A reappraisal of the Alpine structure of the Alpujárride Complex in the Betic Cordillera: Interplay of shortening and extension in the westernmost Mediterranean

Accepted Manuscript A reappraisal of the Alpine structure of the Alpujárride Complex in the Betic Cordillera: Interplay of shortening and extension in the westernmost Mediterranean J.F. Simancas PII:

S0191-8141(18)30142-1

DOI:

10.1016/j.jsg.2018.08.001

Reference:

SG 3720

To appear in:

Journal of Structural Geology

Received Date: 8 March 2018 Revised Date:

31 July 2018

Accepted Date: 1 August 2018

Please cite this article as: Simancas, J.F., A reappraisal of the Alpine structure of the Alpujárride Complex in the Betic Cordillera: Interplay of shortening and extension in the westernmost Mediterranean, Journal of Structural Geology (2018), doi: 10.1016/j.jsg.2018.08.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Abstract

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The Alpujárride Complex has concentrated discussion on the extent and role of orogenic

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extension in the Betic-Rif Orogen (westernmost Mediterranean). Structural analysis on Permo-

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Triassic rocks of the Alpujárride Complex is a firm basis to assess extension and to integrate it in

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the plate tectonic scenario. The main Alpine deformation was dominated by top-to-the-NE

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shearing, with rare map-scale folds; as a whole, this deformation attests ductile extension during

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exhumation of previously subducted rocks. Some authors have suggested that the Alpujárride

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Complex continued evolving extensionally since this early exhumation, but in this paper a stage of

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regional shortening is documented. Thus, km-scale overturned NW-vergent folds are interpreted

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as witnessing shortening. Subsequently, post-metamorphic low-angle faults of Burdigalian-

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Langhian age cut the train of folds in two ways: firstly, top-to-the-N thrusts generated

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stratigraphic and metamorphic superpositions; then, top-to-the-N low-angle normal faults,

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kinematically congruent with the thrusts, formed due to accretion at deep levels of the orogenic

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wedge when the Nevado-Filábride Complex underthrust the Alpujárride Complex. The shortening

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stage occurred at latest Oligocene early-middle Miocene and can be related to fast convergence

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rates between Africa and Iberia. Later, a drop in the convergent rates prompted lithospheric

A reappraisal of the Alpine structure of the Alpujárride Complex in the Betic Cordillera: interplay of shortening and extension in the westernmost Mediterranean J.F. Simancas Departamento de Geodinámica, Universidad de Granada, Spain

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Phone number: +34 958243353 email: [email protected] Address: Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada. 18071 Granada, Spain

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Keywords: Orogenic extension and shortening, crustal deformations and plate kinematics, Alpujárride Complex, western Mediterranean

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rollback and top-to-the-SW crustal extension since Serravallian time, coexisting with moderate

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orthogonal shortening that gave way to E-W trending upright folds.

36 1. Introduction

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Mountain belts, the ultimate consequence of orogenic evolution, are mainly characterized by

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intense crustal shortening and uplift. However, a major advance in understanding the orogenic

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evolution has been the recognition that extensional processes may also play a significant role in

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shaping mountain belts. The amount and timing of extensional deformation in the orogens is very

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variable, as well as the extension scale (crustal or lithospheric) and the driving mechanisms

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(collapse of a thickened crust, bending of a belt, upwelling of asthenospheric mantle, subduction

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rollback, delamination, etc.). Therefore, the correct recognition and characterization of shortening

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vs. extension, and the interpretation of all these deformations in terms of tectonic processes is a

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main goal of structural studies in orogens.

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The Mediterranean region is a key area to study the interplay of orogenic shortening

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construction and extensional collapse during the Alpine orogeny (e.g., Jolivet and Faccenna, 2000;

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Faccenna et al., 2004). This paper, by presenting a re-appraisal of the ductile-to-brittle structural

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evolution of the Alpujárride Complex from the Betic-Rif orogen of Western Mediterranean (Fig. 1),

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is aimed at constraining the role of shortening vs. extension in the Alpine tectonic evolution of the

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Internal Zone of this orogen. To do so, this study focuses on the description and interpretation of

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geometric/kinematic data of the Alpine structure that have been mistaken or overlooked in

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previous papers. The revised structural data presented here are strong argument to decide on the

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extensional versus contractional nature of some deformational events in this region, which, in

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turn, is significant for unravelling the tectonic evolution of the Alpujárride Complex and the Betic

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Internal Zone as a whole. In the light of the new interpretation of the Alpine internal structure of

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the Alpujárride Complex, a comprehensive tectonic evolution will be finally discussed.

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2. Geological setting 2.1. Boundary conditions and deep structure of the Betic-Rif Orogen A first main constraint to the Betic-Rif orogenic evolution is the N-S to NW-SE Africa-Eurasia plate convergence during Cenozoic time (Dewey et al., 1989; De Mets et al., 1990; Rosenbaum et

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al., 2002), responsible for significant shortening in that direction. A second constraint is the

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westward displacement of the Internal Zone of the orogen (Sanz de Galdeano, 1990; Galindo-

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Zaldívar et al. 2015; González-Castillo et al., 2015), which gave way to clockwise rotation of

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paleogeographic units of the External Zone and eastward-directed subduction. The interplay of

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these two main constraints has originated a complex kinematics of deformations.

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In agreement with the complex crustal kinematics, the deep structure under the Betic-Rif

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orogen (Fig. 1A) is not simple, as revealed by geophysical studies. Thus, i) the Variscan crust of the

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Iberian Massif penetrates at least under the Betic External Zone, as demonstrated by

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aeromagnetic anomalies and a magnetotelluric NW-SE transect at the western Betics (Galindo-

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Zaldívar et al., 1997; Ruiz-Constán et al., 2012); ii) seismicity of intermediate depth and seismic

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surveys in the westernmost Mediterranean indicate continental subduction towards the E-SE

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(Lonergan and White, 1997; Morales et al., 1999; Gutscher et al., 2002); iii) deep seismicity (640-

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670 km) and seismic tomography suggest the existence of a lithospheric slab in the mantle

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underlying the westernmost Mediterranean (Blanco and Spakman, 1993; Bezada et al., 2013); iv)

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P-receiver functions suggest subduction rollback with edge delamination under the Betics and Rif

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(Mancilla et al., 2015).

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2.2. The Betics and The Betic Internal Zone

Five geological domains are usually distinguished in the Betics, the Iberian part of the Betic-Rif

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orogen (Vera and Martín-Algarra, 2004, and references therein): i) the Guadalquivir foreland

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basin; ii) the External Zone, which correspond to the paleomargin of southern Iberia and is made

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up of Triassic to Miocene sedimentary rocks; iii) The Flysch Trough Units, constituted by detached

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and dismembered Meso-Cenozoic rocks that represent the sedimentary cover of a subducted

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discontinuous band of mainly Mesozoic rocks firstly imbricated towards the foreland and later

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backfolded and backthrust over the Internal Zone, together with the Flysch Trough Units; and v)

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the Internal Zone, dominated by metamorphic rocks of Paleozoic and Mesozoic (mostly Triassic)

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age. Continental collision with the Maghrebian paleomargin occurred at Miocene time, after

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subduction of the Flysch Trough basement.

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The Betic Internal Zone (Fig. 1A) is constituted by Mesozoic and Paleozoic rocks usually

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metamorphosed and intensely deformed. The large-scale structure is a tectonic pile with three

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main tectonometamorphic units, namely, from top to bottom, the Maláguide Complex, the

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Alpujárride Complex and the Nevado-Filábride Complex. The paleogeographic location of these

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three complexes is uncertain, but it is commonly assumed that they were located eastwards of

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their present-day position, so that their westward migration would have contributed to shaping

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the curvature of the Betic-Rif Orogen.

