Foreland-forearc collisional granitoid and mafic magmatism caused by lower-plate lithospheric slab breakoff: The Acadian of Maine, and other orogens

Abstract

Figure 1. Schematic cross section of Chesuncook Dome-Katahdin area showing relationship of mafic and granitic magmas to upper and lower plates of Acadian orogen. Modified from Bradley and Tucker (2002). ABSTRACTDuring collisional convergence, failure in extension of the lithosphere of the lower plate due to slab pull will reduce the thickness or completely remove lower-plate lithosphere and cause decompression melting of the asthenospheric mantle; magmas from this source may subsequently provide enough heat for substantial partial melting of crustal rocks under or beyond the toe of the collisional accretionary system. In central Maine, United States, this type of magmatism is first apparent in the Early Devonian West Branch Volcanics and equivalent mafic volcanics, in the slightly younger voluminous mafic/silicic magmatic event of the Moxie Gabbro-Katahdin batholith and related ignimbrite volcanism, and in other Early Devonian granitic plutons. Similar lower-plate collisional sequences with mafic and related silicic magmatism probably caused by slab breakoff are seen in the Miocene-Holocene Papuan orogen, and the Hercynian-Alleghenian belt. Magmatism of this type is significant because it gives evidence in those examplesof whole-lithosphere extension. We infer that normal fault systems in outer trench slopes of collisional orogens in general, and possibly those of oceanic subduction zones, may not be primarily due to flexural bending, but are also driven by whole-lithosphere extension due to slab pull. The Maine Acadian example suggests that slab failure and this type of magmatism may be promoted by preexisting large margin-parallel faults in the lower plate.

Linking process, dimension, texture, and geochemistry in dolomite geobodies: A case study from Wadi Mistal (northern Oman)

“…Normal fault systems develop commonly in outer trench slopes of collisional orogens as a result of flexural bending, and thus, faulting in the upper part of the lithosphere (Chapple and Forsyth, 1979). Normal faulting in similar environment could also be caused by whole-lithosphere extension due to slab pull, though (Schoonmaker et al, 2005). Another hypothesis links the normal faulting to the Maastrichtian postobduction ENE-WSW extension or Oligocene NNE and NW extension (Fournier et al, 2006).…”

Petrology of the Grays River volcanics, southwest Washington: Plume-influenced slab window magmatism in the Cascadia forearc

“…Magmas produced in this setting are generally small in volume but diverse in composition, ranging from basalts with mid-ocean-ridge (MORB) or ocean-island (OIB) traits (Kimura et al, 2005) to adakites (Benoit et al, 2002), boninites and high-Mg andesites (Deschamps and Lallemand, 2003), and a variety of felsic rocks (Madsen et al, 2006). Mechanisms proposed to account for forearc magmatism are also diverse, the most common being subduction of a spreading ridge (Madsen et al, 2006), rifting of the overriding plate (Davis et al, 1995), breakoff of the downgoing plate (Schoonmaker et al, 2005), magma leakage along transform faults (Gill, 1981), and the presence of a mantle plume (MacPherson and Hall, 2001). Because forearc igneous rocks record events and processes operating at the entryway to a subduction zone, they are particularly useful for understanding subduction processes as well as for reconstructing plate motions and geometries.…”

Early Middle Triassic mafic dikes from the Baoshan subterrane, western Yunnan: implications for the tectonic evolution of the Palaeo-Tethys in Southeast Asia

“…However, the magmatism expected in a foredeep setting would be submarine and would be interbedded with deep-water sediments (e.g. Hoffman 1987;Schoonmaker et al 2005;Bradley 2008). Therefore, we suggest that the Caojian mafic dikes were not emplaced at the passive margin of the Palaeo-Tethys Ocean.…”

“…There are three possible tectonic settings for emplacement of magma at passive margins: first, a initial rift setting, where continental rifting begins to create a new ocean basin; second, are-rift setting, where a ribbon continent or microcontinent splits off from an existing passive margin (Bradley 2008 and references therein); and third, a foredeep setting (e.g. Hoffman 1987;Schoonmaker et al 2005), where whole-lithosphere extension related to slab pull causes decompression melting of the asthenospheric mantle. The Middle Triassic age of the mafic dikes can rule out the first possibility, because the Palaeo-Tethys opened by at least Devonian time (e.g.…”

“…It has long been recognized that magmatism is usually absent at passive continental margins, unless it is related to the rifting processes, re-rifting or foredeep settings (e.g. Hoffman 1987;Schoonmaker et al 2005;Bradley 2008). However, magmatism is ubiquitous in the passive margins of subduction systems in collisional or post-collisional settings (e.g.…”