A REVERSE ENERGY CASCADE FOR CRUSTAL MAGMA TRANSPORT (Invited Presentation)

Direct constraints on magma ascent through Earth's crust come primarily from exhumed intrusions. Connecting these structures to transport processes that ultimately control the occurrence, longevity, and style of volcanism remains a seminal problem. In long-lived provinces such as arcs, it is particularly unclear why so much mantle-derived melt is ultimately stored and not erupted at the surface. We combine an exhaustive field data analysis with new theory to show that this tendency for magma storage is a crustal property which evolves in response to magma transport. Intrusions preserved as plutonic complexes in the North American Cordillera exhibit two dominant classes of structures that are consistent with a transition in the rheological response of crustal rocks to intrusion. We propose that, in a long-lived crustal magma transport network, energy delivered from the mantle to open small dikes and sills will be transferred to intrusions of increasing size through a “reverse energy cascade” via mechanical merging, assimilation and mixing of small intrusive bodies into larger ones, as well as by cooling and solidification. Although the size distribution of intrusions less than ~100 m in minimum dimension is complex, larger intrusion sizes follow a power law scaling consistent with model predictions. Mantle magma supply over 10s to 100s of kyr should trigger the reverse energy cascade. Magmatic systems evolving within this regime preserve mantle input fluctuations that occur more slowly than the crust can mechanically mix, or that are large enough to activate transport pathways throughout the crust. Identifying regimes of magma transport provides a framework for inferring subsurface magmatic processes from surface patterns of volcanism, information preservation in the plutonic record, and related effects including climate. A young arc, erupting near the rate at which magma is supplied, should exhibit intrusions dominated by elastic deformation and no energy exchange between scales. Surface eruptions should mimic the spatial distribution of mantle input. For the same mantle supply, a longer-lived transport network will evolve to a rheological regime favoring storage, and a power law intrusion size distribution with maximum intrusion scales approaching crustal thickness.