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The Maláguide Complex overthrust the Alpujárride Complex, though the original contact

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between them has been modified by later extensional tectonics (Aldaya et al., 1991; Lonergan and

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Platt, 1995; González-Lodeiro et al., 1996; Booth-Rea et al., 2003). The Maláguide units have a

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Paleozoic sole affected by pre-Alpine (Variscan) penetrative deformations and metamorphism.

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This Paleozoic basement is unconformably overlain by Triassic-Miocene sedimentary rocks

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affected by NW-vergent thrusts and folds (Lonergan, 1993; Booth-Rea et al., 2002).

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The Nevado-Filábride Complex occupies the lowest structural position in the Betic Internal

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Zone and crops out at the core of large-scale antiformal ridges. This complex is perhaps the most

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controversial in the internal Betic Zone, being at present the subject of a strong debate regarding

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stratigraphy, subdivision of tectonic units, internal structure and even the existence or not of an

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Ophiolitic Unit (De Jong, 1993; Gómez-Pugnaire et al., 2000; Puga et al., 2002; Martínez-Martínez

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et al., 2002, 2007). However, there is agreement on the existence of a pre-Alpine (Variscan) low-

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pressure metamorphism and an Alpine metamorphic evolution consisting of an initial stage of

ACCEPTED MANUSCRIPT high-pressure occurred at 15-18 Ma (López Sánchez-Vizcaíno et al., 2001; Platt et al., 2006),

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followed by intermediate and low-pressure conditions. The most evident deformations are late-

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metamorphic extensional shear zones that have erased previous deformations, apparently

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concentrated on the top of the Nevado-Filábride tectonic pile. Shear zones contain many minor

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folds subparallel to the stretching lineation and a few major NW-vergent tight-to-isoclinal folds;

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kinematic criteria indicate top-to-the-W sense of movement (Galindo-Zaldívar et al., 1989; Jabaloy

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et al., 1993; De Jong, 1993; Martínez-Martínez et al., 2002). At structural levels beneath the

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extensional shear zones, there are several shear zones of similar kinematics but interpreted as

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contractional, since they separate stratigraphic recurrences and rocks with higher metamorphic

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grade superposed on rocks with lower metamorphic grade (Martínez-Martínez, 2007).

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The Alpujárride Complex, at an intermediate position in the tectonic pile of the Betic Internal Zone, is the one with more widespread outcrops (Fig. 1A). The thrust contact that placed the

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Alpujárride rocks onto the Nevado-Filábrides ones has been obliterated by the extensional shear

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zones referred above. The lithostratigraphic sequence of the Alpujárride tectonic units is more

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complete in the western units (Fig. 1C), with a common lithostratigraphy that consists in, from

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bottom to top: i) dark schists and quartzschists of Paleozoic age, sometimes with gneissic rocks at

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the base; ii) light-colored schists, quartzschists and quartzites, also of Paleozoic age; iii) Permo-

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Triassic phyllites and quartzites; and iv) Triassic calcitic and dolomitic marbles. However, a very

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significant difference (not shown in Fig. 1C) appears in the westernmost Alpujárride unit, which

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includes a sole of sub-continental ultramafic rocks (the so-called Ronda peridotites) and high-

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grade gneisses. The metamorphism of the Alpujárride units varies from low- to high-grade,

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though its characterization in the Paleozoic schists is complicated by the existence of pre-Alpine

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parageneses. This paper studies the Alpine structure of the Alpujárride units that crop out in a

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central transect shown in Fig. 1B, which is then used to discuss a tectonic interpretation of the

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Internal Betic Zone.

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2.3. Sequence of deformations in the Alpujárride Complex

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The following sequence of deformational events has been determined in Permo-Triassic

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phyllites and marbles of the central transect of the Alpujárride Complex (Fig. 1B), being probably

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valid for eastern and western transects too. The succession of deformations is, from older to

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younger (the suffix A in the labels refers to Alpine):

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- D1(HP)A, which corresponds to the high-pressure/low-temperature metamorphism

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defined by carpholite-bearing assemblages in phyllites (Azañón and Goffé, 1997).

- D2A, characterized by a non-coaxial and very penetrative deformation that gave way to a planar-linear fabric (foliation S2A, stretching lineation Ls2A) with minor folds subparallel to the

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stretching lineation (F2A) (Avidad and García-Dueñas, 1981; Simancas and Campos, 1993; Balanyá

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et al., 1997; Rossetti et al., 2005).

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- D3A, which consists in km-scale, overturned, NW-vergent folds trending NNE-SSW to ENEWSW, with a spaced axial planar foliation (S3A) (Simancas and Campos, 1993). - D4A, constituted by top-to-the-N low-angle faults (Aldaya, 1981; Avidad and GarcíaDueñas, 1981; Simancas and Campos, 1993; Crespo-Blanc et al., 1994).

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-Finally, there is a set of recent (Serravallian to present) but kinematically varied

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structures, which are here grouped as DN. This set includes: i) Major low-angle normal fault zones

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with dominant top-to-the-SW kinematics (Fig. 1B), the main one being the current Nevado-

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Filábride/Alpujárride boundary (Jabaloy et al., 1993; Martínez-Martínez et al., 2002, 2004); ii) E-W

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trending upright open folds, among which the Sierra Nevada antiform is the biggest one

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(Martínez-Martínez et al., 2002, 2004; Sanz de Galdeano and Alfaro, 2004; Pedrera et al., 2007);

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iii) NE-SW left-lateral faults with subordinate NW-SE to E-W right-lateral ones, and high-angle

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normal faults of variable orientation (see compilation by Sanz de Galdeano and Peláez, 2011).

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Present-day deformation in the Betics seems to be dominated by the SW-directed extension, with

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NW-SE shortening being subordinate (Azañón et al., 2015; Galindo-Zaldívar et al., 2015; González-

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Castillo et al., 2015). However, in the eastern Betics active tectonics is dominated by NW-SE

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shortening, with folds, thrusts and strike-slip faults (Bousquet, 1979; Silva et al., 1993; Alfaro et

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al., 2012; Echeverría et al., 2015).

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This paper only deals with the pre-Serravallian D2A, D3A, D4A1 and D4A2 deformational events. Therefore, DN structures, which attest the recent and present-day tectonic activity in the region,

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are beyond the scope of this work. The tectonic discussion presented at the end of this paper will

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show that the distinction between pre- and post-Serravallian structures is not arbitrary,

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corresponding to a change in the large-scale plate tectonic scenario. 2.4. Main issues in the Alpujárride Complex

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2.4.1.Variscan versus Alpine imprint

Recent investigations have definitely shown the importance of the Variscan imprint in the

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Paleozoic rocks of the Alpujárride Complex, thus bringing into question the age of a part of

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mineral growth and tectonic fabrics (Acosta et al. 2014; Sánchez-Navas et al. 2014). This is the

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reason why in this paper the structural analysis has been focused on Permo-Triassic rocks, where

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the entire tectonometamorphic imprint must be necessarily Alpine. The presumed Variscan-

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Alpine unconformity, which should be located somewhere between the black Paleozoic schists

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and the Permo-Triassic phyllites, has not been located yet.

