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Comparison of Dike Intrusions in an Incipient Seafloorspreading Segment in Afar, Ethiopia: Seismicity Perspectives

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Comparison of Dike Intrusions in an Incipient Seafloorspreading Segment in Afar, Ethiopia: Seismicity Perspectives JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B06405, doi:10.1029/2010JB007908, 2011 Comparison of dike intrusions in an incipient seafloor‐spreading segment in Afar, Ethiopia: Seismicity perspectives Manahloh Belachew,1,2 C. Ebinger,1 D. Coté,1 D. Keir,3 J. V. Rowland,4 J. O. S. Hammond,5 and A. Ayele2 Received 3 August 2010; revised 28 December 2010; accepted 15 March 2011; published 24 June 2011. [1] Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. Between 2005 and 2009 the ∼70 km long Dabbahu‐Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started from a <5 km radius zone at the middle of the Dabbahu‐Manda Hararo segment, and traveled northward and southward along the rift axis. Small magnitude earthquakes associated with the margins of the propagating dike tips are followed by the largest magnitude, primarily low‐frequency earthquakes. The seismic moment distributions show >80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low‐frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment. Citation: Belachew, M., C. Ebinger, D. Coté, D. Keir, J. V. Rowland, J. O. S. Hammond, and A. Ayele (2011), Comparison of dike intrusions in an incipient seafloor‐spreading segment in Afar, Ethiopia: Seismicity perspectives, J. Geophys. Res., 116, B06405, doi:10.1029/2010JB007908. 1. Introduction contributed significantly to the understanding of dike intru- sion related processes [Björnsson et al., 1977; Abdallah et al., [2] About two thirds of Earth’s crust is created at diver- 1979; Einarsson and Brandsdóttir, 1980; Tryggvason, 1984; gent plate boundaries [e.g., Kidd, 1977; Delaney et al., Klein et al., 1987; Einarsson, 1991; Fox et al., 1995; Rubin 1998; Macdonald, 1998]. Exposures of sheeted dikes in et al., 1998; Dziak et al.,2004;Doubre et al.,2007a;Traversa ophiolite complexes and the seismically identified layer 2b and Grasso, 2009]. In most cases dike intrusions are char- show the importance of dike intrusions for crustal accretion acterized by deflation‐inflation sequences of the dike feeder [e.g., Macdonald, 1998]. However, such processes are often chamber and associated subsidence‐uplift cycles above it undetected mainly due to lack of access to the active oceanic and by lateral migration of earthquake swarms as magma is rifts and our ability to detect signals remotely. Studies of removed from the chamber and intruded into the rift [Moore subaerial volcanotectonic rifting processes, however, have and Krivoy, 1964; Brandsdóttir and Einarsson, 1979; Einarsson, 1991; Rubin et al., 1998; Toda et al., 2002]. Elastic models of propagation of fluid‐filled cracks show 1Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA. fault slip ahead of a laterally propagating dike and graben 2Inistitute of Geophysics, Space Science and Astronomy, Addis Ababa formation above it account for the majority of seismicity [e.g., University, Addis Ababa, Ethiopia. Rubin and Pollard, 1988; Rubin, 1992; Rubin and Gillard, 3 School of Earth and Environment, University of Leeds, Leeds, UK. 1998]. Such models require an ambient stress distribution 4School of Environment, University of Auckland, Auckland, New Zealand. that is near failure, and existing fractures that extend to the 5Department of Earth Science, University of Bristol, Bristol, UK. dike plane at depth. Seismicity during dike intrusions is also attributed to the volumetric response of the solid matrix to Copyright 2011 by the American Geophysical Union. the emplacement of magma [e.g., Traversa et al., 2010]. 