Forearc structure in the Lesser Antilles inferred from depth to the Curie temperature and thermo-mechanical simulations
Introduction
Magnetic anomalies can provide information on lithology, temperature and hydration of the lithosphere, complementary to other geophysical techniques (Manga et al., 2012, Feuillet et al., 2010). Sources of long wavelength magnetic anomalies were generally assumed to be concentrated in the crust, and mainly in the lower crust, implying that the lithospheric mantle contribution should be negligible (Warner and Wasilewski, 1995, Wasilewski, 1987, Wasilewski and Mayhew, 1992, Wasilewski et al., 1979). However, as proposed by Dyment et al. (1997), some recent works on mantle xenoliths (Ferré et al., 2013, Ferré et al., 2014) also suggest that the upper mantle may also contribute to magnetic anomalies. Spectral methods currently allow studying deep sources and have been used to provide additional information on mantle magnetization in various geodynamic settings: an old oceanic lithosphere (Arnaiz-Rodríguez and Orihuela, 2013), a fore-arc region in a subduction zone (Manea and Manea, 2011), a hot spot area (Gailler et al., 2016). These studies are based on the determination of the Curie Point Depth (CPD) using classical spectral analysis of magnetic anomalies (Spector and Grant, 1970), with the assumption that crustal magnetization is a completely uncorrelated function of position (e.g., Connard et al., 1983, Blakely, 1988, Tanaka et al., 1999, Ross et al., 2006). More recent studies have introduced a more complex representation where crustal magnetization follows fractal behavior (e.g., Pilkington et al., 1994, Maus and Dimri, 1995, Maus and Dimri, 1996, Maus et al., 2007, Lovejoy et al., 2001, Pecknold et al., 2001, Gettings, 2005, Bouligand et al., 2009). For both approaches, one may consider that these studies calculate a depth to the “magnetic sources bottom” rather than a “Curie Point Depth” sensu stricto (Rajaram et al., 2009, Ravat et al., 2011, Salem et al., 2014, Wasilewski et al., 1979). These two interfaces can differ for petrological reasons, lithological variations or because of structural effects. Nevertheless, such methods have been used to characterize regional thermal structures since Curie isotherm provides valuable information on both the present geothermal gradient, and the regional temperature field (Arnaiz-Rodríguez and Orihuela, 2013, Li et al., 2013, Bouligand et al., 2009, Blakely, 1988, Campos-Enriquez et al., 1989, Campos-Enriquez et al., 1990, Shuey et al., 1977). To first order, the Curie point depth is a thermal boundary above which ferromagnetic minerals (the dominant carriers of magnetism) lose their ferromagnetic properties and become paramagnetic (e.g., the Curie temperature for magnetite is 580 °C). The lithosphere is therefore virtually nonmagnetic below the CPD. Numerous previous studies have found a CPD consistent with the geological contexts, being shallower in volcanic and geothermal areas, and deeper in stable continental areas (e.g. Ates et al., 2005, Bhattacharyya and Leu, 1975, Blakely, 1988, Connard et al., 1983, Okubo et al., 1985, Okubo et al., 1989, Shuey et al., 1977, Tsokas et al., 1998). Several magnetic minerals may influence the magnetization of the upper mantle as summarized by Ferré et al. (2014). However, their preliminary results suggest that magnetite is systematically present, and dominates the remnant magnetization of mantle xenoliths. With the depth of the Curie-temperature isotherm for ferromagnetic minerals lying well below the crust-mantle interface in many geologic settings, a contribution of the mantle lithosphere to the magnetic signal is thus possible as also shown by Gailler et al. (2016).