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2.4.2.Timing of the early Alpine events

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The metamorphic record in the Alpujárride Complex attests an Alpine early history of burial at high-pressure/low-temperature conditions followed by exhumation (Goffé et al., 1989; Azañón

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and Goffé, 1997; Azañón et al., 1998; Booth-Rea et al., 2002). The timing of the high-pressure

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event is uncertain, with muscovite 40Ar-39Ar dating having yielded tentative ages of c. 48 Ma (Platt

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et al., 2005). If the high-pressure event took place at c. 48 Ma and the subsequent deformational

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events started at earliest Miocene time (Platt et al., 2005), a time gap of unknown tectonic

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meaning would have existed in-between. On the other hand, the age of the high-pressure

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metamorphism in the Nevado-Filábride Complex has been found to be as young as 15-18 Ma (U-

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Pb on zircon, López Sánchez-Vizcaíno et al., 2001; Lu-Hf on garnets, Platt et al., 2006), hence

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evidencing that the high-pressure events in the Alpujárride and Nevado-Filábride complexes are

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unrelated.

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2.4.3.Extension versus shortening The recognition of important low-angle normal faults in the Betic Internal Zone gave way to a profound rethink of the orogenic evolution. Thus, the view of the Internal Zone of the orogen as

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the result of thrust-stacking was abandoned in favor of a collapsed tectonic pile made up of

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sheets bounded by low-angle normal faults (Galindo Zaldívar et al., 1989; Aldaya et al., 1991;

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García-Dueñas et al. 1992; Jabaloy et al., 1993; Crespo-Blanc et al., 1994; Crespo-Blanc, 1995;

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Lonergan and Platt, 1995; Vissers et al., 1995; Martínez-Martínez et al., 2002). Some authors

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consider that continuous extension has dominated the Betics since the end of the subduction-

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related high-pressure event (Platt and Vissers, 1989; Vissers et al., 1995; Orozco et al., 1998, 2004,

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2017; Williams and Platt, 2017), while others propose alternating contractional and extensional

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events (Tubía et al., 1992; Simancas and Campos, 1993; Azañón et al., 1997; Balanyá et al., 1997;

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Azañón and Crespo-Blanc, 2000; Rossetti et al., 2005). Among the latter, there is remarkable

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confusion concerning the contractional or extensional nature of many tectonic contacts in the

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Alpujárride Complex. In order to decide which one of these disparate interpretations is more

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appropriate, this paper firstly provides with a precise geometric and kinematic description of the

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structures, and then integrate these structures in a proposal of tectonic evolution for the

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Alpujárride Complex and the Betic Internal Zone.

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3. Description of Alpine deformations in the Alpujárride Complex

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3.1. Early Alpine syn-metamorphic deformations (D1(HP)A; D2A)

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Relics of an earliest foliation probably related to the high-pressure/low-temperature event

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are exceptionally preserved at microscopic scale (e.g. Azañón et al., 1997; Rossetti et al., 2005);

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this is apparently what Williams and Platt (2017) have noticed as cleavage arcs surrounded by the

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main foliation in chloritoid-kyanite schists. This relic foliation that precedes the first recognizable

ACCEPTED MANUSCRIPT folds is labelled here as S1A. Moreover, in metapelites (but not in quartzites or marbles) the first

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recognizable folds affect a planar fabric defined by preferred orientation of phyllosilicates, parallel

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to thin grain-size layering, which could represent either the very first tectonic foliation (S1A) or a

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recrystallized sedimentary (compaction) fabric (Hobbs et al., 1976). In any case, there are no

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observable folds or shear-sense indicators related to that relic or presumed tectonic foliation,

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being not possible to decipher any geometric or kinematic information.

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The main deformational fabric in the Permo-Triassic rocks, i.e. the first one associated with

recognizable folds, is frequently LS-type, characterized by foliation (S2A) and associated stretching

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lineation (Ls2A); S2A is continuous and axial planar of tight microfolds; Ls2A orientates NE-SW (Fig.

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1D; Simancas and Campos, 1993). Asymmetric tails in porphyroclasts indicate top-to-the-NE non-

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coaxial flow (Fig. 2A). In the Paleozoic schists, S2A appears as an intense crenulation cleavage

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enclosing a presumably Variscan relic foliation within microlithons. Nevertheless, in strain

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shadows around competent Paleozoic igneous bodies intruded in the schists, the first Alpine

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deformation may be weak, the Variscan foliation prevailing at outcrop-scale (Sánchez-Navas et al.,

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2014). In the central and eastern Alpujárride units, D2A metamorphism is of low grade in phyllites

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and carbonates, reaching medium grade in dark (Paleozoic) schists (Aldaya, 1981; Avidad and

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García-Dueñas, 1981).

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Tight micro- and meter-scale folds of this deformational event are ubiquitous (Fig. 2B), but map-scale folds are exceptional. Indeed, inverted stratigraphy always appears in connection with

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the short limbs of F3A folds (see next section), except in the sector east of Sierra de Lújar (Estévez

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et al., 1985), where map-scale F2A folds appear coaxially overprinted by F3A folds (Figs. 3A and 4A).

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In this sector, F2A axes trend NNE-SSW with gentle but variable plunge; folds are close-to-tight,

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overturned, east-vergent (Figs. 3B and 4) and subparallel to the stretching lineation observed in

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other sectors (compare Figs. 1D and 3B). Interestingly, the deformation associated with these F2A

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folds lacks stretching lineation or asymmetric microstructures, so that folds probably nucleated at

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oblique angle to the regional stretching, then undergoing only moderate passive rotation of hinge

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lines. After removing the effect of superposed F3A folding, F2A folds mapped east of the Sierra de

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Lújar form a train of east-vergent overturned folds with inverted limbs less than 1 km-long (Fig.

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4A). It is worth noting that F3A folds east of the Sierra de Lújar (Figs. 3A and 4) are second-order

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folds of the km-scale fold that mostly shapes that Sierra.

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3.2. Late-metamorphic penetrative folding phase (D3A)

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The existence of km-scale folds widely distributed in the Alpujárride Complex, which fold the main foliation and are cut by low-angle faults, was initially pointed out by Estévez et al. (1985)

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and Simancas and Campos (1993). The selected cross-sections located in Fig. 1B and displayed in

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Fig. 5 show the geometry and arrangement of these folds. F3A folds are close, overturned and NW-

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vergent (Fig. 2C); their dominant trend is ENE-WSW, with dominant gentle to moderate plunge

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towards WSW (Fig. 1D). The axial planar foliation of F3A folds (S3A) is a spaced crenulation cleavage

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in the phyllites and a spaced disjunctive (pressure solution) cleavage in the carbonate rocks, more

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penetrative in hinges and inverted limbs. At the convex side of hinges in carbonate rocks, the

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adjacent phyllites show small triangular zones of weak strain, which are characteristic of buckling

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with moderate competence contrast (Fig. 2D); moreover, a certain resemblance of F3A folds to the

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cuspate (anticlines) and lobate (synclines) style (Figs. 5 and 6) also indicates some rheological

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contrast, with the Triassic carbonate rocks being more competent than the underlying phyllites.

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F3A folds were formed at low-grade metamorphic conditions. The timing of these folds is not

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accurately stablished but 40Ar-39Ar data suggest an earliest Miocene age (Platt et al., 2005).