0148‐0227/11/2010JB007908 B06405 1of23 B06405 BELACHEW ET AL.: COMPARISON OF DIKE INTRUSIONS IN AFAR B06405 Geodetic and seismic observations in different rift settings two subsequent dikes recorded on a sparse temporary array and on basaltic volcanoes also support these modeling results and a permanent station by Keir et al. [2009]. Hamling et al. [e.g., Rubin and Pollard, 1988; Brandsdóttir and Einarsson, [2009] and Grandin et al. [2010a] use surface deformation 1979; Rubin et al., 1998; Traversa and Grasso, 2009]. The patterns to predict magma volumes, dike dimensions and temporal coincidence between deflation signals and migrat- magma source zones for the dike intrusions, but without ing earthquakes observed in some rifts suggest that dike constraints from seismicity data. Ebinger et al. [2010] widening stops when deflation stops at the source [e.g., summarized briefly the onset/end times and migration Einarsson, 1991; Yamaoka et al., 2005]. directions of the seismicity associated with the dikes between [3] Within tectonic rifts, dike feeder zones are located 2007 and 2009 but left the detailed comparison for this within the crust, and dikes are injected along the rift axis paper. We present, for the first time, detailed analyses of the perpendicular to the rift opening direction. The 20 dike seismicity patterns accompanying Dikes 7–13 recorded on a intrusions and fissural eruptions, for instance, between 1975 44‐station seismic network, and interpret them in the and 1984 in Krafla, Iceland were fed from a shallow magma growing body of structural and geodetic studies from this chamber located at ∼3 km depth beneath the Krafla caldera region, and of models of rift zone dike intrusions worldwide. [Einarsson, 1978; Brandsdóttir et al., 1997], which itself [6] In this paper we compare and contrast the detailed was probably fed from a subcrustal source [Björnsson, 1985; temporal and spatial distribution of intense seismic activity de Zeeuw‐van Dalfsen et al., 2004]. Shallow axial magma from 7 of the 14 dike intrusions that took place between chambers were also identified beneath the Asal rift [Doubre 2007 and 2009, and were detected on the same 44‐station et al., 2007b], on the Juan de Fuca Ridge [Bohnenstiehl seismic array. We include seismicity patterns for Dikes 2 et al., 2004] and beneath Kilauea caldera [Owen et al., and 3 [Keir et al., 2009] and parameters from elastic models 2000; Cervelli et al., 2002]. of InSAR deformation patterns [Hamling et al., 2009; [4] Magma breaches the walls of a chamber and propa- Hamling, 2010] in our effort to understand the Dabbahu‐ gates along the rift when the magma chamber pressure Manda Hararo dike sequences. In addition, interdike seis- reaches a threshold value, commonly known as breakout micity distribution is presented. Our aim is to gain insight pressure [Rubin, 1995; Buck et al., 2006]. Among the fac- into the timing and dynamics of the magma intrusion pro- tors that control propagation of dikes are the rate of pressure cess, feeder zone(s) location, repeat intervals of the dike drop in the source, stress level to overcome resistance at the intrusions, rates and directions of propagation, and faulting dike tip, the hydraulic connection between the source and processes above and ahead of the propagating dikes, and to dike, regional tectonic stress level, viscous resistance of the evaluate the relative importance of factors controlling dike magma and presence of stress barriers [Delaney and lengths. These interpretations of the multiple dike intrusion Pollard, 1982; Lister and Kerr, 1991; Rubin, 1995; Segall episodes within the Dabbahu‐Manda Hararo segment pro- et al., 2001; Behn et al., 2006; Buck et al., 2006; Rivalta vide an unprecedented opportunity to understand how and Segall, 2008]. Buck et al. [2006] used an elastic magma is emplaced, and to test and refine models for the model and showed the evolution of tectonic stress due to along‐axis segmentation of the incipient slow‐spreading successive dike intrusions and the decrease in magma seafloor spreading center in Afar that shares many mor- chamber pressure due to magma withdrawal control the phological characteristics of slow‐spreading mid‐ocean pattern of multiple dike intrusions. By assuming a constant ridges [e.g., Hayward and Ebinger, 1996]. Thus, studies of breakout pressure, their model predicts intrusion of a this rift segment provide fundamental constraints on the sequence of dikes propagating in one direction followed by initiation and maintenance of mid‐ocean ridge segments, intrusions in the opposite direction. In each sequence the and the role of intense rifting episodes in the development of first dike is the longest and subsequent intrusions propagate rift faults. shorter distances. The model also predicts that fissural eruptions occur in the vicinity of the feeder zone toward the 2. Tectonic Setting end of the rifting episode, presumably
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