Title: Melanges and their bearing on late Mesozoic and Tertiary subduction and interplate translation at the west edge of the North American plate
Description: Melanges are commonly considered to be material scraped off an oceanic plate descending at a subduction zone, tectonically churned, and accreted to the underside of the overriding plate. Yet the correlation of Late Cretaceous and Tertiary melanges of western North America with subduction zones of that age is poor. During much of the middle and late Tertiary, this area was continuously or discontinuously bordered by a subduction zone within which the Farallon plate and much of its successor, the Juan de Fuca plate, were consumed. Yet known melanges of this age that can reasonably be linked to this process are rare and limited to those of the Olympic Peninsula of Washington. Melanges are also present within the Franciscan Complex of western California and within the Otter Point Formation of southwestern Oregon, mostly Eocene or older.

An alternative to the subduction-complex theory is that melanges are material that was broken and sheared as it was plowed aside and either coasted or was rammed inland at a triple junction migrating along the edge of the continental plate. The required triple junction is of a singular dynamic type, referred to as a Humboldt-type, formed where an oceanic plate obliquely underthrusts a continental plate and advances laterally along the edge of that plate while ·following a retreating oceanic (or possibly continental) plate. The triple junction may be formed through the interection of either (1) a spreading ridge, transform fault, and subduction zone or (2) two transform faults and a subduction zone. The Franciscan Complex includes rocks that contain detritus eroded from preexisting melanges or detritus deposited by normal sedimentary processes on top of preexisting melange.

These sequences were subsequently sheared, fragmented, and intermixed to form new melanges or broken formations, strata similar to melanges but containing no exotic blocks. The Franciscan in places contains a record of two or more distinct cycles of melange development. Evaluation of such constraints as are known on the ages of these cycles suggests three diachronous events, believed to represent the transit along the western margin of the continent of Humboldt-type triple junctions in Cretaceous and early Tertiary time. The youngest of these is fairly well bracketed by ages of nonpenetratively deformed rocks and penetratively deformed melange or broken formation near Morro Bay, Calif., and less satisfactorily in the Covelo-Clear Lake area of California. The ages suggest that the most recent period of formation of the Franciscan Complex and correlative rocks was during the Campanian at Morro Bay and early Eocene or perhaps later time near Covelo.

Farther north, the age of the most recent overthrusting and imbrication of Franciscan-like rocks near Bandon, Oreg., also is bracketed within the early Eocene, but it is not certain that melange or broken formation formed contemporaneously with the thrusting. In California, the final episode of allochthonous deformation was probably a diachronous upheaval producing melange and broken formation that transited the continental margin at a rate of roughly 4 ern/ yr, reaching northern California by the early Eocene. This timing nearly coincides with the transit of the Kula-Farallon-North American triple junction, as inferred by Tanya Atwater in her constant-motion model of Late Cretaceous and Tertiary plate geometry.

In early Eocene time, however, this transit apparently evolved into an event in which coastal areas of southwestern Oregon and northwestern California were contemporaneously deformed and the allochthonous oceanic crust now underlying northwestern Oregon and western Washington was formed and accreted to the craton. The basement rock of trus Oregon-Washington borderland consists of oceanic tholeiitic basalt of early and middle Eocene age, which, from published paleomagnetic data, is believed to have been rotated clockwise as much as about 70° by middle Tertiary time. The contact of the oceanic crust with the craton to the east is apparently defined by a zone of steep negative gravity gradients. The angular to jagged outline of this contact as inferred from published gravity maps suggests that the borderland is an aggregation of variably rotated blocks, rather than a single elongate and coherent crustal block.

The reported attitude of source fissures of the tholeiite suggests derivation in part in a stress system with a tensional direction comparable to that of the Kula-Pacific ridge rather than of the supposedly nearby KulaFarallon ridge. Prior to 56 Ma (million years before A.D. 1950), the paths of Pacific and North American plates may have been convergent rather than parallel to the trend of the Queen Charlotte and San Andreas faults, as they have been for the past 25-30 million years. If they w, .ere convergent, the allochthonous crust of the borderland could have been accreted to the North American plate during a collision between that plate and the Pacific plate. It has been proposed that the Kula-Pacific spreading ridge vanished abruptly shortly after 56 Ma.

The presence within the oceanic crust of the Oregon-Washington borderland of a tensional orientation comparable to that of this ridge suggests that the ridge, instead of vanishing, jumped northward, intersecting the North American plate northwest of the former Kula-Farallon-North American triple junction. The Pacific plate, enlarged by the addition of part of the Kula plate, then sideswiped the North American plate, driving a widening wedge of recently formed oceanic crust inland while crunching and stacking adjacent rocks of the craton in Oregon and Washington. By the impact of this collision, the Pacific and North American plates were deflected into their present paths parallel to their bounding transforms as the allochthonous wedge of oceanic crust was sheared off, fragmented, and rotated clockwise to form the basement of the Oregon-Washington borderland.

The core rocks of the Olympic Peninsula consist of melange and broken formation, infaulted or imbricated with blocks of intact strata. Rocks peripheral to the core consist of the oceanic tholeiitic basement of the Oregon-Washington borderland with interfingering clastic deposits, mainly overlain by shallow-water marine-shelf deposits. The core rocks are bathyal marine turbidite deposits. Melanges of the western core contain fossils whose reported ages are as young as early or middle Miocene. Published potassium-argon ages of the rocks of the eastern core suggest metamorphism after 29 Ma and cooling about 17 Ma. Magnetic lineations of the northeastern Pacific step right laterally across the Aja fracture zone. From the age of these anomalies, it appears that north of the Aja, the spreading ridge system and coexisting subduction zone shrank, then vanished about 21 1/2 Ma.

The Aja would then intersect the Queen Charlotte fault and the subduction zone to the south and, with continued right-lateral movement of the Pacific plate, would form a Humboldt-type triple junction. That triple junction would then persist through about 5 1/2 m.y., finally dying 16 Ma as the ridge system south of the Aja stepped eastward and intersected the subduction zone. This timing nearly coincides with potassiumargon ages of cooling of the youngest melanges in the eastern core of the Olympic Peninsula. To account for the structural fabric and geographic extent of the Olympic melanges of Miocene age through the tectonism associated with this triple junction, the junction must have been situated immediately west of the Olympic Peninsula.

If this spatial and temporal relation is valid, northwestward movement of the Pacific plate relative to the North American plate has averaged about 6 crn/yr at least since middle Miocene time, a rate comparable to accepted estimates of the rate of movement of these plates averaged over the past 2 m.y.
Subjects: Geology, Melanges (Petrology), Plate tectonics, Stratigraphic Geology, Subduction zones
Places: North America
Time periods: Mesozoic, Tertiary