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The biggest F3A folds are found in the lowermost tectonic unit of the Alpujárride Complex, the

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so-called Lújar unit, due perhaps to a thicker competent “layer” of carbonate rocks (Figs. 5E and

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5F). In this unit, three successive km-scale folds are distinguished, from NW to SE: the Sierra de

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Lújar fold, the Sierra de Turón fold and the Sierra Alhamedilla fold (Figs. 1B, 5E and 5F; Simancas

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and Campos, 1993). Furthermore, F3A folds in the Lújar unit are bigger than the F2A folds

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previously developed in the same area (compare Fig. 4A with Figs. 5E and 5F), which suggests

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different mechanisms of formation. The major F3A folds appear on map as remarkably continuous

ACCEPTED MANUSCRIPT until being cut by the low-angle faults of the D4A event (see axial traces in Fig 1B); accordingly,

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each one of the tectonic sheets contains different fold portions, though restoration rearrange the

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pieces to reveal the original train of folds (Fig. 6). In this respect, there are significant differences

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between the schematized structural map of Williams and Platt (2017) and the one in Fig. 1B. The

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nomenclature of the tectonic sheets distinguished in this paper is shown at the bottom of Fig. 5.

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3.3. Top-to-the-N low-angle faulting (D4A)

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Low-angle faults of two different types, thrusts (D4A1) and normal faults (D4A2), cut the F3A folds (see cross-sections in Fig. 5). The kinematics of thrusts and low-angle normal faults is identical

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(top-to-the-NNW or -N; Fig. 1D); both types of faults seem to have developed close in time though

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consecutive, the thrusts being systematically cut by the low-angle normal faults. Associated fault-

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rocks are random fabric crush breccias in carbonate rocks and foliated gouges or spaced C´-type

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shear bands in phyllites/schists (Figs. 2E and 2F); thrust fault zones contain less or no gouge and

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more C´-type shear bands than normal fault zones. All of these low-angle faults were

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subsequently cut off by top-to-the-SW low-angle normal faults (Fig. 1B) and finally folded by open

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upright folds.

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3.3.1.Thrusts (D4A1)

The observation of repeated stratigraphy above and below the low-angle normal faults was

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the reason why all these faults were originally interpreted as thrusts (e.g., Navarro-Vilá and García

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Dueñas, 1980; Aldaya, 1981; Avidad and García-Dueñas, 1981). Later on, detailed observations on

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the contact between the Nevado-Filábride and the Alpujárride complexes proved the extensional

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character of this main fault zone (Galindo-Zaldívar et al., 1989; Jabaloy et al., 1993; Martínez-

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Martínez et al., 2002), and this extensional interpretation was quickly applied by many authors to

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practically all low-angle faults in the Betic Internal Zone. However, the structural data provided

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here show that there is a good deal of thrust contacts preserved in the Alpujárride pile, such as: i)

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a foreland dipping duplex in the Salobreña tectonic sheet (Fig. 5A; ii) imbricate fans in various

292

tectonic sheets (Figs. 5D, 5E and 5F); and iii) ascending frontal ramps in Triassic carbonates (Figs.

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5B, 5C and 5D). Accordingly, some illustrative examples of thrust superposition are as follows: the

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thrust of the Guájares sheet onto the Salobreña one (Fig. 5B), the thrust of the Salobreña sheet

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onto the Herradura one (Fig. 5C), the thrust of the Herradura sheet onto the Lújar one (Fig. 5D),

296

and the large-scale imbricated structure of the Lújar unit (Fig. 5E, L2 onto L1; Fig.5F, L3 onto L2). 3.3.2.Low-angle normal faults (D4A2)

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The extensional character of a part of the top-to-the-N low-angle faults is inferred from their

299

descending trajectories with respect to both thrusts and stratigraphy in normal limbs of F3A folds

300

(Figs. 5A, 5B, 5D and 5E). Since the low-angle normal faults are folded by late open upright folds,

301

their variable dip is not a reliable feature to stablish the extensional character (see Figs. 5B and

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5E). The map in Fig. 1B shows the distinction made in this paper between thrusts and low-angle

303

normal faults: it is clear from this map that a large part of the extensional brittle deformation

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affecting the Alpujárride Complex is not due to the top-to-the-N faults but to subsequent top-to-

305

the-SW or W low-angle normal faults (García-Dueñas et al. 1992; Jabaloy et al., 1993; Lonergan

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and Platt, 1995; Martínez-Martínez et al., 2002).

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The relationships between faults and Miocene sediments suggest that the top-to-the-N lowangle faults are Burdigalian-Langhian in age, while younger top-to-the-SW low-angle normal faults

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are Serravallian (Mayoral et al., 1994; Crespo-Blanc, 1995).

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3.3.3.The current tectonic pile The present-day geometry of the Alpujárride Complex is dominated by a tectonic pile due to

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the interaction of the low-angle faults described above (Fig. 1B). In this respect, two main

313

considerations must be done: i) no tectonic superpositions ascribable to the D2A event (which

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should be folded by F3A folds) has been mapped yet; ii) despite the relevance of the low-angle

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normal faults, the tectonic pile is still dominated by stratigraphic repetitions in most sectors.

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A complete understanding of the meaning of mapped tectonic sheets can only be attained

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through restoration of the structure; unfortunately, this is possible only in a few areas of well

318

constrained and continuous structure. The restoration in Fig. 6 shows the position that the

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Herradura, Salobreña and Guájares tectonic sheets had in the D3A fold train prior to the D4A event.

320

Fig. 6 also shows that a significant N-S shortening of approximately 50 km took place in the

321

analyzed geological transect, though this is only a fraction of the total N-S orogenic shortening. Since middle Miocene to present-day, after the top-to-the-N low-angle faults in the

323

Alpujárrides, low-angle normal faults with SW-NE extensional kinematics started to operate

324

affecting the entire Internal Zone of the Betics (Fig. 1B), overprinting previous structures (Galindo-

325

Zaldívar et al., 1989; Jabaloy et al., 1993). Coevally, orthogonal shortening gave way to major E-W

326

trending upright folds (Martínez-Martínez et al., 2002, 2004; Azañón et al., 2015).

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4. Discussion: tectonic interpretation of the Alpine deformations in the Alpujárride

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Complex

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4.1. D2A event

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PT paths in the Permo-Triassic metapelites (phyllites) attest a pressure drop of 5-6 Kbar associated with the development of the main foliation (S2A), i.e. peak pressures were of 8-10 Kbar

333

while the main foliation developed at 3- 4 Kbar (Azañón et al., 1998; Booth-Rea et al., 2005;

334

Williams and Platt, 2017). This means that the main tectonic fabric in these rocks is related to

335

exhumation, a process hard to evaluate because equating metamorphic pressures and burial

336

depth may result in significant overestimations (Yamato and Brun, 2017).

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Regarding deformation, D2A generated an LS fabric with asymmetric microstructures and

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minor folds. Major folds have only been identified east of Sierra de Lújar (Figs. 1B and 3A), where

339

they show an eastern vergence that may be compatible with the top-to-the-NE kinematics

340

inferred from the LS fabric. To further characterize this deformation, it must be noted that: i) the

341

occasional development of the large-scale F2A folds described in this paper suggests that bedding

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would have been rarely positioned in the shortening field during D2A shearing; ii) no tectonic

343

superposition ascribable to the D2A event (which should appear folded by the subsequent F3A

344

folds) has been described yet; and iii) the vertical proximity of isograds suggests (though attention

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has not been paid to separate the Variscan metamorphism) ductile vertical thinning of the

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Alpujárride units (Azañón et al., 1998; Williams and Platt, 2017). Accordingly, the overall

347

extensional interpretation that is generally favored for the D2A deformation is also supported in

348

this paper. 4.2. The D3A deformational event

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The first late- to postmetamorphic, pre-middle Miocene structures are km-scale, ENE-

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WSW trending, overturned NW-vergent folds (F3A), with moderate or faint hinge curvature. These

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folds are widely and evenly distributed in the area studied (Fig. 1B).

