Tectonic evolution of the Mariana convergent margin

Within this project, the processes within the upper plate and along the subduction channel at an active subduction zone and the associated transformation of the Philippine plate`s mantle will be investigated using drill cores, which were recovered during the International Ocean Discovery Program Expedition 366. The transformation of mantle rocks to serpentinite and serpentinite mud is related to of deep aqueous fluids. These transformation products occur as serpentinite mud volcanoes on the ocean floor and additionally bring along a series of rock fragments from several kilometers of depth. Thus, it is possible to analyse deep processes within the Earth`s crust and the Earth`s mantle, and along subduction zones. For this purpose, ocean rock rocks were drilled and sampled at eight drilling sites in the area of the Mariana Trench north-east of Guam. The fluids rising with the serpentinite mud originate from the subducted lower plate (Pacific Plate). By structural geological and geochemical analyses, systematic variations are to be investigated with the distance from the Mariana trench, and thus with the depth of the subducted lower plate. Using data from trace element analyzes, including rare earth elements, the interactions and chemical reactions between aqueous fluids and the earth`s mantle, and temperatures at the fluid source can be reconstructed. The period of their activity can be determined by means of micro-paleontological investigations on deep sea sediments in the area of the serpentinite mud seamounts.

This project investigated the processes within the upper plate and along the subduction channel at an active subduction zone and the associated transformation of the Philippine plate's mantle using drill cores from 3 serpentinite mud seamounts situated on the Mariana forearc which were recovered during International Ocean Discovery Program (IODP) Expedition 366. Samples comprise serpentinite mud with lithic clasts from the underlying forearc lithosphere and from the subducting Pacific plate. The science objectives include (1) processes of mass transport and geochemical cycling within the forearc, (2) the spatial variability of slab-related fluids within the forearc, and water-rock-reactions in subduction and supra-subduction zone settings, (3) the metamorphic and tectonic history of the subduction channel, (4) the timing and rates of these processes, and (5) the influences of serpentine mud volcanism on environmental conditions. A trend of decreasing serpentinization temperature and increasing alteration degree was identified with decreasing distance to the Mariana trench. Mafic rock clasts were analyzed for reconstruction of their metamorphic and deformational overprint in order to reveal the tectono-metamorphic conditions at the metamorphic peak within the subduction channel and the subsequent low-grade overprint during exhumation. Blueschist facies metamorphic rocks, being affected by metamorphic pressures in the range of 11 to 13.8 kbar at minimum, were very likely exhumed from greater depth within the subduction channel before being captured by uprising, localized serpentine mud flows, indicating evidence that corner flow is actually taking place along the Mariana convergent margin. Integrated calcareous nannofossils and planktonic foraminifera biostratigraphy provide valuable information on the latest phase of the seamount activity. The onset of the latest stage of the serpentinite mud production was recorded between 6.10 million years (late Miocene, Messinian) and 4.20 million years (early Pliocene, Zanclean). The emplacement of the serpentinite mud production has been found to concur with the inception of the rifting in the Mariana Trough dated at 7-6 million years. An abrupt change in the sedimentation rates (11.80 to 94.71 m per million years) between different mud deposits was observed, possibly related to regionally occurring tectonic processes (variation of forearc extension due to the slab rollback and/or phases of seamount accretion and subduction). Statistical analyses on planktonic foraminifera assemblages differentiated two sample groups related to the ratio between thermocline/mixed-layer taxa, which indicate fluctuations in the depth of the thermocline (DOT) during the Pleistocene. Variations in the DOT reflect changes in the intensity of the North Equatorial Current associated with El-Niño/La-Niña conditions. Mudflows do not influence the ecology of planktonic foraminifera but possibly enhance their preservation against dissolution. Benthic foraminifers are severely affected by the serpentinite mud volcanism showing high diversity pre/post-volcanism, indicating oligotrophic bottom-water conditions, but rare in the serpentinite mud.