Development of surface folds on rhyolitic lava: paleomagnetic evidence

Surface folds are common deformation structures in rhyolitic lava flows; however, the thermal conditions under which such structures develop remain poorly understood because direct observations during lava emplacement are rare. In this study, we investigate the formation of surface folds in the ~3.1-ka Kawagodaira rhyolitic lava flow, central Japan, using paleomagnetic analyses. Stepwise thermal demagnetization of lava crust samples reveals multiple remanent magnetization components with distinct directional characteristics, whereas a localized protrusion records a single magnetization component consistent with the contemporaneous geomagnetic field. The reference geomagnetic field direction at the time of eruption was determined from small pyroclastic clasts associated with the eruption. At the lava crust sites, the remanence directions systematically diverge from the reference direction from the low-temperature to the medium- and high-temperature components. The geometry of these directional changes indicates progressive rotation predominantly about a sub-horizontal axis, consistent with the folding-type deformation of the lava crust. The sense of rotation systematically differs between the northern and southern limbs of the fold structure; higher-temperature components record progressively steeper inclinations on the northern limb and shallower inclinations on the southern limb. The relationship between remanence directions and unblocking temperatures indicates that lava folding occurred after cooling below the Curie temperature of magnetite (~ 580 °C). The earliest deformation events were recorded shortly below this temperature, at about 540–500 °C, whereas later events occurred at temperatures as low as ~200 °C or even below ~100 °C. These results indicate that the deformation of the lava surface continued well after crustal solidification and persisted over a broad temperature range during cooling, likely driven by continued lava supply and internal motion beneath the crust. Our findings demonstrate that paleomagnetism provides a useful tool for reconstructing the thermal and kinematic conditions associated with deformation in silicic lava flows. Graphical abstract