Lithospheric Control on the Spatial Pattern of Azores Hotspot Seafloor Anomalies: Constraints from a Model of Plume-Triple Junction Interaction Jennifer E
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Old Dominion University ODU Digital Commons OEAS Faculty Publications Ocean, Earth & Atmospheric Sciences 2011 Lithospheric Control on the Spatial Pattern of Azores Hotspot Seafloor Anomalies: Constraints from a Model of Plume-Triple Junction Interaction Jennifer E. Georgen Old Dominion University, [email protected] Follow this and additional works at: https://digitalcommons.odu.edu/oeas_fac_pubs Part of the Geophysics and Seismology Commons, and the Tectonics and Structure Commons Repository Citation Georgen, Jennifer E., "Lithospheric Control on the Spatial Pattern of Azores Hotspot Seafloor Anomalies: Constraints from a Model of Plume-Triple Junction Interaction" (2011). OEAS Faculty Publications. 221. https://digitalcommons.odu.edu/oeas_fac_pubs/221 Original Publication Citation Georgen, J. E. (2011). Lithospheric control on the spatial pattern of Azores hotspot seafloor noma alies: Constraints from a model of plume-triple junction interaction. Geophysical Research Letters, 38, L19305. doi:10.1029/2011gl048742 This Article is brought to you for free and open access by the Ocean, Earth & Atmospheric Sciences at ODU Digital Commons. It has been accepted for inclusion in OEAS Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact [email protected]. GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L19305, doi:10.1029/2011GL048742, 2011 Lithospheric control on the spatial pattern of Azores hotspot seafloor anomalies: Constraints from a model of plume‐triple junction interaction Jennifer E. Georgen1 Received 11 July 2011; revised 7 September 2011; accepted 8 September 2011; published 11 October 2011. [1] The Azores hotspot is located near a plate boundary [2004], this study treats the TER as an ultra‐slow diverging triple junction (TJ) consisting of the Terceira Rift (TER) ridge with a half‐rate of 0.4 cm/yr. Spreading has occurred and two branches of the Mid‐Atlantic Ridge (MAR). The along the TER over the last ∼25 Ma [Luis and Miranda, seafloor expression of the Azores hotspot has a complex 2008]. spatial pattern. Latitudinal anomalies in seafloor depth and [3] The Azores region has been marked by excess volca- other data along the MAR extend farther to the south of nism for an extended period of geologic time. A main phase the inferred location of the mantle heterogeneity than to in the formation of the Azores plateau commenced ∼10 Ma the north. Longitudinal anomalies span a greater distance [Cannat et al., 1999; Escartín et al., 2001], although anom- to the east of the MAR (along the TER) than to the west. alous volcanism in the broader northern Atlantic region A finite element model is used to investigate how the occurred prior to then [Gente et al., 2003]. This chief plateau‐ divergence of three plates away from a TJ may affect the building episode ended ∼3–7 Ma, depending upon latitude spatial dispersion of thermally buoyant material simulating a [Cannat et al., 1999; Escartín et al., 2001; Gente et al., 2003; mantle plume. Prescribed plate motion vectors approximate Maia et al., 2007]. Since then, rifting has been a dominant the kinematics of the Azores TJ during a main phase of process shaping the plateau, although several islands in the plateau formation ∼7 Ma. The plume is located off axis archipelago remain active in recent times. to the southeast of the simulated triple junction, following [4] Several lines of evidence point to the existence of a several studies that suggest that the present‐day conduit mantle heterogeneity in the Azores area [e.g., Schilling, 1991; is located near the islands of Faial and Pico. Asymmetry in Dosso et al., 1993; Detrick et al., 1995]. For example, seis- the divergence of the three plates with respect to the triple mic and gravity determinations of crustal thickness range to junction tends to drive plume material preferentially up to ∼8–12 km [Searle, 1976; Detrick et al., 1995; Luis southward and eastward, consistent with observed anomalies. et al., 1998; Gente et al., 2003; Georgen and Sankar, Citation: Georgen, J. E. (2011), Lithospheric control on the spatial 2010]. Bathymetric V‐shaped ridges, similar to those south pattern of Azores hotspot seafloor anomalies: Constraints from a model of Iceland, mark a pulse of elevated magmatism [Cannat of plume‐triple junction interaction, Geophys. Res. Lett., 38, L19305, et al., 1999; Escartín et al., 2001]. Although seismic doi:10.1029/2011GL048742. tomography studies of the upper mantle differ in some respects, investigations generally find a low‐velocity region 1. Geological Setting of the Azores near the Azores [e.g., Montelli et al.,2006;Silveira et al., Triple Junction 2006]. Overall, many of these observations are consistent with the existence of a thermal plume under the Azores [2] Significant variations in seafloor depth, geochemistry, plateau. Although