Subduction dynamics and rheology control on forearc and backarc subsidence: Numerical models and observations from the Mediterranean

<p>The dynamics of oceanic and continental subduction zones is linked to the rise and demise of forearc and backarc basins in the overriding plate. Subsidence and uplift rates of these distinct sedimentary basins are controlled by variations in plate convergence and subduction velocities and determined by lithospheric rheological structure and different lithospheric thicknesses.</p><p>In this study we conducted a series of high-resolution 2D numerical models applying the thermo-mechanical code 2DELVIS (Gerya and Yuen, 2007). The model, based on finite differences and marker-in-cell techniques, solves the mass, momentum, and energy conservation equations for incompressible media; assumes elasto-visco-plastic rheologies and involves erosion, sedimentation and hydration processes.</p><p>The models show the evolution of wedge-top basins lying on top of the accretionary wedge and retro-forearc basins in the continental overriding plate, separated by a forearc high. These forearc regions are affected by repeated compression and extension phases. Higher subsidence rates are recorded in the syncline structure of the retro-forearc basin when the slab dip angle is higher and the subduction interface is stronger and before the slab reaches the 660 km discontinuity. This implies the importance of the slab suction force as the main forcing factor creating up to 3-4 km negative dynamics topographic signals.</p><p>Extensional back-arc basins are either localized along inherited crustal or lithospheric weak zones at large distance from the forearc region or are initiated just above the hydrated mantle wedge. During trench retreat and slab roll-back the older volcanic arc area becomes part of the back-arc region. Back-arc subsidence is primarily governed by crustal and lithospheric thinning controlled by slab roll-back. Onset of continental subduction and soft collision is linked to the rapid uplift of the forearc basins; however, the back-arc region records ongoing extension. Finally, during hard collision the forarc and back-arc basins are ultimately under compression.</p><p>Our results are compared with the evolution of the Mediterranean and based on the reconstructed plate kinematics, subsidence and heat flow evolution we classify the Western and Eastern Alboran, Paola and Tyrrhenian, Transylvanian and Pannonian Basins to be genetically similar forearc–backarc basins, respectively.</p>

Morphotectonic Analysis of the East Manus Basin, Papua New Guinea

Backarc basin systems are important sites of extension leading to crustal rupture where basin development typically occurs in rifting phases (or stages) with the final successful stages identified by the formation of spreading ridges and new oceanic crust. The East Manus Basin is a young (<1 Ma), active, rapidly rifting backarc basin in a complex tectonic setting at the confluence of the oblique convergence of the Australian and Pacific plates. Here we undertake the first comprehensive spatial-temporal morphotectonic description and interpretation of the East Manus Basin including a link to the timing of, and tectonic controls on, the formation of seafloor massive sulfide mineralization. Key seafloor datasets used in the morphotectonic analysis include multi-resolution multibeam echosounder seafloor data and derivatives. Morphotectonic analysis of these data defines three evolutionary phases for the East Manus Basin. Each phase is distinguished by a variation in seafloor characteristics, volcano morphology and structural features: Phase 1 is a period of incipient extension of existing arc crust with intermediate to silicic volcanism; Phase 2 evolves to crustal rifting with effusive, flat top volcanoes with fissures; and Phase 3 is a nascent organized half-graben system with axial volcanism and seafloor spreading. The morphotectonic analysis, combined with available age constraints, shows that crustal rupture can occur rapidly (within ∼1 Myr) in backarc basins but that the different rift phases can become abandoned and preserved on the seafloor as the locus of extension and magmatism migrates to focus on the ultimate zone(s) of crustal rupture. Consequently, the spatial-temporal occurrence of significant Cu-rich seafloor massive sulfide mineralization can be constrained to the transition from Phase 1 to Phase 2 within the East Manus Basin. Mineralizing hydrothermal systems have utilized interconnected structural zones developed during these phases. This research improves our understanding of the early evolution of modern backarc systems, including the association between basin evolution and spatial-temporal formation of seafloor massive sulfide deposits, and provides key morphotectonic relationships that can be used to help interpret the evolution of paleo/fossilized backarc basins found in fold belts and accreted terrains around the world.

