East Asian orogenic collapse caused by oblique subduction and reduced boundary force

<p>East Asia experienced compressional deformation in the early Mesozoic, across the South China Block, North China Craton (NCC) and the part of the Central Asian Orogenic Belt to the north of the NCC. Deformation and magmatism resulted from Triassic collisions that accreted the continental blocks, and also Izanagi (Paleo-Pacific) Plate subduction from the east. We suggest that there was a single East Asian orogenic plateau by the Middle Jurassic, from NE Russia to SW China, with a length of ~4000 km. The causes and timings of the destruction of this plateau are unclear, especially loss of the lower lithosphere of the NCC. Here, we synthesize evidence for late Mesozoic and early Cenozoic crustal thinning via extension and denudation, to quantify the previous crustal thickness. We find that there was a ~50 km thick crust by the Middle Jurassic across much of the area between NE Asia and SW China, which has since undergone ~30% thinning. A force balance indicates that the buoyancy force produced by the gravitational potential energy of this thick crust drove extension from the latest Jurassic - Early Cretaceous (~145 Ma), when a rapid switch from orthogonal to oblique subduction at the Asia-Izanagi plate margin reduced the compressive boundary force by ~30%. Mantle lithosphere thinning of the NCC exceeds crustal thinning by a factor of ~2; extensional collapse cannot be the only cause of cratonic destruction, but played a major role, and potentially triggered mantle instability. Early Cretaceous extension was accompanied by a flare-up in volcanism along East Asia, which we speculate contributed to the Cretaceous hothouse climate.</p>

The cosmological constant as a zero action boundary

ABSTRACT The cosmological constant Λ is usually interpreted as Dark Energy (DE) or modified gravity (MG). Here, we propose instead that Λ corresponds to a boundary term in the action of classical General Relativity. The action is zero for a perfect fluid solution and this fixes Λ to the average density ρ and pressure p inside a primordial causal boundary: Λ = 4πG <ρ+3p >. This explains both why the observed value of Λ is related to the matter density today and also why other contributions to Λ, such as DE or MG, do not produce cosmic expansion. Cosmic acceleration results from the repulsive boundary force that occurs when the expansion reaches the causal horizon. This universe is similar to the ΛCDM universe, except on the largest observable scales, where we expect departures from homogeneity/isotropy, such as CMB anomalies and variations in cosmological parameters indicated by recent observations.

The origin of secondary microseism Love waves

The interaction of ocean surface waves produces pressure fluctuations at the seafloor capable of generating seismic waves in the solid Earth. The accepted mechanism satisfactorily explains secondary microseisms of the Rayleigh type, but it does not justify the presence of transversely polarized Love waves, nevertheless widely observed. An explanation for two-thirds of the worldwide ambient wave field has been wanting for over a century. Using numerical simulations of global-scale seismic wave propagation at unprecedented high frequency, here we explain the origin of secondary microseism Love waves. A small fraction of those is generated by boundary force-splitting at bathymetric inclines, but the majority is generated by the interaction of the seismic wave field with three-dimensional heterogeneity within the Earth. We present evidence for an ergodic model that explains observed seismic wave partitioning, a requirement for full-wave field ambient-noise tomography to account for realistic source distributions.

Forced subduction initiation at passive continental margins: Numerical modeling

<p>Subduction initiation (SI) induced by the tectonic boundary force may play a significant role in the Wilson cycle. In the previous analog and numerical models, the constant convergent velocity is generally applied, which may lead to large boundary forces for SI. In this study, we begin with testing the simple case of SI at passive margin with constant convergent force. The results indicate that the boundary force required to trigger the SI at passive margin with a thin and young oceanic lithosphere is much lower than that with a thick and old one. It is consistent with the multiple Cenozoic subduction zones in the Southwest Pacific, which are young ocean basin within 40 Ma and compressed by the India-Australia plate. Furthermore, we extended our model to explore a more complex case, forced SI during the collision-induced subduction transference, which is critical for Tethyan evolution. Both collision and SI processes are integrated in the numerical models. The results indicate that the forced convergence, rather than pure free subduction, is required to trigger and sustain the SI in the neighboring passive margin after collision of terrane. In addition, a weak passive margin can significantly promote the occurrence of subduction initiation, by decreasing required boundary force within reasonable range of plate tectonics. However, the lengths of subducted oceanic slab and accreting terrane play secondary roles in the occurrence of SI after collision. Under the favorable conditions of collision-induced subduction transference, the time required for subduction initiation after collision is generally within 10 Myrs, which is consistent with the general geological records of Neo-Tethys. In contrast, both Atlantic passive margin and Indian passive margin are old and stable with absence of subduction initiation in the present, which remains an open question.</p>