TY - JOUR A1 - Ishizuka, Osamu A1 - Hickey-Vargas, Rosemary A1 - Arculus, Richard J. A1 - Yogodzinski, Gene M. A1 - Savov, Ivan P. A1 - Kusano, Yuki A1 - McCarthy, Anders A1 - Brandl, Philipp A. A1 - Sudo, Masafumi T1 - Age of Izu-Bonin-Mariana arc basement JF - Earth & planetary science letters N2 - Documenting the early tectonic and magmatic evolution of the lzu-Bonin-Mariana (IBM) arc system in the Western Pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the Pacific and Philippine Sea plates. Forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation (52 Ma) and production of "forearc basalt". Ocean floor drilling (International Ocean Discovery Program Expedition 351) recovered basement-forming, low-Ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin (nominally reararc) of the future IBM arc (Amami Sankaku Basin: ASB). Radiometric dating of this basement gives an age range (49.3-46.8 Ma with a weighted average of 48.7 Ma) that overlaps that of basalt in the present-day IBM forearc, but up to 3.3 m.y. younger than the onset of forearc basalt activity. Similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore-to reararc of the present-day IBM arc. Given the age difference between the oldest forearc basalt and the ASB crust, asymmetric spreading caused by ridge migration might have taken place. This scenario for the formation of the ASB implies that the Mesozoic remnant arc terrane of the Daito Ridges comprised the overriding plate at subduction initiation. The juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic. (C) 2017 The Authors. Published by Elsevier B.V. KW - subduction initiation KW - Izu-Bonin-Mariana arc KW - arc basement KW - Ar-40/Ar-39 age Y1 - 2017 U6 - https://doi.org/10.1016/j.epsl.2017.10.023 SN - 0012-821X SN - 1385-013X VL - 481 SP - 80 EP - 90 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Scharf, Andreas A1 - Sudo, Masafumi A1 - Pracejus, Bernhard A1 - Mattern, Frank A1 - Callegari, Ivan A1 - Bauer, Wilfried A1 - Scharf, Katharina T1 - Late Lutetian (Eocene) mafic intrusion into shallow marine platform deposits north of the Oman Mountains (Rusayl Embayment) and its tectonic significance JF - Journal of African earth sciences N2 - A silica undersaturated alkali-olivine basanitic magma intruded the late Paleocene/early Eocene Jafnayn Formation near Muscat. Geochemical analyses indicate that a significant amount of host rock (limestone) was assimilated into the magma. We dated the basanite as 42.7 +/- 1.0 Ma (2 sigma error; late Lutetian), using the whole rock Ar-40/Ar-39 step-wise heating technique. Intrusion occurred in the hanging wall of a major regional extensional shear zone (Frontal Range Fault, FRF) bounding the northern margin of two domes within the Oman Mountains (Jabal Akhdar and Saih Hatat domes). Two shear intervals along the FRF have been documented. The first interval lasted immediately after emplacement of the Semail Ophiolite (latest Cretaceous-early Eocene) while the second and poorly constrained interval was assumed to have occurred during the Oligocene. The proximity of the basanite to the FRF suggests that magma used extensional faults for the upper part of its ascent path. Reactivated Permian rift faults of the Pangaea rift or other preexisting faults may have been used for the lower ascent part. We conclude that the basanite intrusion coincided with the onset of the second deformation interval along the FRF, because (1) the position of the basanite is near a dextral releasing bend, associated with the second shear interval, (2) the overlap of our Ar-40/Ar-39 age with the cooling curves for rocks from the nearby Jabal Akhdar Dome, and (3) the basanite postdates the first FRF deformation episode by > 10 Ma. Thus, the second interval along the FRF had started already during the late Lutetian and probably lasted into the Miocene. KW - Ar-40/Ar-39 age KW - Jafnayn formation KW - gravitational collapse KW - Basanite KW - extension KW - Limestone assimilation in basanite Y1 - 2020 U6 - https://doi.org/10.1016/j.jafrearsci.2020.103941 SN - 1464-343X SN - 1879-1956 VL - 170 PB - Elsevier CY - Oxford ER -