Abstract
Abstract. The anisotropy of magnetic susceptibility (AMS) technique provides an effective way to measure fabrics and, in the process, interpret the kinematics of actively deforming orogens. We collected rock fabric data of alluvial fan sediments surrounding the Sierra Nevada massif, Spain, and a broader range of Cenozoic sediments and rocks across the Northern Apennine foreland, Italy, to explore the deformation fabrics that contribute to the ongoing discussions of orogenic kinematics. The Sierra Nevada is a regional massif in the hinterland of the Betic Cordillera. We recovered nearly identical kinematics regardless of specimen magnetic mineralogy, structural position, crustal depth, or time. The principal elongation axes are NE–SW in agreement with mineral lineations, regional GPS geodesy, and seismicity results. The axes trends are consistent with the convergence history of the Africa–Eurasia plate boundary. In Italy, we measured AMS fabrics of specimens collected along a NE–SW corridor spanning the transition from crustal shortening to extension in the Northern Apennines. Samples have AMS fabrics compatible only with shortening in the Apennine wedge and have locked in penetrative contractional fabrics, even for those samples that were translated into the actively extending domain. In both regions, we found that specimens have a low degree of anisotropy and oblate susceptibility ellipsoids that are consistent with tectonic deformation superposed on compaction fabrics. Collectively, these studies demonstrate the novel ways that AMS can be combined with structural, seismic, and GPS geodetic data to resolve orogenic kinematics in space and time.
Highlights
A number of circum-Mediterranean orogens are associated with rapid slab rollback, resulting in paired compressional and extensional domains in the orogenic wedge of the retreating upper plate (Elter, 1975; Carminati and Doglioni, 2012)
Anisotropy of magnetic susceptibility (AMS) results offers an alternative proxy for grain-preferred orientation, and rock strain, to determine the tectonic fabric in these orogens where other deformation markers are not available (Borradaile and Jackson, 2004, 2010; Borradaile and Henry, 1997; Averbuch et al, 1992; Parés, 2004)
There is no correlation between the bulk magnetic susceptibility and the anisotropy of the magnetic ellipsoid (Pj ), so a comparison of the principal axis of susceptibility across the various structural positions around the Sierra Nevada massif specimens can provide useful kinematic information (Fig. 6a)
Summary
A number of circum-Mediterranean orogens are associated with rapid slab rollback, resulting in paired compressional and extensional domains in the orogenic wedge of the retreating upper plate (Elter, 1975; Carminati and Doglioni, 2012). Examples include the Calabria Arc–Tyrrhenian Sea (Beccaluva et al, 1985; Milia et al, 2009), the Hellenic Arc–Aegean Sea (Le Pichon and Angelier, 1979; Papazachos et al, 2000), and the Gibraltar Arc–Alboran Sea (Lonergan and White, 1997; Platt et al, 2006; Fernández-Ibáñez and Soto, 2008) Along these tectonic boundaries, the temporal and spatial relationship between thrust belt contraction, wedge-top basin evolution, hinterland extension, and orogenic uplift are the subjects of continuing controversy. Anisotropy of magnetic susceptibility (AMS) results offers an alternative proxy for grain-preferred orientation, and rock strain, to determine the tectonic fabric in these orogens where other deformation markers are not available (Borradaile and Jackson, 2004, 2010; Borradaile and Henry, 1997; Averbuch et al, 1992; Parés, 2004). We show how AMS can extend the temporal reach of GPS geodesy back in time in orogenic studies of the Betic Cordillera, Spain, and in the Northern Apennines, Italy (e.g., Sagnotti et al, 1998; Mattei et al, 2004; Fig. 1)
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