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The structure of the Sierra de Lújar area deserves attention because it has been presented as an example of mega-sheath folds inside a large-scale, top-to-the-N, extensional shear zone

355

(Orozco et al., 2004, 2017). However, the data presented here do not support that interpretation.

356

Firstly, the structural pattern of this area is not due to a single structure but to fold superposition

357

(Figs. 3A, 4A and 4B); if folding superposition is not considered, the geometric appraisal is

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incomplete and a mixture of fold styles, minor structures, axial traces and stratigraphic polarities

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results (both F2A and F3A folds produce local inversions). Furthermore, the rocks do not exhibit the

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intense strain and stretching lineation that characterizes sheath folds: the Sierra de Lújar is mainly

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a major F3A syncline (Fig. 5E), with minor folds of this phase featuring moderate strain and spaced

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foliation; on the contrary, F2A folds (Fig. 4A) show a more penetrative foliation. One more

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geometrical constraint to be considered is the hinge curvature of the F3A Sierra de Lújar fold,

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which is rather moderate, trending between N20E at the eastern border (Fig. 3B) and N80E (WSW

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plunging) at the southern border of the Sierra. The cause of the hinge curvature is uncertain, but a

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possible explanation is that the uncommon N20E trend results from adjustment to the anisotropy

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introduced by F2A folds in this area. Interestingly, the km-scale Sierra de Turón overturned

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syncline, located to the southeast of the Sierra de Lújar fold, can be followed along more than 20

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km without showing significant hinge curvature (Figs. 1B and 5F); the same can be said about the

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F3A folds mapped in the tectonic sheets west of the Sierra de Lújar (Figs. 1B and 1D). Finally, it is

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ACCEPTED MANUSCRIPT worth noting that the top-to-the-N shear deformation is not ductile and pervasive but semibrittle

372

and discrete (see below). To conclude, this paper does not sustain the interpretation by Orozco et

373

al. (2017) and Williams and Platt (2017) on the F3A folds mapped in Fig. 1B as mega-sheath folds

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formed in a large-scale, ductile extensional shear zone. Quite the contrary, the data presented

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here strongly suggest that F3A folds were formed by regional NNW-SSE shortening, which, in turn,

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implies that the continuous extension envisaged by some authors since the end of the high-

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pressure event (Platt and Vissers, 1989; Platt et al., 1998; Orozco et al., 1998, 2017, Williams and

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Platt, 2017) must be reconsidered.

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4.3. The D4A deformational event

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The superposition of tectonic sheets that characterize the Alpujárride Complex is due to a

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deformational stage characterized by kinematically coherent (N-directed) thrusts (responsible for

382

stratigraphic and metamorphic repetitions) and low-angle normal faults (Figs. 5 and 6). Contrary

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to this interpretation, some authors have cast doubt on the truthfulness of post-metamorphic

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superpositions (Platt and Vissers, 1989; Orozco et al., 1998, 2004; Williams and Platt, 2017). These

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authors claim that stratigraphic superposition is sometimes explained due to early “ghost” thrusts

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formed during a subduction-related thickening stage and completely destroyed by transposition

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and recrystallization later on. Furthermore, since early (pre-metamorphic) thrusts cannot explain

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metamorphic superposition, normal faults complicating geometric relationships are invoked.

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However, detailed structural mapping does not support these alternative interpretations, as can

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be illustrated with the example of the area north of Motril (Fig. 7). The structural map in Fig. 7

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shows that: i) the internal structure of the Herradura unit schists consists in F3A folds and an

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imbricate fan of D4A1 thrusts; and ii) the Paleozoic medium-grade schists of the Herradura unit

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overlie the Triassic low-grade carbonates of the Escalate unit, as demonstrated at the northern

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frontal footwall-ramp and at the eastern oblique footwall-ramp, this latter re-activated as normal

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fault. This structure is summarized in the cross-section of Fig. 5D. For the same area, Williams and

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Platt (2017) suggest the following interpretation: i) pre-metamorphic thrusting between dark and

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ACCEPTED MANUSCRIPT light-colored schists; and ii) east-dipping normal faults (opposite to what is observed in the field)

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downthrowing the Triassic carbonates with respect to the Paleozoic schists. It can be hence

399

concluded that in the area north of Motril (Fig. 7), as in other areas of the Alpujárride Complex,

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post-metamorphic superposition is well documented (e.g., Avidad and García-Dueñas, 1981;

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Simancas and Campos, 1993; Rossetti et al., 2005).

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The structural association of kinematically coherent thrusts and low-angle normal faults is not uncommon (e.g., Çoruh et al., 1988; Yin and Kelty, 1991; Butler, 1992; England and Molnar,

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1993), the mechanics of thrust wedges offering a number of potential explanations (Davis et al.,

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1983; Platt, 1986). Since there is no factual support for a change of the friction coefficient or the

406

fluid pore-pressure at the basal decollement, these factors are not invoked. Instead, accretion at

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deep levels of the wedge is very appealing because it is well known that the Nevado-Filábride

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Complex underthrust the Alpujárride Complex coevally (15-18 Ma, López Sánchez-Vizcaino et al.,

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2001; Platt et al., 2006) with the top-to-the-N normal faults (Burdigalian-Langhian; Mayoral et al.,

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1994; Crespo-Blanc, 1995). If the surface slope angle of the wedge increased due to

411

underthrusting of the Nevado-Filábride Complex, development of structures that could diminish

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the surface slope would result, such as normal faults on top of the wedge. Additionally, the

413

deeper location of the basal decollement might have resulted in ductile deformation at the basal

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slope, also prompting reduction of the surface slope. Thus, underthrusting of the Nevado-

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Filábride Complex may have induced the development of top-to-the-N low-angle normal faults in

416

the Alpujárrides.

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4.4. Shortening evaluation

An accurate evaluation of the NNW-SSE total shortening due to D3A folds and D4A1 thrusts

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is not possible for any transect of the orogen, though the following considerations may serve as

420

an approximation. The restoration of the continuous cross-section shown in Fig. 6 yields a

421

shortening of 49 km. However, this is only a part of the total shortening, due to the following

422

reasons: i) the analyzed cross-section does not completely cross the Alpujárride Complex (see

ACCEPTED MANUSCRIPT location in Fig. 1B); ii) whether or not the tectonic pile made up of Guájares + Salobreña +

424

Herradura units (Fig. 6) overstepped the Lújar unit is unknown, a superposition that should be

425

added to compute shortening; iii) the whole Alpujárride Complex thrust over the Nevado-Filábride

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Complex, the corresponding tectonic window (Figs. 1A and 1B) providing an additional shortening

427

of 50 km; iv) the initial part of the NNW-directed shortening in the External Zone of the Betics

428

(Crespo-Blanc et al., 2007) was coeval with similar shortening in the Alpujárrides, which implies an

429

additional increase in global shortening. Summing up, a minimum figure of around 100 km of

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NNW-SSE total shortening can be taken for granted in the Alpujárride Complex. This calculation is

431

not affected by the fact that the present-day contact between the Nevado-Filábride and

432

Alpujárride complexes is not the original thrust but a major extensional fault zone with 116 km of

433

W-SW horizontal displacement and substantial ductile thinning of the footwall rocks (Martínez-

434

Martínez et al., 2002); it simply means that the Alpujárride Complex has to be placed before

435

extension more than 100 km east of their present-day location and over a thicker pile of Nevado-

436

Filábride rocks.

