Deposition and Deformation of Modern Accretionary-Type Forearc Basins: Linking Basin Formation and Accretionary Wedge Growth

Since a comprehensive review of forearc basins was published by Dickinson more than 20 years ago, a significant amount of new data about them have been published. These recent studies revealed details of depositional and deformation styles in the forearc basins, suggesting the formation processes were not unique. In this chapter, we reviewed modern forearc basins to understand how is the basin stratigraphy related with growth of accretionary wedges. The results indicate forearc basin can be classified into two (single‐ and two‐wedge models) plus one (strike‐slip model): (1) the single‐wedge model which is characterized by landward tilting of the basin strata ascribed to asymmetrical doubly vergent (single‐vergent) uplift of the outer arc high with forethrusts (seaward‐vergent thrusts in the pro‐wedge); (2) the two‐wedge model which is marked by contractional deformation caused by symmetrical doubly vergent uplift of the wedge with forethrusts in the prowedge and back‐thrusts (landward‐vergent thrusts) in the retro‐wedge; and (3) the strike‐slip model which is an additional one being represented by transpressional and/or transtensional deformations due to oblique subduction. We speculate that these models spatially and temporally depend on material fluxes at the plate interfaces that affect geometry and mechanical strength of backstops

Volcanic magma reservoir imaged as a low-density body beneath Aso volcano that terminated the 2016 Kumamoto earthquake rupture

Additional file 2: Figure S2. Calculated density contrast models beneath Aso volcano in horizontal slices at depths of 3.1, 4.9, and 8.2 km: (a), (b), (c), (d), (e), and (f) correspond to density contrasts of ±0.15, ±0.20 ±0.25, ±0.30, ±0.35, and ±0.40 g/cm3, respectively (Additional file 5: Table S1)

Granular experiments of thrust wedges: Insights relevant to methane hydrate exploration at the Nankai accretionary prism

The accumulation mechanism of methane hydrates has been a central issue in previous hydrate research regarding the Nankai accretionary prism, southwest of Japan. Expulsion of formation fluids is significant during the prism accretion process, and the migration of these methane-bearing fluids exerts a strong control on the accumulation of hydrates. Two types of fluid pathways, inter-granular porosity and faults, need to be evaluated to understand hydrate accumulation. Fluid migration along faults can be partly modeled by examining faulting activity. Our study modeled the accretion process by using two granular methods that approximated the geologic body as an assemblage of particles: (1) analog experiments using granular materials, and (2) a numerical simulation based on the distinct element method. The analog experiments closely reproduced the prism geometry observed in seismic profiles across the Nankai accretionary prism. Digital image correlation analysis indicated that the frontal thrust is generally active but older structures are also frequently reactivated. The numerical simulations produced prism geometries similar to those of the analog experiments. The velocity distributions of the particles showed evidence of episodic faulting and reactivation, but the internal stress field exhibited little change in the deeper part of the prism during deformation. The frequent and substantial changes in fault activity displayed by the models indicate episodic fluid flow along fault surfaces. Active frontal thrusting suggests that formation fluids generally migrate from deep within the prism to the deformation front, but may move along reactivated older faults. Inter-granular permeability also fluctuates, as it is controlled by temporal and spatial variations in the internal stress field. However, fluid flow is likely to be relatively stable in the deeper segment of the prism

Temporal changes in the internal stresses and pore pressures in a large-scale submarine mass transport deposit

We examined the temporal changes in the internal stresses and pore fluid pressures of a submarine mass transport deposit (MTD) in the Akkeshi Formation of the Upper Cretaceous–Paleocene Nemuro Group, eastern Hokkaido Island, Japan. We first analyzed previous paleostress field results from meso-scale faults in the MTD blocks, which indicated two phases during the evolution of the debris flow: phase I, radial spreading of the flow body during downslope movement; phase II, the flow body underwent compression during deposition on the basin plain. We also estimated the pore fluid pressure ratio from the fault orientation distribution. There was a large increase in the pore fluid pressure ratio during the transition from phase I to phase II that continued to rise during the initial stage of phase II and then decreased in its latter stages, whereas the maximum horizontal compressive stress increased throughout phase II. This variation in pore fluid pressure relates to the dynamics and evolution of the debris flow, where the clasts in the central part of the flow were supported by the excess pore pressure due to the compression of the debris flow as the flow head decelerated. Although pore fluid pressure plays a critical role in the dynamics of debris flows, there was no previous methodology to quantify both the stress fields and pore fluid pressures in large debris flows and their resultant MTDs. Our results implemented for outcrop studies imply that meso-scale faults in MTDs can provide clues to better understand these paleoflow mechanisms

