SEDIMENTARY SIGNATURES AND PALEOGEOGRAPHIC IMPLICATIONS OF SUBDUCTION INCEPTION: TWO PACIFIC MARGIN EXAMPLES

From initiation to abandonment, continental convergent margins are complex and dynamic settings whose basin fills record margin history. In the case of subduction inception, published geodynamic models suggest that both induced and spontaneous processes are possible; these models serve to predict the stratigraphic fingerprints one might expect from such events. Spontaneous subduction should lead to subsidence and early magmatism in the proto-forearc prior to development of the magmatic arc. In contrast, where subduction is induced, there should be evidence for rapid uplift (shallowing/unconformity), then rapid subsidence (fining- and deepening-upward succession), followed by building of the arc edifice. Indeed, the Permian meta-sedimentary succession of the El Paso Mountains, California, exhibits such a record. Within this marine sequence, there is sedimentological evidence of uplift and erosion (conglomerate with clasts of deep-water facies), then subsidence (calcareous to non-calcareous argillite), followed by shallowing (calcareous argillite, increasing carbonate and quartzose turbidites, transitioning to volcaniclastic sediment). Continued gradual shallowing was accompanied by progressive development of a magmatic edifice as indicated by the thick succession of tuffaceous turbidites overlain and intruded by andesitic lava flows. Various paleontological and geochronological data constrain this succession to ~280-260 Ma, a short timeframe predicted by modeling. Ophiolite obduction, marked by subaerial exposure and erosion of oceanic crust, is also a possible signature of subduction initiation. In a New Zealand example, mafic serpentine-bearing gravels (Miocene Ihungia Conglomerate) were likely sourced from an emergent ophiolite-bearing tectonic highland (island) arguably emplaced during subduction inception. This unit has paleogeographic significance in that similar islands produced to the northwest of the study area could have served as important faunal and floral refugia during a time period (Late Oligocene) when the New Zealand region was otherwise mainly a submerged marine shelf. Such islands would thus help explain the enigmatic persistence of non-marine fauna and flora across the Oligocene/Miocene boundary in this region as demonstrated in the fossil record.