 2003] suggest that the left-lateral offset of the Klamath salient took place over the interval $150-140 Ma as the crustal assembly of oceanic terranes and superjacent strata gradually migrated westward off the subducting plate's deep-seated magmagenic zone. This seaward transport of the Klamath Province apparently occurred during a relatively brief period characterized by sinistral slip along the western margin of the continent [Saleeby, 1992;Saleeby et al, 1992], and terminated at the time of development of the KimmeridgianTithonian cusp in the North and South American apparent polar wander paths [May and Butler, 1986;Schettino and Scotese, 2005].…”

Mesozoic transpression, transtension, subduction and metallogenesis in northern and central California

“…A rifted mafic arc or forearc model is consistent with geological constraints for the entire North ForkEastern Hayfork assembly. Juxtaposition of the similar, coeval Rattlesnake Creek ophiolitic terrane (Hacker et al, 1995) outboard from the Hayfork belt, and stranding of the Western Hayfork-Eastern HayforkNorth Fork complex against the Klamath margin between 169 and 193 Ma (Wright and Wyld, 1994) may have caused the plate junction to step seaward, preserving the older Stuart Fork blueschists, and suturing WTrPz rifted oceanic arc terranes in their present positions.…”

“…Klamath terranes contain abundant mafic igneous rocks, fine-grained clastic strata, and chert-argillite-limestone units containing Tethyan fauna, deposited on a structurally-dismembered maficultramafic basement. Individual terranes were stranded by a component of east-descending underflow during mid-Paleozoic to mid-Mesozoic time (Irwin, 1981;Hacker et al, 1993Hacker et al, , 1995Wallin and Metcalf, 1998). Coeval intermediate and felsic igneous units are rare, whereas postkinematic, Late Jurassic and Early Cretaceous granitic stocks intruded the imbricated orogen (Lanphere et al, 1968;Irwin and Wooden, 1999).…”

Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

“…The Klamath Mountains consist of a series of terranes representing a Permian-Triassic subduction complex that developed west of a Permian-Triassic arc system (e.g., Irwin, 1981;Saleeby et al, 1992). These terranes were structurally imbricated by a major east-dipping thrust system and overprinted by arc-related magmatism (Burchfiel et al, 1992;Saleeby et al, 1992 Hacker et al, 1995;Irwin and Wooden, 1999, and references therein).…”