Abstract Tidal channels exert a crucial control on sediment transport and drive geomorphic changes in the tidal environment. Despite their ubiquitous occurrence, long-term morphodynamics and processes driving the morphologic changes remain less well understood than fluvial counterparts. Spanning from straight to dendritic, Korean tidal channels become more sinuous and densely populated with elevation due to higher mud content. Mutually evasive current patterns resulted in a cuspate meander bend, where a flood barb develops at the seaward side of the bend. Multiannual observation revealed that tidal channels migrate from up to 80 m per year in the lower intertidal zone of open-coast sandy tidal flats to nearly stationary for several years in the upper intertidal zone of protected muddy tidal flats. Migration rates are temporarily pronounced during the summer monsoon, when heavy rainfall-induced surface runoff intensifies ebb tidal asymmetry and promotes headward erosion. Meander bends are mostly landward-skewed and shift downstream, implying that ebb currents primarily drive the long-term channel morphodynamics. Tidal point bars commonly display ebbward dipping, inclined heterolithic stratification (IHS) dominated by bedforms formed by subordinate flood tidal currents. An overall ebb dominance and mutually evasive current patterns account for the counterintuitive stratigraphic architecture of point bars in Korean tidal channels.

Abstract Continental arcs are key sites of granitic magmatism, yet details of the origins of these magmas, including the role and contribution of mafic magma, the timing and location of initial zircon formation and how zircon isotopic signatures relate to granite formation, remain as challenges. Here we use U–Pb dating, trace elements and Hf isotopic systematics of zircon in mafic microgranular enclaves (MMEs), from the convergent plate margin Satkatbong diorite (SKD) in Korea to understand lower arc magmatism and zircon production. The host granitic body and MMEs display similar major element evolutionary trends and similar ranges of Sr, Nd and Hf isotopes, implying a cognatic relationship. Zircons show a large variability in ε Hf (t) (c. 6 units) and age (>30 Ma). We propose that the SKD and MMEs originated from the same, long-lasting, lower crustal mush reservoir, enabling long and variable residence times for zircons. Prolonged zircon ages, combined with the Hf isotope variability within a single pluton (SKD and its MME), indicate that not all zircons were instantaneously crystallized in a rapidly cooling shallow magma chamber but were continuously formed in a long-lasting hot source. A low-melt-fraction mush type reservoir in a deep crustal hot zone provides a viable model for the source setting. Continuous replenishment of mafic magmas acts as the main re-activator of the reservoir, and provide a critical role in spawning zircons that record a long age span, because (1) the magma adds Zr into the reservoir, enabling it to reach zircon saturation and (2) the generated zircon grains are transported upward as antecrysts by flow inside of the reservoir. This means that antecrysts with different ages may mix with each other in the ascending magma body. The significance of this model is that a conclusive time of intrusion cannot be constrained by such zircon ages, as these antecrysts constitute inherited grains.