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

The character of magmatism associated with the early stages of subduction zone and island arc development is unlike that of mature systems, being dominated in the Izu-Bonon-Mariana (IBM) case by low-Ti-K tholeiitic basalts and boninites. Basalts recovered by coring the basement of the Amami Sankaku Basin (ASB), located west of the oldest remnant arc of the IBM system (Kyushu-Palau Ridge; KPR), were erupted at ~49 Ma, about 3 million years after subduction inception. The chain of stratovolcanoes defined by the KPR is superimposed on this basement. The basalts were sourced from upper mantle similar to that tapped following subduction inception, and represented by forearc basalt (FAB) dated at ~52-51 Ma. The mantle sources of the ASB basalt basement were more depleted by prior melt extraction than those involved in the vast majority of mid-ocean ridge (MOR) basalt generation. The ASB basalts are low-Ti-K, aluminous spinel-olivine-plagioclase-clinopyroxene-bearing tholeiites. We show this primary mineralogy is collectively distinct compared to basalts of MOR, backarc basins of the Philippine Sea Plate, forearc, or mature island arcs. In combination with bulk compositional (major and trace element abundances plus radiogenic isotope characteristics) data for the ASB basalts, we infer the upper mantle involved was hot (~1400oC), reduced, and refractory peridotite. For a few million years following subduction initiation, a broad region of mantle upwelling accompanied by partial melting prevailed. The ASB basalts were transferred rapidly from moderate pressures (1-2 GPa), preserving a mineralogy established at sub-crustal conditions, and experienced little of recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

PALEODICTYON IN SHALLOW-MARINE SETTINGS – AN EVALUATION BASED ON EOCENE EXAMPLES FROM IRAN

ABSTRACT The graphoglyptid trace fossil Paleodictyon, characterized by stratiform hexagonal meshes, typically occurs preserved at the base of deep-marine turbidites. There is, however, a growing number of occurrences of Paleodictyon in shallow-marine deposits as evidenced by new finds in the Eocene of Iran. The Paleodictyon-containing Asara Shale Member of the Karaj Formation accumulated in a shallow backarc basin. Parallel-crested wave-ripple marks and microbially induced sedimentary structures occur closely above and below the Paleodictyon-bearing strata. Shallow-marine Paleodictyon have so far been reported from morphologically structured, extensive, epicontinental seas, rift basins, and young, prograding passive continental margins, but mainly from foreland and backarc basins. In the two latter cases, the Paleodictyon producers appear to represent adaptive survivors. Initially they settled in abyssal to bathyal turbiditic settings that rapidly aggraded and/or became tectonically uplifted with slight changes to depositional conditions. Finally, the Paleodictyon producers lived in rather shallow water and became preserved by tempestites. This scenario argues against the continuous presence of Paleodictyon producers in shallow-marine settings, suggesting instead they appeared there recurrently.

Pacific subduction control on Asian continental deformation including Tibetan extension and eastward extrusion tectonics

The India-Asia collision has formed the highest mountains on Earth and is thought to account for extensive intraplate deformation in Asia. The prevailing explanation considers the role of the Pacific and Sunda subduction zones as passive during deformation. Here we test the hypothesis that subduction played an active role and present geodynamic experiments of continental deformation that model Indian indentation and active subduction rollback. We show that the synchronous activity and interaction of the collision zone and subduction zones explain Asian deformation, and demonstrate that east-west extension in Tibet, eastward continental extrusion and Asian backarc basin formation are controlled by large-scale Pacific and Sunda slab rollback. The models require 1740 ± 300 km of Indian indentation such that backarc basins form and central East Asian extension conforms estimates. Indentation and rollback produce ~260–360 km of eastward extrusion and large-scale clockwise upper mantle circulation from Tibet towards East Asia and back to India.