Paleomagnetism of Izu-Bonin-Mariana Forearc Igneous Rocks from IODP Expedition 352 and Their Geologic and Tectonic Implications

Abstract:

IODP Expedition 352 cored igneous rocks on the Izu-Bonin-Mariana forearc, ~40-70 km west of the trench axis at 28.4°N. Holes U1439C, U1440B, and 1442A penetrated 362 m, 253 m, and 440 m, respectively of igneous section. Recovered rocks are mainly forearc basalts from Hole U1440B and boninites and andesites from Holes U1439C and U1442A. Radiometric dates of emplacement ages are primarily clustered around 50-51 Ma (Reagan et al., Earth Planet. Sci. Lett., 2019). We performed paleomagnetic analyses on 338 samples from these three holes to obtain data about the site paleolatitude and clues about geologic evolution. Most samples (88%) displayed stable demagnetization behavior. Hysteresis measurements and high and low-temperature magnetization curves show that samples from Hole U1440B undergo magneto-chemical changes when heated and mainly contain single-domain (SD) or pseudo-single-domain (PSD) titanomaghemite. Similar measurements show that most samples from Holes U1439C and U1442A contain either SD or PSD titanomagnetite with very little titanium content, or SD ferromagnetic grains with a large paramagnetic contribution. Characteristic remanent magnetization (ChRM) inclinations are dominantly <±30°, but with outliers that probably result from post emplacement geologic events. Inclination analysis indicates 28 statistically distinct inclination groups. Nevertheless, many inclination groups appear serially correlated, implying that the sections were emplaced within a geologically brief time period. The inclination groups give an average paleolatitude of -0.1°, implying that the site has drifted ~28° northward since emplacement, which is similar to prior published results. Many outlier samples are located near fault zones described in the cores, suggesting that these samples were either rotated or remagnetized. Both Holes U1439C and U1442A, which are only ~10 km apart, have zones with either positive or negative inclinations that match the site geocentric axial dipole inclination, implying the zones were recently remagnetized.