several studies have suggested that the and other types of geological data have been observed along mantle anomaly has a compositional component [e.g., mid‐ocean ridges located near mantle plumes. Over 20% of Bonatti, 1990; Asimow et al., 2004], this modeling study the global mid‐ocean ridge system is plume‐affected, making treats the anomaly as entirely thermal in nature for compu- investigation of ridge‐hotspot interactions critical to under- tational simplicity. standing oceanic crustal accretion. This study focuses on [5] One of the salient characteristics of the Azores Plateau the interaction of the Azores hotspot and a plate boundary is its shape in plan form (Figure 1). Trends in several types triple junction (TJ) formed by two nearby divergent systems, of seafloor data are asymmetric in an east‐west direction the Mid‐Atlantic Ridge (MAR) and the Terceira Rift about the MAR. For example, seafloor depths are shallow (TER) (Figure 1). The MAR spreads with a half‐rate of 1.1– over much of the 550 km length of the TER [Vogt and Jung, 1.2 cm/yr. The precise nature of the TER has been debated. 2004], but bathymetry is anomalous over a shorter distance Some investigations describe the boundary as a zone of to the west of the MAR. Anomalies in gravity, bathymetry, distributed deformation or as an extensional strike‐slip fault and geochemical data are also asymmetric about the inferred [e.g., Luis et al., 1998]. However, similar to Vogt and Jung plume center along the MAR, extending significantly farther southward than northward [e.g., Dosso et al., 1993; Detrick et al., 1995; Thibaud et al., 1998; Goslin et al., 1999; Maia 1 et al., 2007; Shorttle et al., 2010]. Studies using these data Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, Virginia, USA. place the maximum northward extent of Azores influence in the vicinity of the Kurchatov FZ or an axial relay zone at Copyright 2011 by the American Geophysical Union. 42°–43°40′ [e.g., Goslin et al., 1999; Maia et al., 2007]. To 0094‐8276/11/2011GL048742 the south, seafloor anomalies in gravity and depth persist at L19305 1of6 L19305 GEORGEN: AZORES PLUME-TRIPLE JUNCTION INTERACTION L19305 Figure 1. Geological setting of the Azores. Azores TJ is indicated with a dashed circle. MAR = Mid‐Atlantic Ridge, TER. R. = Terceira Rift, E. Az. FZ = East Azores FZ, and J.P. = Jussieu Plateau. (Left inset) Filtered bathymetry of the Azores plateau. A lowpass filter with a cutoff wavelength of 300 km was applied to the bathymetry data of Smith and Sandwell [1997]. Contour lines indicate 1.9, 2.7, and 3.5 km depth. (Right inset) Locations of islands in the Azores Archipelago. S.J., S. Mig, C.B., and Hr. B. are Sao Jorge Island, Sao Miguel Island, Castro Bank, and Hirondelle Basin, respectively. least as far as the Atlantis FZ [e.g., Detrick et al., 1995; plates. In a TJ setting, however, lithospheric cooling away Thibaud et al., 1998]. from three ridge axes results in a 3D structure to the litho- [6] With respect to the MAR, the cause of the asymmetry sphere‐asthenosphere boundary, potentially channeling plume of seafloor anomalies in depth, gravity, and other data has flow in a more complex spatial pattern. generally been attributed to two sources. First, rifting history and relative plate motion (i.e., the southwest motion of the 2. Numerical Model Design MAR away from the Azores hotspot) contribute to the spatial distribution of Azores seafloor volcanism [e.g., [8] A finite element model is used to study the flow field Cannat et al. 1999; Gente et al. 2003]. Second, instanta- of a buoyant plume upwelling under three plates (Figure 2). neous interactions between the hotspot and the MAR have The general numerical approach and governing equations also been asymmetric. Studies that examine specific stages are provided by Georgen and Sankar [2010]. For example, in the emplacement of the plateau at selected chrons find the conservation equations for mass, momentum, and energy that enhanced volcanism was preferentially directed to the are solved using the finite element software package COMSOL. Also, the velocity boundary condition for each south [e.g., Cannat et al., 1999; Escartín et al., 2001; Maia ‐ et al., 2007]. plate is prescribed as a two dimensional vector relative to a fixed TJ point [e.g., Georgen, 2008; Georgen and Sankar, [7] This study uses a steady‐state model of mantle flow to assess how three ridges interacting with a mantle plume may 2010], with magnitude and direction dictated by a plate affect plume dispersion in the upper mantle. Multiple factors kinematic (or TJ velocity triangle) solution [Luis et al., 1994; may make the spatial distribution of a plume rising near a TJ Luis and Miranda, 2008]. Here, only the aspects of the more complex than that of an intraplate plume or a plume methodology that differ from Georgen and Sankar [2010] are interacting with a single ridge. First, material can upwell described. One significant difference is the addition of a along three extensional plate boundaries.