Normal-faulting stress state associated with low differential stress in an overriding plate in northeast Japan prior to the 2011 Mw 9.0 Tohoku earthquake

Abstract Spatial and temporal variations in inland crustal stress prior to the 2011 Mw 9.0 Tohoku earthquake are investigated using focal mechanism solutions for shallow seismicity in Iwaki City, Japan. The multiple inverse method of stress tensor inversion detected two normal-faulting stress states that dominate in different regions. The stress field around Iwaki City changed from a NNW–SSE-trending triaxial extensional stress (stress regime A) to a NW–SE-trending axial tension (stress regime B) between 2005 and 2008. These stress changes may be the result of accumulated extensional stress associated with co- and post-seismic deformation due to the M7 class earthquakes. In this study we suggest that the stress state around Iwaki City prior to the 2011 Tohoku earthquake may have been extensional with a low differential stress. High pore pressure is required to cause earthquakes under such small differential stresses

Thermal maturity structures in an accretionary wedge by a numerical simulation

Abstract This study investigates the thermal maturity structure of the accretionary wedge along with the thermal history of sediments during wedge formation using a numerical simulation. The thermal maturity, which is described in terms of vitrinite reflectance, is determined using the temperature and duration of exposure based on the particle trajectories within the accretionary wedge. This study revealed the variability in the thermal maturity even though sediments are observed to originate at an identical initial depth and thermal conditions. We propose two end-member pathways of sediment movement in the accretionary wedge during wedge growth: a shallow, low thermal maturity pathway and a deep, high thermal maturity pathway. These shallow path sediments, which move into the shallow portion of the wedge during wedge growth through accretion, rarely experience high temperatures; therefore, their thermal maturity is low. However, the sediments subducted in the deep portion of the wedge experience high temperatures and obtain high thermal maturity as a function of the deep high thermal maturity pathway. Simultaneously, a geological deformation event, such as faulting, defines the steps of thermal maturity. The small step of thermal maturity is formed by the frontal thrusting and can be preserved as a function of the shallow low thermal maturity pathway. However, the step is overprinted and is observed to disappear through the deep high thermal maturity pathway. The large step of thermal maturity is formed by long-term displacement along an out-of-sequence thrust (OOST) in the deep portion of the wedge

The Role of Crystal Accumulation and Cumulate Remobilization in the Formation of Large Zoned Ignimbrites: Insights From the Aso-4 Caldera-forming Eruption, Kyushu, Japan

The Aso-4 caldera-forming event (86.4 ± 1.1 ka, VEI-8) is the second largest volcanic eruption Earth experienced in the past 100 ka. The ignimbrite sheets produced during this event are some of the first ever described compositionally zoned pyroclastic flow deposits exhibiting clear compositional, mineralogical and thermal gradients with stratigraphic position. Large quantities of the deposits are composed of crystal-poor, highly evolved juvenile pumices, while late-erupted pyroclastic flows are in many cases dominated by crystal-rich and less silicic scoria. These petrological gradients in the Aso-4 deposits have been linked to extensive magma mixing of two compositionally distinct magmas in a complex upper crustal reservoir. However, new studies on several other zoned ignimbrites suggest that magma mixing alone is not sufficient to fully explain such strong compositional gradients in the deposits. These gradients are expected to be dominantly caused by the recharge-induced reactivation of extracted melt caps and their complementary cumulate in the upper crust. Here, we investigate bulk rock and matrix glass data with detailed analyses of mineral chemistry in order to re-evaluate the Aso-4 deposits in light of these latest developments. Reverse chemical zoning in phenocrysts, Sr enrichment in euhedral rims of plagioclase and the presence of mafic minerals (clinopyroxene, olivine) indicate recharge of hot, mafic magmas shortly prior to eruption, inducing a mixing signature. However, the marked enrichment in some elements in bulk-rock analyses and the presence of highly evolved minerals (some in the form of glomerocrysts) in the late-erupted, crystal-rich units, provide clear evidence for crystal accumulation in these scoria. Mass balance modeling of P2O5, Sr and SiO2 supports the extraction of melt-rich lenses within an upper crustal mush zone, leaving a partly cumulative evolved crystal residue. We therefore propose an origin of the compositionally zoned Aso-4 ignimbrite largely by erupting a heterogeneous upper crustal reservoir, consisting of crystal-poor rhyodacitic melt caps within its associated cumulate mush. This complex reservoir was reactivated by mafic recharge shortly prior to eruption, imparting an additional mixing signature to the deposits.ISSN:2296-646