The Great Terrane Wreck of the Cordillera Allen J. McGrew April 2005

What is a Tectonostratigraphic Terrane? Terranes can be: Native – showing shared traits with North American crust, indicating an origin adjacent to North America. Exotic – Far-traveled, not born adjacent to North America Superterranes – amalgamated with other terranes before accretion to the continental margin.

Evidence of origins? Stratigraphic similarities Faunal affinities Sedimentary provenance e.g.,detrital zircon dating Paleomagnetic evidence

Paleomagnetic Evidence Paleomagnetism Inclination Declination Polarity Paleopoles and Paleolatitude

Apparent Polar Wander Paths Comparison with APWP for North America allows assessment of whether terrane could have been close to No. America at time of deposition of the measured rock.

Limitations of Paleomagnetic Analysis Paleolatitude North or South? No constraint on paleolongitude. What if paleomagnetism was not acquired when we thought it was? (fold tests) How well do we know paleohorizontal at the time magnetism was acquired? What if paleomagnetic vectors changed in orientation after deposition? (e.g., due to flattening or deformation)

An Example: Paleomagnetic Reconstruction of the Alexander Terrain (after Butler, Gehrels & Bazard, GSAB, 1997) Ordovician-Silurian: No dependable paleolatitude. Early Devonian: Karheen Fm 14º ± 4º possibly Australia or pref. Baltica Middle Devonian - Pennsylvanian: no dependable information.

Alexander Paleogeographic Reconstructions (cont’d) Permian: Halleck volcanics & three other formations indicate paleoposition of amalgamated Alexander-Wrangellia terrane at 25º - 30º N off the west coast of No. America. Triassic - 10º - 20º N at paleolatitude of northern Oregon (Seven Devils & Huntington Arcs). Jurassic & Cretaceous – no primary magnetizations; Alexander-Wrangellia accreted to North American continental margin and dispersed northward as far as Alaska by translation on strike-slip faults.

Recognizing Amalgamation & Accretion “Stitching Plutons” – plutons intruded into adjacent terranes demonstrating that they had to be adjacent at the time of pluton injection. “Overlap Assemblages” Stratigraphic packages deposited across terrane boundaries indicating that terranes had to be adjacent at time of deposition.

A History of Terrane Accretion

Mesozoic Paleogeography and Tectonic History of Western North America Images and text modified from a poster session presented to the Annual Meeting of the Geological Society of America, Seattle, Nov. 2003 by Ron Blakey and Paul Umhoefer, Department of Geology, NAU http://jan.ucc.nau.edu/~rcb7/mz_paleogeog_wus.html

Permian Tectonics and Paleogeography (290 Ma) Mc Cloud arc fragmented during late Paleozoic truncation of SW North America Assembly of western Pangaea completed Transform margin coupled McCloud arc with arcs built on Chortis and South America Havallah back arc basin (HB) and Slide Mtn back arc separated northern arc, Quesnellia (Q), from Antler belt (A) and western North America Blakey and Umhoefer, 2003

Triassic Tectonics and Paleogeography (240 Ma) Early to Middle Triassic (240 Ma) McCloud arc fragmented and accreted to North America resulting in Sonoman orogeny Caborca terrane moved SE along truncated continental margin Subsequent Cordilleran arc was continental to south and marine to north Blakey and Umhoefer, 2003

Early Jurassic Tectonics and Paleogeography (180 Ma) Early to Middle Jurassic (180 Ma) ?? Fringing Cordilleran arc accreted to western North America including Bridge River, North Cascades, western parts of Eastern Oregon terranes, and western terranes of Klamaths and Sierra Nevada ?? Older Mesozoic back arc basins closed Blakey and Umhoefer, 2003

Middle Jurassic Tectonics and Paleogeography (160 Ma) Middle to Late Jurassic (160 Ma) Major arc magmatism Initial collision between southern Wrangellia and Cordilleran margin at approximately the latitude of Klamaths and Sierras Fringing arcs south of collision zone Ophiolites obducted in collision zone and in inter arc region between Cordilleran arc and fringing arcs Foreland basin in Utah and thrusting in Nevada Rift basin in southwestern North America related to opening Gulf of Mexico Blakey and Umhoefer, 2003

Late Jurassic Tectonics and Paleogeography (145 Ma) Major plate reorganization -- change to sinistral motion of Farallon plate relative to North America causes Wrangellia to move southward Complex series of events at SW margin of North America: Chortis and related terranes shift SE as Atlantic and Proto-Caribbean expand; transform fault and oblique rift systems developed along waning continental arc (McCoy-Bisbee; Mojave-Sonoran megashear); early Franciscan mÈlange formed farther outboard Nutzotin Ocean between northern Wrangellia and North America remained open Oblique convergence shut down much of Cordilleran arc Blakey and Umhoefer, 2003

Early Cretaceous Tectonics and Paleogeography (125 Ma) Guerrero arc collided and subduction zone rebuilt to west. South-moving Wrangellia linked to North Cascades, Tyaughton-Methow, and adjacent terranes to form Baja BC Tectonic escape of Intermontane terrane and parts of California northward Great Valley forearc basin and Franciscan subduction complex built on reorganized Cordilleran margin; as Baja BC moved southward, northern Great Valley a transpressive basin North Slope terrane rotated CCW as Canadian basin opened Early Sevier thrust belt and foreland basin formed Blakey and Umhoefer, 2003

Late Early Cretaceous Tectonics and Paleogeography (105 Ma) Two large volume magmatic belts formed (Peninsular, Sierra Nevada, Idaho; Coast Plutonic complex built on Baja BC); plutonic complexes may have been fed by volatiles from under-thrusting of North American miogeocline South-moving Baja BC approached maximum S position at latitude of southern Arizona Baja BC west of classic Great Valley and Franciscan complexes; Great Valley as interarc basin(?) Rotated North Slope terrane collided with Yukon-Tanana and related terranes Sevier orogeny continued and foreland basin expanded eastward Blakey and Umhoefer, 2003

Late Cretaceous Tectonics and Paleogeography (85 Ma) Plate reorganization as Kula separated from Farallon Plate; dextral transpression with North America; Kula-Farallon ridge moved northward along coast Baja BC transported northward with obliquely converging Kula plate Major magmatic activity in two belts (110-85 Ma) followed by strong decline and null in magmatic activity (80-40 Ma) Andean-style segmentation of Farallon arc from Central America to Southern California Sevier thrusting continued and foreland basin shifted locus of subsidence southward in response to shallow Laramide ‘flat-slab’ subduction Blakey and Umhoefer, 2003

Cretaceous-Tertiary Tectonics and Paleogeography (65 Ma) Triple junction and Baja BC moved rapidly northward Intermontane terranes translated to north Forearc sedimentation (Franciscan-Great Valley) continued along southern margin Uplift of Coast plutonic complex generated large deep-sea fan deposits (Chugach flysch) Strong Sevier thrusting and foreland basin Laramide orogeny: Farallon Plate shallow subduction caused expansion NE of foreland uplifts and basins Blakey and Umhoefer, 2003

Eocene Tectonics and Paleogeography (50 Ma) Intermontane and Baja BC blocks continue northward translation along with north-migrating Kula-Farallon plate boundary; many terranes approach their present latitude with respect to North America Olympic terrane accreted Extensive fore arc sedimentation along most of Cordilleran margin Rocky Mountain foreland uplift, deformation, and shallow-subduction-related magmatism; interior sedimentation mostly restricted to Rocky Mountain foreland basins Blakey and Umhoefer, 2003