Gravity and Magnetic Anomalies in the Allochthonous Rdenes

TECTONICS, VOL. 31, TC5007, doi:10.1029/2011TC003093, 2012 Gravity and magnetic anomalies in the allochthonous Órdenes Complex (Variscan belt, northwest Spain): Assessing its internal structure and thickness José R. Martínez Catalán,1 Fernando Álvarez Lobato,1 Victor Pinto,2 Juan Gómez Barreiro,1 Puy Ayarza,1 Juan José Villalaín,3 and Albert Casas2 Received 31 December 2011; revised 24 August 2012; accepted 31 August 2012; published 16 October 2012. [1] The Órdenes Complex is the largest Variscan allochthonous structure of NW Iberia, and preserves the suture of a long-standing Paleozoic ocean, probably the Rheic. Gravity and magnetic data, the latter specifically acquired on land for this study, show that the complex occupies the core of an open synform with a maximum depth of 9–10 km, which contrasts with the flat geometry of the lower crust and Moho discontinuity beneath. The maximum depth reached by the ophiolitic rocks marking the suture is around 7 km. The allochthonous units formed by basic and ultrabasic rocks are lens-shaped in section, and occur not only at the periphery of the complex, but form wide ribbons trending NE-SW to N-S. The Bouguer anomaly related with the longest of them, the Fornás Unit, previously used to support an autochthonous interpretation of the complexes, is modeled as a rootless, massive amphibolite body with a maximum thickness of 6 km located at the downthrown block of a large normal fault cutting across previous thrust faults and extensional detachments. The main magnetic anomalies are associated with ultrabasic rocks cropping out in the NW and SE, but a weak, wide anomaly in the central part of the complex is related with one or more thin layers of amphibolite partly mineralized with massive sulphides. The weakly arcuate geometry of this anomaly and of the Bouguer anomaly caused by the Fornás Unit may reflect the NE flank of the Central Iberian arc, an orocline whose core is occupied by the allochthonous complexes. Citation: Martínez Catalán, J. R., F. Álvarez Lobato, V. Pinto, J. Gómez Barreiro, P. Ayarza, J. J. Villalaín, and A. Casas (2012), Gravity and magnetic anomalies in the allochthonous Órdenes Complex (Variscan belt, northwest Spain): Assessing its internal structure and thickness, Tectonics, 31, TC5007, doi:10.1029/2011TC003093. 1. Introduction [Van Overmeeren, 1975] and the southern Órdenes Complex (25 mGal) [Keasberry et al., 1976] as indicators of deeply [2] The allochthonous complexes of NW Iberia (Spain rooted structures favoring an autochthonous interpretation. and Portugal) consist of dense basic and ultrabasic units Both complexes were explained as resulting from a mantle alternating with units mostly consisting on lighter metase- plume or diapir emplaced during the early Paleozoic [van diments and orthogneisses. The complexes are surrounded by Calsteren and Den Tex, 1978; van Calsteren et al., 1979]. low-density metasediments, felsic metavolcanics, migmatites [3] The alternative interpretation of the complexes repre- and granites. The contrasts in density make the complexes senting remnants of a Variscan thrust plate [Ries and appropriated for gravity investigations, and thus, they were Shackleton, 1971] was supported by a re-evaluation of the surveyed in the 1970’s to elucidate their autochthonous or Cabo Ortegal gravity anomaly by Bayer and Matte [1979], allochthonous character. In Spain, Bouguer gravity anoma- and by lithological and structural criteria, such as the presence lies were interpreted in the Cabo Ortegal Complex (40 mGal) of late Paleozoic ophiolites, high-pressure metamorphism, nappe staking, thrust imbricates, and tectonic mélanges. 1 [4] Castaño et al. [1981] modeled the Bouguer gravity Departamento de Geología, Universidad de Salamanca, Salamanca, anomaly of the Cabo Ortegal Complex with a folded klippe Spain. 2Departamento de Geoquímica, Petrología y Prospección Geofísica, with geometry similar but more detailed than that used by Universidad de Barcelona, Barcelona, Spain. Bayer and Matte [1979], and carried out a magnetic survey 3Laboratorio de Paleomagnetismo, Departamento de Física, Universidad which also supported an allochthonous interpretation. But no de Burgos, Burgos, Spain. attempts have been made to date to re-interpret the gravi- Corresponding author: J. R. Martínez Catalán, Departamento de metric anomalies of the southern Órdenes Complex since the Geología, Universidad de Salamanca, ES-37008 Salamanca, Spain. pioneering work of Keasberry et al. [1976], and no magnetic ([email protected]) survey has been carried out before to the scale of the whole ©2012. American Geophysical Union. All Rights Reserved. complex. 