Postmagmatic Tectonic Evolution of the Outer Izu‐Bonin Forearc Revealed by Sediment Basin Structure and Vein Microstructure Analysis: Implications for a 15 Ma Hiatus Between Pacific Plate Subduction Initiation and Forearc Extension

Abstract

International Ocean Discovery Program Expedition 352 recovered sedimentary-volcaniclastic successions and extensional structures (faults and extensional veins) that allow the reconstruction of the Izu-Bonin forearc tectonic evolution using a combination of shipboard core data, seismic reflection images, and calcite vein microstructure analysis. The oldest recorded biostratigraphic ages within fault-bounded sedimentary basins (Late Eocene to Early Oligocene) imply a~15 Ma hiatus between the formation of the igneous basement (52 to 50 Ma) and the onset of sedimentation. At the upslope sites (U1439 and U1442) extension led to the formation of asymmetric basins reflecting regional stretch of~16-19% at strain rates of~1.58 × 10 −16 to 4.62 × 10 −16 s −1 . Downslope Site U1440 (closer to the trench) is characterized by a symmetric graben bounded by conjugate normal faults reflecting regional stretch of~55% at strain rates of 4.40 × 10 −16 to 1.43 × 10 −15 s −1 . Mean differential stresses are in the range of~70-90 MPa. We infer that upper plate extension was triggered by incipient Pacific Plate rollback 15 Ma after subduction initiation. Extension was accommodated by normal faulting with syntectonic sedimentation during Late Eocene to Early Oligocene times. Backarc extension was assisted by magmatism with related Shikoku and Parece-Vela Basin spreading at~25 Ma, so that parts of the arc and rear arc, and the West Philippine backarc Basin were dismembered from the forearc. This was followed by slow-rift to postrift sedimentation during the transition from forearc to arc rifting to spreading within the Shikoku-Parece-Vela Basin system. Plain Language SummaryThis study examines the stress and deformation conditions and timing of extension in the Izu-Bonin forearc subsequent to subduction initiation by combining seismic images and microstructure analyses on veins and fault zones. By that we also examine a hiatus of 15 Ma between the formation of igneous forearc crust and the formation of sediment basins by forearc extension. This is implemented into an overall tectonic model at lithospheric scale.

Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

“…This implies that the rate of mineral growth approximated the rate of fracture opening (Fisher & Brantley, 1992;Hilgers et al, 2001). Extensional veining at Sites U1439-U1441 fits well into the concept of an extensional forearc regime where normal faults, grabens, and half-grabens (Christeson et al, 2016;Robertson et al, 2018) related to slab rollback and trench retreat are observed Kurz et al, 2019). In accordance with this interpretation, the Izu-Bonin rear arc Site U1438 lacks syntaxial veins as well as large-scale extensional structures (…”

“…Transitional veins occur predominantly in the Izu-Bonin forearc Sites U1439-U1441, whereas the rear arc Site U1438 exposes a high number of purely antitaxial veins with length-width ratios >100. We therefore suggest that the extensional regime in the Izu-Bonin forearc (Christeson et al, 2016;Kurz et al, 2019) inhibited widespread antitaxial veining. Transitional veins may accordingly reflect antitaxial veining that was preceded, accompanied, or succeeded by extensional fracturing, whose path of propagation was predetermined by zones of weakness such as preexisting (antitaxial) veins.…”

“…The postmagmatic phase of the Izu-Bonin forearc is characterized by graben and half-graben formation along conjugate normal faults that acted as prominent fluid paths along which secondary minerals precipitated in veins (Christeson et al, 2016;Kurz et al, 2019;Reagan et al, 2015). These structures are late-stage features that concur with Eocene-Miocene Pacific slab rollback Kurz et al, 2019;Robertson et al, 2018). Drill cores and seismic cross sections from the rear arc in contrast lack any major fault structures (Arculus, Ishizuka, Bogus, Gurnis, et al, 2015;Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015).…”

“…The forearc, in contrast, is characterized by multiple normal faults (Christeson et al, 2016;Kurz et al, 2019;Reagan et al, 2015) that might have channelized advective fluid circulation, which in turn hampered antitaxial veining. Hence, blocky veins dominate the forearc, whereas well-developed antitaxial veins are rare.…”

“…The postmagmatic phase of the Izu‐Bonin forearc is characterized by graben and half‐graben formation along conjugate normal faults that acted as prominent fluid paths along which secondary minerals precipitated in veins (Christeson et al, ; Kurz et al, ; Reagan et al, ). These structures are late‐stage features that concur with Eocene‐Miocene Pacific slab rollback (Faccenna et al, ; Kurz et al, ; Robertson et al, ).…”