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4.5. Crustal deformation and plate kinematics

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The Betic-Rif Orogen exemplifies the interaction of complex boundary conditions constraining crustal deformation. The two most important constraints are the N-S to NW-SE

440

Africa-Eurasia plate convergence during Cenozoic time and the westward displacement of the

441

Internal Zone of the orogen. These two main constraints, together with associated processes of

442

lithospheric rollback and edge tearing (Mancilla et al. 2015) have originated a complex kinematics

443

of deformations. Despite this limitation, essaying a correlation between N-S to NW-SE orogenic

444

shortening and the Africa-Eurasia plate convergence seems still justified.

445

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There are significant differences in the Paleogene-Neogene Africa/Iberia convergence

446

rates evaluated by different authors (Dewey, 1989; Rosenbaum et al., 2002; Vissers and Meijer,

447

2012); fortunately, some common patterns can be extracted and considered to relate crustal

448

deformations in the Alpujárride rocks to plate-scale kinematics. The starting point is the

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computation that Africa/Eurasia motions between 80 and 40 Ma were accommodated in the

450

Pyrenean domain, i.e. NNW-SSE convergence and crustal shortening in the Betics should be

451

younger than 40 Ma (Vissers and Meijer, 2012).

452

High convergence rates exceeding 10 km/m.y with NW-SE trending vectors have been computed between ≈ 30 and 20 Ma (Rosenbaum et al., 2002). As the age tentatively attributed to

454

D2A folds is earliest Miocene (Platt et al., 2005), there is coincidence in broad terms between

455

shortening in the Alpujárride Complex and high plate-convergence rates. Actually, the overall

456

timing of the shortening period (D3A folds and D4A low-angle faults) can be estimated on a

457

different ground: i) for more than 100 km of NNW-SSE shortening, as discussed above, convergent

458

velocities ≈ 10 km/m.y need around 10 m.y to complete that figure; and ii) if the subsequent

459

extensional episode started at ≈ 15 Ma (Crespo-Blanc et al., 1994), shortening could have taken

460

place between ≈ 26 and 16 Ma. At some stage of this shortening episode, the entire Alpujárride

461

Complex thrust over the Nevado-Filábride Complex, which reached then its maximum depth (peak

462

pressure dated at 15-18 Ma; López Sánchez-Vizcaino et al., 2001; Platt et al., 2006). Moreover, the

463

new geometry of the thicker orogenic wedge would provoke NNW-directed extension at the

464

upper part of the tectonic pile.

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Unlike the high convergence rates for the ≈ 30-20 Ma period, relatively slow convergence

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rates suddenly started to dominate since early-middle Miocene time (Rosenbaum et al., 2002;

467

Vissers and Meijer, 2012). Slow convergence rates could have promoted lithospheric rollback and

468

extension at surface (Royden, 1993; Rosenbaum et al., 2002). Accordingly, while the NNW-SSE

469

shortening in the Alpujárride Complex fits well with fast Africa-Iberia convergence at latest

470

Oligocene to early Miocene time, the top-to-the-SW extension dominant since Langhian-

471

Serravallian time fits well with a slowdown in the convergence rate, which, in turn, would have

472

triggered west-directed lithospheric rollback (Lonergan and White, 1997; Duggen et al., 2008;

473

Mancilla et al., 2015). Thus, at Serravallian time, the new tectonic scenario of slab rollback would

474

have imposed a kinematically distinctive westward extension with exhumation of the Nevado-

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ACCEPTED MANUSCRIPT Filábride Complex (Martínez-Martínez et al., 2002, 2004). Notwithstanding, before the dominance

476

of westward extension (or partly coeval to it), contractional shears must have operated to cause

477

the superposition within the Nevado-Filábride Complex of higher pressure metamorphic rocks

478

over lower pressure ones. Shear zones separating these contrasting Nevado-Filábride units have

479

been described by Martínez-Martínez (2007), with the same top-to-the-W kinematics as the

480

extensional shear zones affecting the upper levels of the Nevado-Filábride Complex. However, the

481

similar kinematics raises the possibility that all of the W-directed displacements in the Nevado-

482

Filábride might correspond to extensional tectonics obliterating previous contractional shears.

483

Whichever the case, if partially coeval contractional and extensional shear zones would have

484

existed, they would constitute an efficient crustal flow for the very fast Nevado-Filábride

485

exhumation (Martínez-Martínez, 2007). Since Serravallian time, after the Alpujárrides/Nevado-

486

Filábride superposition, both complexes share the same set of deformational structures.

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487 5. Conclusions

489

i) In accordance with previous interpretations, the main penetrative and synmetamorphic

490

Alpine deformation in the Alpujárride rocks is extensional and probably related to exhumation

491

after an initial continental subduction stage of uncertain age. The new map-scale folds described

492

in this paper are rather of local development, denoting that layering would have been rarely

493

positioned in the shortening field during D2A shearing.

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ii) Late-metamorphic NW-vergent buckling folds are ubiquitous in the Alpujárride Complex

495

and denote shortening at latest Oligocene-earliest Miocene time. Subsequent semi-brittle and

496

brittle low-angle faults with top-to-the-N kinematics (thrust and low-angle normal faults) cut this

497

train of folds.

498

iii) The top-to-the-N thrust system became blurred but not obliterated by the later low-angle

499

normal faults, with ascending ramps, imbricate fans and duplex structures having been preserved

500

at many places. The documented thrusts are responsible for the stratigraphic and metamorphic

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repetitions observed in the current tectonic pile. Separate mapping of thrusts and low-angle

502

normal faults has been achieved in the central sector of the Alpujárride Complex.

503

iv) The top-to-the-N low-angle normal faults cutting the thrust system might have formed due to accretion at deep levels of the orogenic wedge during underthrusting of the Nevado-Filábride

505

Complex at Burdigalian-Langhian time.

506

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v) A minimum of 100 km of NNW-SSE shortening has been evaluated from restoration of the Alpujárride tectonic pile and the NNW-SSE length of the underlying Nevado-Filábride tectonic

508

window.

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vi) The internal structure of the Alpujárride Complex can be roughly correlated with the

510

Paleogene-Neogene movements between Africa and Iberia. Thus, the NNW-SSE shortening

511

episode (folds and thrusts) can be related to high convergence rates at latest Oligocene to earliest

512

Miocene time. Thrusting culminates at 16-18 Ma with superposition of the Alpujaride Complex

513

onto the Nevado-Filábride Complex, which in turn reached peak-pressure at that time. Later on,

514

relatively slow Africa/Iberia convergence rates enabled the previously subducted slab to rollback

515

and delaminate, with top-to-the-SW crustal extension becoming dominant since Serravallian time.

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Acknowledgments

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This paper has received financial support from the Spanish Ministry of Economy and

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Competitiveness through Grant CGL2015-71692-P. Thanks to A. Azor for his revision of the

522

manuscript. F. Rossetti and J.M. Martínez-Martínez are also thanked for constructive reviews that

523

have improved the manuscript.

