MAGMA PROPAGATION AND STRESS INFLUENCES: A SPATIAL ANALYSIS OF THE AKAROA VOLCANIC COMPLEX

Magmatic intrusions and eruptions are dictated by attributes including volume, composition, the tectonic environment, and gravitational stress (edifices). The balance of tectonic and gravitational stress is critical to understanding new growth, yet is incompletely characterized. This study investigates the influence of gravity and tectonics on magma propagation (V and L-type) in the Miocene Akaroa Volcanic Complex, Banks Peninsula, New Zealand.

An analysis of the orientations (valleys, ridges, dikes), compositions, and locations of features (dikes, domes, and eruptive vents), as well as relationships between physical and geochemical attributes reveals a segregation of features both by proximity to eruptive centers and elevation. Basaltic vents occur consistently at low elevations, while undifferentiated dikes exist from the shore platform to upper ridges. Additionally, trachytic features are restricted to, and evenly distributed in, an elevation zone of 400 to 700m. The segregation of features by elevation indicates a close tie between gravitational stress and magma propagation. The zonation of trachytic domes is likely due to a combined critical load stress and buoyancy preventing magma ascension. Low elevation eruptive vents display tectonic control in their alignment with co-eruptive faults. The dikes however, are grouped radially and are unaligned with faults, indicating gravitational control.

In light of this apparent segregation, a model of the Akaroa Volcanic Complex magmatic system as vertically segregated magma reservoirs stemming from deep magma is proposed. Key processes include: Primitive basalt rising from depth (V-type) via open fractures (magmas are highly buoyant, with limited degassing and/or fractional crystallization), magma pooling in dike plexuses (promoting fractional crystallization and degassing), solidification and sealing of vertical conduits driven by increases in gravitational stress (cone growth), lateral L-type intrusions of evolved (trachytic) magma from upper plexuses, and continued magma propagation via new pathways. Over time and with rejuvenation and migration of magma, this process repeated on Akaroa to build an aggregate volcanic complex.