Topographic controls on dike injection in volcanic rift zones

Abstract

Abstract:Dike emplacement in volcanic rift zones is often associated with the injection of "bladelike" dikes, which propagate long distances parallel to the rift, but frequently remain trapped at depth and erupt only near the tip of the dike. Over geologic time, this style of dike injection implies that a greater percentage of extension is accommodated by magma accretion at depth than near the surface. In this study, we investigate the evolution of faulting, topography, and stress state in volcanic rift zones using a kinematic model for dike injection in an extending 2-D elastic-viscoplastic layer. We show that the intrusion of blade-like dikes focuses deformation at the rift axis, leading to the formation of an axial rift valley. However, flexure associated with the development of the rift topography generates compression at the base of the plate. If the magnitude of these deviatoric compressive stresses exceeds the deviatoric tensile stress associated with far-field extension, further dike injection will be inhibited. In general, this transition from tensile to compressive deviatoric stresses occurs when the rate of accretion in the lower crust is greater than 50-60% of the far-field extension rate. These results indicate that over geologic time-scales the injection of blade-like dikes is a self-limiting process in which dike-generated faulting and topography result in an efficient feedback mechanism that controls the time-averaged distribution of magma accretion within the crust.

Spatial relationship between earthquakes and volcanic vents in the central-northern Main Ethiopian Rift

“…The greater width of the active zone of faulting, shown from the maximum in seismicity intensity, than the width of the zone of vents is consistent with formation of 60-degree dipping normal faults that dip inwards and towards a central zone of diking (Rubin and Pollard, 1988;Behn et al, 2006). Along the western margin of the rift valley the correlation between vents and earthquakes is relatively poor.…”

Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present

“…These propagating dikes are sometimes accompanied by fissural eruptions (e.g., Einarsson & Brandsdóttir 1980). Dikes are characterized by uplift adjacent to the dike intrusion zone; faulted dikes show subsidence on the hanging-wall side, but the asymmetry is smaller than that predicted by fault slip (e.g., Rubin & Pollard 1988, Behn et al 2006. Thus, parallel zones of uplift, comparatively small seismic moment release, and slowly migrating swarms of volcano-tectonic earthquakes characterize dike intrusions.…”

“…(b) Magmatic and faulting processes achieving plate boundary deformation in response to far-field extensional forces and buoyancy forces of lateral density contrasts (magma, topography, crustal thinning, mantle thinning). Vertical crustal movements predicted by dike intrusion alone are shown above the rift valley topography (vertical scale in meters; after Behn et al 2006). Interstitial mantle melt percolates upward along pressure gradients and then ponds in magma chambers.…”

“…The temporal and spatial patterns of faulting and magmatism maintain the along-axis morphology of mid-ocean ridges from the onset of seafloor spreading, as seen in the Afar rifting episode from 2005 to the present outlined below. The production, storage, and eruption of magma shape the structure and morphology of the plate boundary, and the buoyancy force of the magma adds to the tectonic forces driving plate divergence (e.g., Buck 2004, Behn et al 2006. Fluids and heat from magma-filled cracks, dikes, and sills as well as long-lived magma chambers modify the physical properties of the rocks they intrude, temporarily altering the way they respond to tectonic forces (e.g., Holtzman et al 2003, Evans et al 2004.…”

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

“…Fractional crystallization of basaltic magma rather than anatexis of continental crust was the major source for the silicic magmas of the Afar and rift valley margins Peccerillo et al, 2003;Peccerillo et al, 2007;Ayalew and Gibson, 2009;Rooney et al, 2012b). A lessened magma supply rate, resulting in magmatic intrusion at greater depths and slower cooling (Behn et al, 2006), fostered crystal fractionation processes together with lithospheric assimilation. Additionally, rift faulting and fissuring within the continental crust facilitated shallow-level magmatic intrusion and differentiation (Antonellini and Cambray, 1992), together with stagnation of laterally migrating magma in the footwall of the faults (Bonini et al, 2001).…”