0278-7407/12/2011TC003093 TC5007 1of18 TC5007 MARTÍNEZ CATALÁN ET AL.: ÓRDENES COMPLEX, GRAVITY AND MAGNETISM TC5007 [5] While there are no longer doubts about the allochtho- kilometres thick, and an allochthonous nappe stack cropping nous character of the NW Iberian complexes, and the surface out in five allochthonous complexes. geology of the Órdenes Complex is presently known in [11] The autochthonous sequence consists of metasedi- detail [e.g., Martínez Catalán et al., 2002], its geometry in ments, metavolcanics and orthogneisses. It developed during depth was only based in an approximated down-plunge the Late Proterozoic and Paleozoic in northern Gondwana, continuation of the folded structures whose axes are as indicated by sedimentary and faunal evidence and by known to vary quickly along and across the complex. Many detrital zircon age populations [Martínez Catalán et al., allochthonous units show a lens-shaped geometry in map 2004]. The parautochthon or Schistose Domain consists of view, for which no structural criteria permit a sound down- Ordovician, Silurian and perhaps Early Devonian metasedi- dip continuation, and the actual depth reached by the com- ments and volcanics, with stratigraphic and igneous affini- plex was unknown. ties with the Iberian autochthon. It represents a distal part of [6] On the other hand, the allochthonous complexes of the Gondwanan continental margin (references in Martínez NW Iberia occupy the core of the Central Iberian arc Catalán et al. [2009]). Deformation and metamorphism are (Figure 1a), an orocline delineated by fold axes and mag- Variscan, and Early Carboniferous syn-orogenic, flysch-type netic anomalies in the surrounding autochthon [Martínez deposits related to the emplacement of the allochthonous Catalán, 2011, 2012]. The arc was found by Staub [1926, complexes occur in both the autochthon and parauthochthon 1928a] when he visited Spain with occasion of the XIV domains [Martínez Catalán et al., 2008]. International Geological Congress, in 1926. He based his [12] The complexes consist of allochthonous units charac- finding on the trends of Variscan folds, named the arc the terized by a lithologic association and a particular tectono- Castilian bend, and included it on geological sketches of metamorphic evolution. These units are grouped according to Europe in his book “Der Bewegungsmechanismus der Erde” their lithologic affinities and their relative position in the [Staub, 1928b]. original nappe pile, and are separated from each other by [7] However, Lotze [1929, 1945a] discussed the existence faults, either thrusts or extensional detachments. The alloch- of the arc arguing that the folds maintained a NW-SE atti- thonous units include pieces of the outermost basement of tude in the SE of the Iberian Massif without being bent in the Gondwana, ophiolites derived from a Paleozoic ocean, and Toledo region. The influential contribution on the division fragments of an ensialic Cambro-Ordovician island arc. of the Iberian Variscides by Lotze [1945b] did not consider Several of these units underwent subduction in early- the existence of an arc in Central Iberia and the same Variscan times, and are considered to represent the suture occurred with the successive subdivisions of the Iberian of the Rheic Ocean, formed after its closure during the Massif [Julivert et al., 1972; Farias et al., 1987], all based Variscan collision [Arenas et al., 2007a; Martínez Catalán on Lotze’s work. The arc remained ignored until Aerden et al., 2007; Sánchez Martínez et al., 2007]. [2004] noticed its existence on the patterns delineated by [13] Variscan granitoids are abundant in late-Variscan the Variscan folds and the magnetic anomalies in the Central extensional domes, many of them gneiss domes, which Iberian Zone. Figure 1b shows the map of magnetic anoma- alternate with synforms and structural basins whose core is lies of the Iberian Peninsula, based on aeromagnetic data by occupied in NW Spain and northern Portugal by the Ardizone et al. [1989] and Miranda et al. [1989]. Figure 1c allochthonous complexes. depicts the axial trace of the main Variscan folds, according to Martínez Catalán [2012], and explains the discrepancy 3. Geology of the Órdenes Complex in Staub’s [1926, 1928a] and Lotze’s [1929, 1945a] obser- [14] Órdenes is the largest of the Iberian complexes, with vations: the first folds (D1) were bent by the arc in the SE 2 of the Iberian Massif, while the long and continuous late an outcropping area of 4,600 km , and shares with the other folds (D3) have an axial planar attitude in relation to it, so four the same types of allochthonous units and their stacking that they were not bent by the Central Iberian arc, although sequence. Following an overview of the main structural they were by the Ibero-Armorican arc. features of the complex, three sets of allochthonous units are described from top to

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