In this work, we apply the determination of the CPD to the Lesser Antilles Arc (LAA), especially in its northern part, by combining data from global Earth Magnetic Anomaly Grid (EMAG2; Maus, 2009) and data from aeromagnetic surveys and marine campaigns recently compiled in the study area. This approach will help characterizing the regional thermal structures of the LAA but also better constraining the very complex structure of the fore-arc domain in its northern part where: (1) almost no heat flow data are available (synthesis in Manga et al., 2012), (2) an Oligocene–Miocene remnant arc is present East of the present-day active volcanic arc, thus in a fore-arc setting (e.g., Bouysse, 1979, Bouysse et al., 1985, Bouysse et al., 1988, Bouysse and Westercamp, 1988) and (3) extensional tectonics is active since at least the early Miocene (Münch et al., 2014, De Min et al., 2015). One of the main goals of this study is therefore to better understand the origin of magnetic anomalies along the LAA subduction zone. In fore-arc settings it has been proposed that magnetic mantle may be common in relation with the serpentinization of the mantle wedge (Blakely et al., 2005). The serpentinization process, which is related to the hydration of the mantle wedge, generates the production of magnetite that contributes to long-wavelength magnetic anomalies above subduction zones (Blakely et al., 2005). In this paper we seek to improve the understanding of thermal and tectonic structures of the northern LAA taking into account that a remnant arc occurs in the present-day fore-arc domain.
To better interpret the inferred thermal structure, we perform complementary thermo-mechanical simulations to model the thermal structure of the fore-arc/arc domains in conditions close to steady state. The 2D numerical model includes a computation of water transfers, associated with slab dehydration/overlying rocks hydration and a simple water-induced mechanical weakening, in order to test (1) the hypothesis of fore-arc serpentinization suggested by high magnetic anomalies and (2) its possible influence on the thermal state of the arc lithosphere. The time-scale given by the volcanic arc jump in the LAA (35 to 40 Ma ago) is used in this paper as a main constraint to discuss the modeling results, obtained for various mechanical parameters, convergence rates and initial hydration states. Our results are compared with the most recent geophysical interpretations and geodynamical models and lead us to propose a new interpretation of the structure of the volcanic arc domain in the northern part of the LAA.
Section snippets
Geodynamic setting and structure of the LAA
The LAA (Fig. 1) is a 850-km long island arc which results from the oblique subduction of the North-and South-America plates beneath the Caribbean plate, at a mean rate of around 2 cm/yr in the SW direction relative to the Caribbean plate (DeMets et al., 2000, DeMets et al., 2010). The arc, convex towards the east, spans from 12°N to 18°N on the eastern edge of the Caribbean plate, and is made up of 20 islands carrying 21 active volcanoes with > 34 historical eruptions (e.g. Briden et al., 1979,
Thermo-mechanical simulations of the Lesser Antilles subduction zone
In this section, 2D numerical simulations modeling the structure of the Lesser Antilles subduction zone are performed in order to investigate, at first order, which set of thermo-mechanical conditions allows simulating the thermal state of the Lesser Antilles arc inferred from CPD calculations. Specifically, we aim at testing the parameters necessary to reproduce the thermal structure of the central and northern LAA, particularly of the Guadeloupe archipelago characterized by the major CPD
Crustal structure inferred from previous studies at the latitude of the Guadeloupe Island
Our results from two complementary approaches, i.e. the CPD determination and the thermo-mechanical modeling, have several fundamental implications on the LAA structure, particularly in the vicinity of the Guadeloupe archipelago. There, the structure of the fore-arc has been extensively studied concerning its deep parts (Bangs et al., 2003, Christeson et al., 2003, Evain et al., 2013, Kopp et al., 2011, Ruiz et al., 2013, Laigle et al., 2013a) and its shallow sedimentary cover (De Min et al.,
Conclusions
In this work, we address the thermal structure of the northern LAA subduction by combining a Curie Point Depth computation from magnetic spectral analysis, and a thermo-mechanical modeling. Even regarding the overall uncertainty of our CPD estimates, we evidence an anomalous main CPD doming, both in amplitude (CPD rises up to ~ 18.5 km depth) and wavelength, in the inner fore-arc domain at the latitude of the Guadeloupe archipelago and more or less below the ancient and inactive arc. This doming
Acknowledgments
This research was financed by the French Government Laboratory of Excellence initiative n°ANR-10-LABX-245, the Région Auvergne and the European Regional Development Fund. This work has benefited from data acquired by numerous scientific projects. We thank the NOAA and Stephen Maus for the diffusion of the global relief model of Earth's surface, and Earth's magnetic database respectively. We are also grateful to Richard J. Blakely, Serge Lallemand, Jean-Jacques Cornée, for their contribution and
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