Deng Zeng

The Tibetan Plateau has drawn significant research attention for its complex formation and uplift processes. While central and southern Tibet have been relatively well studied, northern Tibet remains less explored. This study integrates zircon and calcite U-Pb dating, detrital zircon analysis, and stable and clumped isotope analyses to refine the chronostratigraphy, paleoelevation, and paleo-drainage change in the Fenghuoshan Basin. U-Pb dating of a volcanic tuff from the lower Fenghuoshan Group yielded an age of 65–64 Ma. The volcanic tuff is underlain by detrital zircon age populations of 81 Ma and 75 Ma, as well as limestone ages of 79–77 Ma, which collectively supports a revised depositional onset of ca. 80 Ma for the Fenghuoshan Group. Clumped isotope analyses from the Erdaogou area reveal extremely high temperatures (>150 °C), which makes those samples unsuitable for paleoelevation reconstruction. However, paleosol carbonate nodules found in the northernmost part of the Erdaogou area provide a more reliable proxy. Using an updated moisture mixing model, we estimate a Late Cretaceous period paleoelevation of 1260 m (+740/−870 m) at ca. 80 Ma. Furthermore, the integration of new detrital zircon data with previous provenance and paleoelevation data reveals a transition from a single northern sediment source originating from the Tanggula Mountains during the Late Cretaceous to contributions from the double flow of the Fenghuoshan Basin in the early Oligocene. This suggests the initial emergence of a topographic configuration similar to the present-day landscape in at least the early Oligocene.

The transition from cold forearc to hot arc is considered fundamental to subduction dynamics. However, the range and depth of the cold nose changing significantly along the strike are difficult to constrain. By incorporating the three‐dimensional high‐resolution slab topography and MORVEL plate motion field to constrain thermomechanical modelling, we estimate the subduction thermal state and petrological dehydration in Izu‐Bonin. The multilayered hydrothermal regime gradually transitions from cold to less cold forearc, associated with a large‐scale subduction dewatering process varying along the strike. The dehydration embrittlement resulting from eclogitization and harzburgitization influences the occurrence of fast to slow earthquakes on megathrusts. Impressively, the cold nose is constrained to a depth of 60 km between the arc and Moho‐depth plate interface. The lowered temperature and delay of rock harzburgitization in the cold center of the subducted plate at depth contribute to the occurrence of deep earthquakes below the hot backarc interface.

The catastrophic 2004 Sumatra‐Andaman Mw9.1 earthquake associated with destructive tsunamis has characterized Sumatra as one of the most dangerous convergence zones. Nevertheless, the effects of the thermohydrous state on the strongly coupled megathrust of the incoming plate remain enigmatic. By using a three‐dimensional (3‐D) thermomechanical model to compute the temperature variation and the complicated phase transition process of the water‐bearing descending plate which generates unstable thrust slips, we find that Sumatran earthquakes at varying depths are likely under the control of the inter or intraplate hydrothermal regime, which occurs in or close to the petrological metamorphism transition area. The slab dehydration of the water‐rich mid‐ocean‐ridge basalts (MORB) and the ultramafic rocks in the oceanic lithosphere releases a large amount of fluid to the continental wedge and further facilitates arc magmatism. The fluids are prone to upwelling following the subduction channel along the plate interface and thus contribute to the clustering of earthquakes updip of the dehydration front beneath offshore Sumatra. Brittle failure and dehydration embrittlement at depth, along with the temperature differences caused by variant slab geometry, are conjectured to greatly influence the occurrence of fast and slow earthquakes in Sumatra.