Derek J. Thorkelson

Subduction of diverging plates and the principles of slab window formation

Consumption of an ocean basin by subduction commonly brings a sea-floor-spreading ridge toward a deep-sea trench. If plate divergence and convergence continue after the ridge intersects the subduction zone, a slab window forms between the subducted parts of the diverging oceanic plates, producing anomalous thermal, physical and chemical effects in the surrounding asthenospheric mantle. In turn, these conditions alter the tectonic and magmatic evolution of the overriding plate, usually disturbing ordinary fore-arc and arc regimes. Differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate. Anomalous magmatism from fore arc to back arc may be accompanied by fore-arc metamorphism, strike-slip faulting, uplift, extension and, in extreme cases, rifting. The shape and size of the window are controlled mainly by the pre-subduction ridge-transform-trench configuration, slab dip angles and vectors of plate convergence. Subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates. Subducted transform faults continue to be active, usually as oblique-slip faults, until the plates separate. As transform faults subduct, they become longer on the plate which occupies the acute angle between ridge and trench, and shorter on the other plate. Trains of isolated windows produced by subduction of a segmented ridge-transform system progressively expand during descent, commonly merging together to form a composite slab window. Oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows, one at each site of ridge-trench intersection. Above a slab window, arc volcanism diminishes and may be replaced by volcanism of mid-ocean ridge or rift affinity. The change in chemical character reflects various processes including elevated heat flow, decreasing hydration of the upper mantle, juxtaposition of supra- and sub-slab mantle reservoris, asthenospheric upwelling and melting of the trailing plate edges. If the slab window migrates, the anomalous magmatic regime may be replaced by renewed arc volcanism. Identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications.

Cordilleran slab windows

The geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics. As a consequence of subduction of the Kula-Farallon and East Pacific rises, slab windows formed and migrated beneath the North American Cordillera. The probable shape and extent of these windows, which represent the asthenosphere-filled gaps between two separating, subducting oceanic plates, are depicted from the Late Cretaceous to the present. Possible effects of the existence and migration of slab windows on the Cordillera at various times include cessation of arc volcanism and replacement by rift or plate- edge volcanism; lithospheric uplift, attenuation, and extension; and increased intensity of compressional tectonism. Eocene extensional tectonism and alkaline magmatism in southern British Columbia and the northwestern United States were facilitated by slab-window development.

Cocos-Nazca slab window beneath Central America

Abstract Integration of petrologic and tectonic data favours a model of slab window formation beneath Central America in the Pliocene-Pleistocene. Central America has been the site of voluminous Cenozoic arc volcanism. The Cocos and Nazca plates, which are subducting beneath Central America, are diverging along the east-trending Cocos-Nazca spreading ridge. Since 25 Ma the Americas have advanced about 1800 km west over the ridge-transform system. Since at least 8 Ma, plate integrity and the ridge-transform configuration have been preserved during convergence, resulting in subduction of the spreading ridge and development of a slab window. The Panama fracture zone, an active transform fault, is the part of the ridge-transform system currently being subducted. The ridge-transform system formerly adjoining the northern end of the Panama fracture zone is likely to have been left-stepping. We use present-day plate motions to design a slab window to fit known variations in igneous composition, hypocentre distribution, and mantle anisotropy. The modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 Ma. Cessation of ridge subduction occurred between 1 and 3 Ma, when subduction of the Panama fracture zone is considered to have begun. The slab window is continuing to expand and migrate northeastward below the Central American volcanic arc. The absence of a Wadati-Benioff zone from southeastern Costa Rica through Panama corresponds to the position of the slab window. Within this region, dacitic and rhyolitic volcanic rocks have “adakitic” compositions, and are thought to result from anatexis of the young, buoyant crust which forms the trailing edges of the slabs bounding the window. Basalts in this area were derived from an enriched ocean-island type mantle source, whereas basalts from the rest of the arc, in Nicaragua, El Salvador and Guatemala, are mainly derived from slab-modified depleted mantle, characteristic of volcanic arcs. The presence of ocean-island type mantle beneath southern Costa Rica and Panama is explained by eastward flow of enriched asthenosphere from the Galapagos plume-head through the slab window and into the volcano source region. Eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the Nazca and Caribbean plates. The flow of peridotite is a consequence of progressive shrinkage of the Pacific mantle reservoir and concurrent expansion of the Atlantic mantle reservoir.

Pan-Continental River System Draining Grenville Orogen Recorded by U-Pb and Sm-Nd Geochronology of Neoproterozoic Quartzarenites and Mudrocks, Northwestern Canada

Regional lithostratigraphic correlation of early Neoproterozoic Sequence B on the northwest margin of Laurentia includes a 0.5-2 km-thick, laterally continuous, quartzarenite marker of mainly fluvial origin. Concordant U-Pb ages of 54 single detrital zircons from five regionally dispersed samples cluster in the Archean (2.8-2.6 Ga), Paleoproterozoic (2.0-1.9 Ga), and Mesoproterozoic (1.6-1.0 Ga). The vast majority (85%) of the zircons are of Mesoproterozoic age, with a high proportion clustering between 1.25-1.0 Ga, closely matching the age of synorogenic intrusions in the Grenville Province of North America. A

Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America

100 3 \pm 4 Ma

Introduction: An overview of ridge-trench interactions in modern and ancient settings

zircon from the Pinguicula group in the eastern Ogilvie Mountains provides a maximum age for the quartzarenite marker. Sm-Nd isotopic data from intercalated mudrocks indicate a major contribution from sources with relatively juvenile model ages (

Mantle flow through the Northern Cordilleran slab window revealed by volcanic geochemistry

T_{DM} = 1.54-1.74 Ga