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Crespo-Blanc, A., Orozco, M., García-Dueñas, V., 1994. Extension versus compression during the Miocene tectonic evolution of the Betic chain. Late folding of normal fault systems. Tectonics 13, 78-88 Davis, D., Suppe, J., Dahlen, F.A., 1983. Mechanics of fold-and-thrust belts and acretionary wedges. Journal of Geophysical Research 88, 1153-1172 De Jong, K., 1993. Large scale polyphase deformation of a coherent HP/LT metamorphic unit: the Mulhacén Complex in the eastern Sierra de los Filabres (Betic Zone, SE Spain). Geol. Mijnb. 71, 327-336 DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophys. J. Int. 101, 425-478 Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W., Knott, S.D., 1989. Kinematics of the Western Mediterranean. In: Coward MP, Dietrich D, Park RG (Ed) Alpine Tectonics. Geological Society, London, Special Publication 45, 265-283 Duggen, S., Hoernle, K., Klügel, A., Gelmacher, J., Thirlwall, M., Hauff, F., Lowry, D., Oates, N., 2008. Geochemical zonation of the Miocene Alborán Basin volcanism (westernmost Mediterranean): geodynamic implications. Contributions to Mineralogy and Petrology 156, 577-593 Echeverría, A., Khazaradze, G., Asensio, E., Masana, E., 2015. Geodetic evidence for continuing tectonic activity of the Carboneras fault (SE Spain) Tectonophysics 663, 302-309, doi: 10.1016/j.tecto.2015.08.009 England, Ph., Molnar, P., 1993. Cause and effect among thrust and normal faulting, anatectic melting and exhumation in the Himalaya. Geological Society London, Special Publication 74, 401-411, doi: 10.1144/GSL.SP.1993.074.01.27 Estévez, A, Delgado, F., Sanz de Galdeano, C., Martín Algarra, A., 1985. Los Alpujárrides al sur de Sierra Nevada. Una revisión de su estructura. Mediterranea 4, 5-32 Faccenna, C., Piromallo, C., Crespo-Blanc, A., Jolivet, L., Rossetti, F., 2004. Lateral slab deformation and the origin of the Western Mediterranean arcs. Tectonics 23, http://dx.doi.org/10.1029/2002TC001488 Galindo-Zaldívar, J., González Lodeiro, F., Jabaloy. A., 1989. Progressive extensional shear structures in a detachment contact in the western Sierra Nevada (Betic Cordilleras, Spain). Geodinamica Acta 3, 73-85 Galindo-Zaldívar, J., Jabaloy, A., González-Lodeiro, F., Aldaya, F., 1997. Crustal structure of the central sector of the Betic Cordillera (SE Spain). Tectonics 16, 18-37 Galindo-Zaldívar, J., Gil, A.J., Sanz de Galdeano, C., Lacy, M.C., García-Armenteros, J.A., Ruano, P., Ruiz, A.M., Martínez-Martos, M., Alfaro, P., 2015. Active shallow extension in central and eastern Betic Cordillera from CGPS data. Tectonophysics 663, 290-301, doi: 10.1016/j.tecto.2015.08.035 García-Dueñas, V., Balanyá, J.C., Martínez-Martínez, J.M., 1992. Miocene extensional detachments in the outcropping basement of the northern Alboran Basin and their tectonic interpretation. Geo-Marine Letters 12, 88-95 Goffé, B., Michard, A., García-Dueñas, V., González-Lodeiro, F., Monié, P., Campos, J., Galindo-Zaldívar, J., Jabaloy, A., Martínez-Martínez, J.M., Simancas, J.F., 1989. First evidence of high-pressure, lowtemperature metamorphism in the Alpujárride nappes, Betic Cordilleras (SE Spain). European Journal of Mineralogy 1, 139-142 Gómez-Pugnaire, M.T., Braga, J.C., Martín, J.M., Sassi, F.P., Moro, A., 2000. Regional implications of a Palaeozoic age for the Nevado-Filábride cover of the Betic Cordillera, Spain. Schw. Min. Petrog. Mitt. 80, 45-52 González-Castillo, L., Galindo-Zaldívar, J., de Lacy, M.C., Borque, M.J., Martínez-Moreno, F.J., GarcíaArmenteros, J.A., Gil, A.J., 2015. Active rollback in the Gibraltar Arc: Evidences from CGPS data in the western Betic Cordillera. Tectonophysics 663, 310-321, doi: 10.1016/j.tect.2015.03.010 González-Lodeiro, F., Aldaya, F., Galindo-Zaldívar, J., Jabaloy, A., 1996. Superposition of extensional detachments during the Neogene in the internal zones of the Betic cordillera. Geol. Rundschau 85, 350-362 Hobbs, B., Means, W.D., Williams, P.F., 1976. An Outline of Structural Geology. John Wiley & Sons, Inc., New York, 571 p. Jabaloy, A., Galindo-Zaldívar, J., González-Lodeiro, F., 1993. The Alpujárride - Nevado-Filábride extensional shear zone, Betic Cordillera, SE Spain. Journal of Structural Geology 15, 555-569 Jolivet, L., Faccenna, C., 2000. Mediterranean extension and the Africa-Eurasia collision. Tectonics 19 10951106, doi: 10.1029/2000TC900018 Lonergan, L., Platt, J.P., 1995. The Malaguide-Alpujarrid boundary: a major extensional contact in the Internal Zone of the eastern Betic Cordillera, SE Spain. Journal of Structural Geology 17, 1655-1671 Lonergan, L., White, N., 1997. Origin of the Betic-Rif mountain belt. Tectonics 16, 504-522

