TECTONOMAGMATIC CONTROLS ON SEQUENCE STRATIGRAPHY AND THE ACTIVE PASSIVE MARGIN

Sequence stratigraphy is a concept used to describe packages of sedimentary deposits bound by unconformities that are related to fundamental changes in sedimentary facies. These sequence boundaries are controlled by changes in rates of sediment supply or accommodation space and are manifested in the form of apparent relative sea level change. Traditionally, major sequence boundaries are thought to be time correlative across vast lengthscales. In addition, thick parasequences and their bounding unconformities are a distinctive feature of passive margins, far from any orogenic activity. This had led to the view that sequence boundaries are controlled by eustasy - changes in ocean volume associated with the waxing and waning of ice sheets. However, major sequence boundaries are common even during greenhouse intervals, such as the Cretaceous, when ice sheets were absent or limited, suggesting that other controls, such as tectonics, must also be considered. But whether tectonics can change at rates fast enough to generate major sequence boundaries is unclear. Here, we explore how far-field effects associated with mountain building can lead to major sequence boundaries on passive margins. During the Cretaceous, subduction along the western margin of North America generated an Andean style magmatic arc and a back-arc basin (interior seaway) undergoing dynamic subsidence. We show that changes in magmatic flux can lead to rapid (My-scale) changes in crustal thickness, elevation and erosion rates. The subsequent changes in crustal thickness can lead to changes in flexural loading of the North American plate, which adds more complexity to the geometry of the dynamically subsiding back-arc basin. We show through high age resolution reconstruction of volcanic ash compositions in the Cretaceous that magmatic arcs can vary in terms of major element composition on My timescales. These compositional changes may be associated with changes in crustal thickness or tectonic stress state, and if so, imply that far-field effects of magmatic arcs on sediment supply and basin geometry, driven by deep Earth processes, can control depositional environments on faster timescales than previously appreciated. Deposition at passive margins is therefore strongly influenced by distal orogenesis and magmatism.