Giulia Palazzin

Long‐term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan

The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic melange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a “simple” giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.

Fluid properties and dynamics along the seismogenic plate interface

Fossil structures, such as exhumed accretionary prisms, are the only direct recorders of the fluids wetting the plate interface near the base of the seismoxad genic zone. By studying exhumed accretionary prisms, it is thus possible to determine the physicochemical properties of fluids and the geometry and dynamics of their circulation. We considered here two transects encompassxad ing the brittlexadplastic transition (BPT) zone, in the FrancoxadItalian Alps and the Shimanto Belt in Japan, and compared our data with a broader set of examxad ples from the literature. On quartz that grew synkinematically at peak burial conditions, we inferred fluid properties indirectly from quartz tracexadelement concentrations (using cathodoluminescence [CL] imaging) and directly from fluidxadinclusion composition and PxadρxadT properties (using Raman and microxad thermometry). At ~250 °C, quartz grew principally through fracturing and two types of quartz, a CLxadbrown and a CLxadblue, precipitated alternately. At ~350 °C, where plastic deformation and recrystallization is pervasive, only a single, hoxad mogeneously CLxadbrown quartz is present. The salinity of the fluid in the incluxad sions shallower than the BPT is consistently of the order or lower than seaxad water, while salinities are very scattered deeper than the BPT and often exceed seawater salinity. The gas dissolved in the fluid is predominantly CH 4 shalxad lower than the BPT, and either CH 4 or CO 2 deeper than the BPT, depending on the nature of the host rock and in particular on the proportion of carbonates. Cathodoluminescence properties, salinity, and nature of the gas all point to a closedxadsystem behavior in rocks deeper than the BPT. In contrast, shallower than the BPT (i.e., at seismogenic depths), textures revealed by CLxadimaging evidence the episodic influx of an external fluid, leading to the crystallization of CLxadblue quartz. The scale of the circulation leading to the generation of the CLxadblue quartz, or its relationship with the seismic cycle, is still unclear. Bexad sides, the fluid pressure recorded in the abundant waterxadrich fluid inclusions is systematically much lower than the corresponding lithostatic pressure, irxad respective of the depth domain considered. For inclusions trapped at large depth, the low fluid pressure recorded in the inclusions reflects postxadentrapxad ment reequilibration. For inclusions trapped at shallower conditions, typically at seismogenic depths, the low fluid pressure may as well be the result of large fluid pressure drop after earthquakes.

Tertiary evolution of the Shimanto belt (Japan): a large-scale collision in Early Miocene

To decipher the Miocene evolution of the Shimanto belt of southwestern Japan, structural and paleo-thermal studies we carried out in the western area of Shikoku Island. All units constituting the belt, both in its Cretaceous and Tertiary domains, are in average strongly dipping to the NW or SE, while shortening directions deduced from fault kinematics are consistently orientated NNW-SSE. Peak paleo-temperatures estimated with Raman spectra of organic matter increase strongly across the southern, Tertiary portion of the belt, in tandem with the development of a steeply-dipping metamorphic cleavage. Near the southern tip of Ashizuri Peninsula, the unconformity between accreted strata and fore-arc basin, present along the whole belt, corresponds to a large paleo-temperature gap, supporting the occurrence of a major collision in Early Miocene. This tectonic event occurred before the magmatic event that affected the whole belt at ~15u2009Ma. The associated shortening was accommodated in two opposite modes, either localized on regional-scale faults such as the Nobeoka Tectonic Line in Kyushu, or distributed through the whole belt as in Shikoku. The reappraisal of this collision leads to reinterpret large-scale seismic refraction profiles of the margins, where the unit underlying the modern accretionary prism is now attributed to an older package of deformed and accreted sedimentary units belonging to the Shimanto belt. When integrated into reconstructions of Philippine Sea Plate motion, the collision corresponds to the oblique collision of a paleo Izu-Bonin-Mariana Arc with Japan in Early Miocene.

Deepwater fold-and-thrust belt along New Caledonia's western margin : relation to post-obduction vertical motions

Classically, deepwater fold-and-thrust belts are classified in two main types, depending if they result from near- or far-field stresses and the understanding of their driving and triggering mechanism is poorly known. We present a geophysical dataset off the western margin of New Caledonia (SW Pacific) that reveals deformed structures of a deepwater fold-and-thrust belt that we interpret as a near-field gravity-driven system, which is not located at a rifted passive margin. The main factor triggering deformation is inferred to be oversteepening of the margin slope by post-obduction isostatic rebound. Onshore erosion of abnormally-dense obducted material, combined with sediment loading in the adjacent basin, has induced vertical motions that have caused oversteepening of the margin. Detailed morpho-bathymetric, seismic stratigraphic and structural analysis reveals that the fold-and-thrust belt extends 200 km along the margin, and 50 km into the New Caledonia Trough. Deformation is rooted at depths greater than 5 km beneath the seafloor, affects an area of 3500 km2, and involves a sediment volume of approximately 13 000 km3. This deformed belt is organized into an imbricate fan system of faults, and one out-of-sequence thrust fault affects the seabed. The thrust faults are deeply rooted in the basin along a low-angle floor thrust and connected to New Caledonia Island along a major detachment. This study not only provides a better knowledge of the New Caledonia margin, but also provides new insight into the mechanisms that trigger deepwater fold-and-thrust belts.