Expedition 351 methods

Timing of Tectonic and Magmatic Events in the Philippine Sea Plate Since 50 Ma From High‐Resolution Magnetostratigraphy of IODP Site U1438

“…The base of Unit III is transitional into Unit IV (1361.4-1461.1 mbsf), which comprises a highly diverse suite of medium to coarse sandstones, siltstones, and radiolarian-bearing mudstones, together with three thin (10-20 cm) intervals of igneous rock. Most of the sedimentary sequence is characterized by horizontal to subhorizontal beds (except for rare intervals showing tilted beds; see Figure F6 in Arculus et al (2015b)), indicating no significant deformation at Site U1438. Below the sedimentary succession is Unit 1 (1461.1-1611.1 mbsf), which is a sparsely vesicular, microcrystalline to fine-grained, aphyric to sparsely porphyritic basalt formed upon decompressional melting of a previously depleted mantle source (Hickey-Vargas et al, 2018;Yogodzinski et al, 2018).…”

“…By combining the ChRM directions from the half core sections and discrete samples from all the analyzed cores, we identified 142 reversals, providing the first high-resolution magnetostratigraphy for IODP Site U1438 (Figure 3). By integrating our magnetostratigraphy with the available biostratigraphic data (radiolarians, nannofossils, and foraminifers) from Arculus et al (2015b), zircon U/Pb ages (Barth et al, 2017;Waldman et al, 2021), and Ar/Ar ages (Ishizuka et al, 2018;Waldman et al, 2021), we correlated 115 reversals from our magnetostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) of Ogg (2020). Below, we describe our proposed magnetostratigraphic correlations in more detail for seven depth intervals, following the subdivision adopted in Figure 3.…”

“…Although known to be a first-order approximation, the precise timing of these tectonic and magmatic events in the PSP over the past ∼50 Ma remains unconstrained due to the lack of a reliable and continuous geological record. In 2014, International Ocean Discovery Program (IODP) Expedition 351 recovered a unique ∼1.6-km-thick sedimentary and volcanic sequence at Site U1438 (Arculus et al, 2015a(Arculus et al, , 2015b from the northern Amami-Sankaku basin at the base of the western flank of the Kyushu-Palau Ridge (Figure 1). Sedimentary rocks at Site U1438 make up the majority of the recovered material, with a ∼1.4-km thick sequence divided into four sedimentary units (Units I to IV), overlying a volcanic basement (Unit 1) (Figure 1c; Arculus et al, 2015b).…”

“…In 2014, International Ocean Discovery Program (IODP) Expedition 351 recovered a unique ∼1.6-km-thick sedimentary and volcanic sequence at Site U1438 (Arculus et al, 2015a(Arculus et al, , 2015b from the northern Amami-Sankaku basin at the base of the western flank of the Kyushu-Palau Ridge (Figure 1). Sedimentary rocks at Site U1438 make up the majority of the recovered material, with a ∼1.4-km thick sequence divided into four sedimentary units (Units I to IV), overlying a volcanic basement (Unit 1) (Figure 1c; Arculus et al, 2015b). Lithostratigraphic Unit I is composed of hemipelagic mud with minor ash beds, Units II and III are dominated by coarser (sand-gravel) volcaniclastic deposits, and Unit IV contains a mixed assemblage of hemipelagic and volcaniclastic sediments.…”

“…Furthermore, the limited time available during shipboard operations did not allow for an accurate visual inspection of the individual demagnetization plots, necessary for a reliable assessment of the magnetic polarity. Lastly, the magnetostratigraphy in Arculus et al (2015b) did not include the bottom of the sedimentary sequence and the underlying volcanic basement.…”