EVIDENCE OF SUBDUCTION INITIATION RECORDED IN THE DADEVILLE COMPLEX OF ALABAMA
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Abstract:
The Appalachian Mountain range formed from multiple orogen-scale collisional events spanning from the Neoproterozoic to the Devonian. As the Iapetus Ocean began to close, early subduction zone magmatism created forearc lithosphere and volcanic arcs that were subsequently obducted onto Laurentia during the formation of Pangaea. These early magmatic products contain geochemical information that informs our understanding of subduction zone formation and evolution; however multiple collisional events, the emplacement of large (and potentially unrelated) magmatic intrusions, and extreme weathering at the southernmost extent of the Appalachians have made identifying these rocks and reconstructing the history of this portion of the margin notoriously difficult. Fortunately, recent studies of the modern Izu–Bonin–Mariana subduction system have shed new light on the geochemical evolution of subduction initiation and have resulted in the development of new criteria for classifying the first magmatic products of subduction–forearc basalts and boninites. These constraints allow for reassessment of subduction-related rocks throughout the Appalachian margin. Only one potential volcanic arc has been identified in the Southern Appalachians. This hypothesized arc, the Dadeville Complex of Alabama and Georgia, has geochemical signatures associated with subduction zone influence and is considered to have formed during subduction of Iapetan lithosphere underneath or adjacent to the Laurentian margin; however, the exact nature of the complex and its geologic history remains unclear. We apply the new understanding of subduction zone magmatic evolution derived from the Izu–Bonin–Mariana system to the Dadeville Complex to further elucidate its origin. Using whole rock and mineral major- and trace-element geochemistry coupled with detailed petrographic analyses, we have identified forearc basalts and boninites; therefore, we interpret these data to reflect formation of the Dadeville Complex during initial subduction in the Iapetus Ocean.Keywords:
Forearc
Volcanic arc
Eclogitization
Forearc basins are large sediment repositories that develop in the upper plate of convergent margins and are a direct response to subduction. These basins are part of the magmatic arc-forearc basin-accretionary prism "trinity" that defines the tectonic configuration of the upper plate along most subduction-related convergent margins. Many previous studies of forearc basins have explored the links between construction of magmatic arcs, exhumation of accretionary prisms, and sediment deposition in adjacent forearc basins. These studies provide an important framework for understanding firstorder tectonic processes recorded in forearc basins that are characterized by long-lived subduction of "normal" oceanic crust. Many convergent margins, however, are complicated by second-order subduction processes, such as flat-slab subduction of buoyant oceanic crust in the form of seamounts, spreading and aseismic ridges, and oceanic plateaus. These second-order processes can substantially modify the tectonic configuration of the upper plate both in time and space, and produce sedimentary basins that do not easily fit into the conventional magmatic arc-forearc basin-accretionary prism trinity. In this chapter, we discuss the modification of the southern Alaska forearc basin by Paleocene-Eocene subduction of a spreading ridge followed by Oligocene- Holocene subduction of thick oceanic crust. This thick oceanic crust is currently being subducted beneath south-central Alaska and has an imaged maximum thickness of 30km at the surface and 22 km at depth. Findings from southern Alaska suggest that forearc basins modified from flat-slab subduction processes may contain a sedimentary and volcanic stratigraphic record that differs substantially from typical forearc basins. Processes and sedimentary features that characterize modified forearc basins include the following: (1) flat-slab subduction of a buoyant, topographically elevated spreading ridge oriented subparallel to the margin prompts diachronous uplift of the forearc basin floor and exhumation of older marine forearc basin strata as the ridge is subducted. Passage of the spreading ridge leads to subsidence and renewed deposition of nonmarine sedimentary and volcanic strata that locally exceeds the thickness of the underlying marine strata. (2) Insertion of a slab window beneath the forearc basin during spreading ridge subduction produces local intrabasinal topographic highs with adjacent depocenters, as well as discrete volcanic centers within and adjacent to the forearc basin. (3) Flat-slab subduction of thick oceanic crust also results in surface uplift and exhumation of forearc basin sedimentary strata. However, the insertion of thick crust throughout the flat-slab region (i.e., lack of a slab window) inhibits subduction-related magmatism adjacent to the forearc basin. In the case of subduction of a >350-km-wide fragment of thick oceanic crust beneath south-central Alaska, exhumation of forearc basin strata located above the region of flat-slab subduction has prompted enhanced sediment delivery to active basins located along the perimeter of the flat-slab region. These perimeter basins record an increase in subsidence and sediment accumulation rates coeval with flat-slab subduction beneath the exhumed, inactive remnant forearc basin.