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López Sánchez-Vizcaíno, V., Rubatto, D., Gómez-Pugnaire, M.T., Trommsdorff, V., Müntener, O., 2001. Midle Miocene high-pressure metamorphism and fast exhumation of the Nevado-Filábride Complex, SE Spain. Terra Nova 13, 327-332 Mancilla, F.L., Booth-Rea, G., Stich, D., Pérez-Peña, J.V., Morales, J., Azañón, J.M., Martin, R., Giaconia, F., 2015. Slab rupture and delamination under the Betics and Rif constrained from receiver functions. Tectonophysics 663, 225-237, doi: 10.1016/j.tecto.2015.06.028 Martínez-Martínez, J.M., 2007. Coexistencia de zonas de cizalla dúctil de extensión y de acortamiento en el domo de Sierra Nevada, Béticas (SE de España). Revista de la Sociedad Geológica de España, 20, 229-246 Martínez-Martínez, J.M., Soto, J.I., Balanyá, J.C., 2002. Orthogonal folding of extensional detachments: Structure and origin of the Sierra Nevada elongated dome (Betics, SE Spain), doi: 10.1029/2001TC0012983 Martínez-Martínez, J.M., Soto, J.I., Balanyá, J.C., 2004. Elongated domes in extended orogens: A mode of mountain uplift in the Betics (southeast Spain). The Geological Society of America, Special Paper 380, 243-266 Mayoral, E., Crespo-Blanc, A., Díaz, M.G., Benot, C., Orozco, M., 1994. Rifting miocène du Domaine d’Alboran: datations de sédiments discordants sur les unités alpujarrides en extension (Sud de la Sierra Nevada, Chaîne Bétique). C. R. Acad. Sci. Paris 319(II), 581-588 Morales, J., Serrano, I., Jabaloy, A., Galindo-Zaldívar, J., Zhao, D., Torcal, F., Vidal, F., González-Lodeiro, F., 1999. Active continental subduction beneath the Betic Cordillera and Alboran Sea. Geology 27, 735-738 Navarro-Vilá, F., García Dueñas, V., 1980. La Peza, Mapa Geológico de España 1:50.000. IGME Orozco, M., Alonso-Chaves, F.M., Nieto, F., 1998. Development of large north-facing folds and their relation to crustal extension in the Alborán domain (Alpujarras region, Betic Cordilleras, Spain). Tectonophysics 298, 271-295 Orozco, M., Álvarez-Valero, A.M., Alonso-Chaves, F.F., Platt, J.P., 2004. Internal structure of a collapsed terrain. The Lújar syncline and its significance for the fold- and sheet-structure of the Alborán Domain (Betic Cordilleras, Spain). Tectonophysics 385, 85-104, doi: 10.1016/j.tecto.2004.04.025 Orozco, M., Alonso-Chaves, F.M., Platt, J.P., 2017. Late extensional shear zones and associated recumbent folds in the Alpujárride subduction complex, Betic Cordillera, southern Spain. Geologica Acta 15, 51-66, doi: 10.1344/GeologicaActa2017.15.1.5 Pedrera, A., Galindo-Zaldívar, J., Sanz de Galdeano, C., López-Garrido, A.C., 2007. Fold and fault interactions during the development of an elongated narrow basin: the Almanzora Neogene-Quaternary Corridor (SE Betic Cordillera, Spain).Tectonics 26, TC6002, doi: 10.1029/2007TC002138 Platt, J.P., 1986. Dynamic of orogenic wedges and the uplift of high-pressure metamorphic rocks. Geological Society of America Bulletin 97, 1037-1053 Platt, JP., Vissers, R.L.M., 1989. Extensional collapse of thickened continental lithosphere: a working hypothesis for the Alboran Sea and the Gibraltar arc. Geology 17, 540-543 Platt, J.P., Soto, J.I., Whitehouse, M.J., Hurford, A.J., Kelley, S.P., 1998. Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean. Tectonics 17, 671-689 Platt, J.P., Kelley, S.P., Carter, A., Orozco, M., 2005. Timing of tectonic events in the Alpujárride Complex, southern Spain. Journal of the Geological Society, London, 162, 451-462. Platt, J.P., Anczkiewicz, S.P., Soto, J.I., Kelley, S.P., Thirlwall, M., 2006. Early Miocene continental subduction and rapid exhumation in the western Mediterranean. Geology 34, 981 Puga, E., Díaz de Federico, A., Nieto, J.M., 2002. Tectonostratigraphic subdivisión and petrological characterization of the deepest complexes of the Betic Zone: a review. Geodinamica Acta 15, 23-43 Rosenbaum, G., Lister, G.S., Duboz, C., 2002. Relative motions of Africa, Iberia and Europe during Alpine orogeny. Tectonophysics 359, 117-129 Rossetti, F.C., Faccenna, C., Crespo-Blanc, A. 2005. Structural and kinematic constraints to the exhumation of the Alpujárride Complex (Central Betic Cordillera, Spain). Journal of Structural Geology 27, 199216, doi: 10.1016/j.jsg.2004.10.008 Royden, L.H., 1993. Evolution of retreating subduction boundaries formed during continental collision. Tectonics 12, 629-638 Ruiz-Constán, A., Pedrera, A., Galindo-Zaldívar, J., Pous, J., Arzate, J., Roldán-García, F.J., Marín-Lechado, J., Anahnah, F., 2012. Constraints on the frontal crustal structure of a continental collision from an

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ACCEPTED MANUSCRIPT integrated geophysical research: The central-western Betic Cordillera (SW Spain). Geochemistry, Geophysics, Geosystems 13 Sánchez-Navas, A., García-Casco, A., Martín-Algarra, A. 2014. Pre-Alpine discordant granitic dikes in the metamorphic core of the Betic Cordillera: tectonic implications. Terra Nova 26, 477-486, doi: 10.1111/ter.12123 Sanz de Galdeano, C., 1990. Geologic evolution of the Betic Cordilleras in the Western Mediterranean, Miocene to the present. Tectonophysics 172, 107-119 Sanz de Galdeano, C., Alfaro, P., 2004. Tectonic significance of the present relief of the Betic Cordillera. Geomorphology 63, 175-190 Sanz de Galdeano, C., López Garrido, A.C., 2014. Structure of the Sierra de Lújar (Alpujárride Complex, Betic Cordillera). Estudios Geológicos 70, e005, doi: 10.3989/egeol.41491.290 Sanz de Galdeano, C., Peláez, J.A., 2011. Fallas activas en la Cordillera Bética. Editorial Universidad de Granada Silva, P.G., Goy, J.L., Somoza, L., Zazo, C., Bardají, T., 1993. Landscape response to strike-slip faulting linked to collisional setting: Quaternary tectonics and basin formation in the Eastern Betics, southeastern Spain. Tectonophysics 224, 289-303 Simancas, J.F., Campos, J., 1993. Compresión NNW-SSE tardi a postmetamórfica y extensión subordinada en el Complejo Alpujárride (Dominio de Alborán, Orógeno Bético). Revista de la Sociedad Geológica de España 6, 23-35 Tubía, J.M., Cuevas, J., Navarro-Vilá, F., Álvarez, F., Aldaya, F., 1992. Tectonic evolution of the Alpujárride Complex (Betic Cordillera, southern Spain). Journal of Structural Geology 14, 193-203 Vera, J., Martín-Algarra, A., 2004. Cordillera Bética y Baleares. Divisiones mayores y nomenclatura. In: Vera, J. (Ed.) Geología de España. SGE-IGME, Madrid, 347-350 Vissers, R.L.M., Meijer, P.Th., 2012. Iberian plate kinematics and Alpine collision in the Pyrenees. EarthScience Reviews 114, 61-83, doi: 10.1016/j.earscirev.2012.05.001 Vissers, R.L.M., Platt, J.P., van der Wal, D., 1995. Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view. Tectonics 14, 786-803 Williams, J.R., Platt, J.P., 2017. Superposed and refolded metamorphic isograds, and superposed directions of shear during late-orogenic extension in the Alborán Domain, southern Spain. Tectonics 36, 756786, doi: 10.1002/2016TC004358 Yamato, P., Brun, J.P., 2017. Metamorphic record of catastrophic pressure drops in subduction zones. Nature geoscience 10, doi: 10.1038/NGE02852

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Fig. 1 A) Location and main geological domains of the Betic Orogen. B) Sketched structural map of the central sector of the Alpujárride Complex. Location of Figs. 3 and 7 is given, as well as the crosssections shown in Figs. 4 and 5. C) Lithological sequence of the Alpujárride Complex; notice tapering to the east. D) Equal-area and lower-hemisphere projections of linear structures of the successive deformational events (see text for further explanation).

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Fig. 2 A) Main D2A deformation in marbles, with stretching lineation (Ls2A) and asymmetric tails indicating top-to-the-NE sense of shear. B) Two generations of microfolds in marbles: F2A, pointed by the arrow, and F3A. C) Hinge of an F3A syncline in marbles, with axial-planar foliation at right angle to layering; the fold is overturned to the NW. D) Second-order M-type folds at the hinge strain shadow of a first-order F3A fold, developed in quartzites and phyllites underlying marbles. E) C’-

ACCEPTED MANUSCRIPT type shear bands in low-grade schists, indicating top-to-the-N semibrittle shearing. F) Crush breccia at the thrust contact between two carbonate slabs.

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Fig. 3 A) Structural map of the area east of the Sierra de Lújar, showing the nearly coaxial but opposite vergent superposition of the D2A and D3A folding phases; the location of the cross-sections in Fig. 4 is indicated. B) Equal-area and lower-hemisphere projections of foliations (S2A and S3A) and fold axes (F2A and F3A) of the two superposed folds. Fig. 4 A) Geological cross-section of the structure east of the Sierra de Lújar (see location in Fig. 3A). B) Detailed cross-section showing local controls of vergence and polarity.

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ACCEPTED MANUSCRIPT Highlights Thrust orogenic wedge obliterated by low-angle normal faults Top-to-the-N extension caused by crustal underplating Top-to-the-SW extension caused by lithospheric rollback Lithospheric rollback prompted by slow Africa-Iberia convergence rates

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