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Abstract Initiation of subduction following the impingement of a hot buoyant mantle plume is one of the few scenarios that allow breaking the lithosphere and recycling a stagnant lid without requiring any preexisting weak zones. Here, we investigate factors controlling the number and shape of retreating subducting slabs formed by plume‐lithosphere interaction. Using 3‐D thermomechanical models we show that the deformation regime, which defines formation of single‐slab or multi‐slab subduction, depends on several parameters such as age of oceanic lithosphere, thickness of the crust and large‐scale lithospheric extension rate. Our model results indicate that on present‐day Earth multi‐slab plume‐induced subduction is initiated only if the oceanic lithosphere is relatively young (<30–40 Myr, but >10 Myr), and the crust has a typical thickness of 8 km. In turn, development of single‐slab subduction is facilitated by older lithosphere and pre‐imposed extensional stresses. In early Earth, plume‐lithosphere interaction could have led to formation of either episodic short‐lived circular subduction when the oceanic lithosphere was young or to multi‐slab subduction when the lithosphere was old.
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Efficient recycling of subducted sedimentary nitrogen (N) back to the atmosphere through arc volcanism has been advocated for the Central America margin while at other locations mass balance considerations and N contents of high pressure metamorphic rocks imply massive addition of subducted N to the mantle and past the zones of arc magma generation. Here, we report new results of N isotope compositions with gas chemistry and noble gas compositions of forearc and arc front springs in Costa Rica to show that the structure of the incoming plate has a profound effect on the extent of N subduction into the mantle. N isotope compositions of emitted arc gases (9-11 N°) imply less subducted pelagic sediment contribution compared to farther north. The N isotope compositions (δ15N = -4.4 to 1.6‰) of forearc springs at 9-11 N° are consistent with previously reported values in volcanic centers (δ15N = -3.0 to 1.9‰). We advocate that subduction erosion enhanced by abundant seamount subduction at 9-11 N° introduces overlying forearc crustal materials into the Costa Rican subduction zone, releasing fluids with lighter N isotope signatures. This process supports the recycling of heavier N into the deep mantle in this section of the Central America margin.
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Abstract A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism.
Volcanic arc
Slab window
Slab
Eclogitization
Eurasian Plate
Convergent boundary
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Initiation of subduction following the impingement of a hot buoyant mantle plume is one of the few scenarios that allow breaking the lithosphere and recycling a stagnant lid without requiring any preexisting weak zones. Here, we investigate factors controlling the number and shape of retreating subducting slabs formed by plume-lithosphere interaction. Using 3-D thermomechanical models we show that the deformation regime, which defines formation of single-slab or multi-slab subduction, depends on several parameters such as age of oceanic lithosphere, thickness of the crust and large-scale lithospheric extension rate. Our model results indicate that on present-day Earth multi-slab plume-induced subduction is initiated only if the oceanic lithosphere is relatively young ( 10 Myr), and the crust has a typical thickness of 8 km. In turn, development of single-slab subduction is facilitated by older lithosphere and pre-imposed extensional stresses. In early Earth, plume-lithosphere interaction could have led to formation of either episodic short-lived circular subduction when the oceanic lithosphere was young or to multi-slab subduction when the lithosphere was old.
Slab
Eclogitization
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Mantle plume
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