Contrasting structural and P-T evolution of tectonic units in the southeastern Betics: Key for understanding the exhumation of the Alboran Domain HP/LT crustal rocks (western mediterranean).

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Contrasting structural and P-T evolution of tectonic units in the southeastern Betics: Key for understanding

the exhumation of the Alboran Domain HP/LT crustal rocks (western mediterranean).

G. Booth-Real, J.M. Azanon, Olivier Vidal, V. Garcia-Duenas, J.M.

Martinez-Martinez

To cite this version:

G. Booth-Real, J.M. Azanon, Olivier Vidal, V. Garcia-Duenas, J.M. Martinez-Martinez. Contrasting

structural and P-T evolution of tectonic units in the southeastern Betics: Key for understanding

the exhumation of the Alboran Domain HP/LT crustal rocks (western mediterranean).. Tectonics,

American Geophysical Union (AGU), 2005, 24, pp.TC2009. �10.1029/2004TC001640�. �hal-00079312�

Contrasting structural and P-T evolution of tectonic units in the southeastern Betics: Key for understanding

the exhumation of the Alboran Domain HP//LT crustal rocks (western Mediterranean)

G. Booth-Rea,

J. M. Azan˜o´n,

J. M. Martı´nez-Martı´nez,

O. Vidal,

and V. Garcı´a-Duen˜as

Received 24 February 2004; revised 18 November 2004; accepted 10 December 2004; published 19 April 2005.

[

] Thermobarometry using mica-chlorite local equilibria and structural analysis of three tectonic units of the Alpujarride complex in the southeastern Betics indicate that rocks that underwent low- pressure/low-temperature metamorphism are found below higher-grade blueschist facies rocks. Moreover, the high-pressure/low-temperature (HP/LT) units underwent contrasting (P-T) and structural evolutions.

The structurally highest unit was exhumed in a typical postorogenic high-temperature geothermal context in the andalusite stability field, while the intermediate unit was exhumed following a cooling and decompression P-T path within the kyanite stability field. The superposition of the three units occurred during the lower Miocene related with north directed brittle- ductile shear zones and associated north vergent overturned folds, after postorogenic exhumation of the top unit. The coexistence of thrusting with lower and middle Miocene brittle extensional systems active in shallower levels of the Alboran orogenic wedge implies that extension was synorogenic. These data strengthen the hypothesis that rocks thinned by postorogenic extension during the latest Oligocene and lower Miocene were entrained in the lower to middle Miocene Alboran orogenic wedge. ^Citation: Booth-Rea, G., J. M. Azan˜o´n, J. M. Martı´nez-Martı´nez, O. Vidal, and V. Garcı´a-Duen˜as (2005), Contrasting structural and P-T evolution of tectonic units in the southeastern Betics: Key for understanding the exhumation of the Alboran Domain HP/LT crustal rocks (western Mediterranean), Tectonics, 24, TC2009, doi:10.1029/2004TC001640.

1. Introduction

[

] The modes of exhumation of high-pressure low- temperature (HP/LT) metamorphic rocks, which form the basement of Mediterranean Tertiary back arc basins, have been the subject of great debate [e.g., Malinverno and Ryan, 1986; Platt and Vissers, 1989; Royden, 1993; Vissers et al., 1995; Avigad et al., 1997; Azan˜o´n et al., 1997; Lonergan and White, 1997; Jolivet and Patriat, 1999; Jolivet et al., 1999; Faccenna et al., 2001; Trotet et al., 2001; Faccenna et al., 2004]. The models proposed for the exhumation of the Alboran crustal domain (ACD), which forms the base- ment of the Alboran basin and of other Neogene uplifted and exposed basins outcropping in the Betics [e.g., Platt and Vissers, 1989; Garcı´a-Duen˜as et al., 1992; Martı´nez- Martı´nez and Azan˜o´n, 1997; Balanya´ et al., 1998; Platt, 1998; Comas et al., 1999; Orozco et al., 1998; Platt et al., 2003a; Martı´nez-Martı´nez et al., 2004], are a clear example of this debate.

[

] The ACD comprises in ascending order the Nevado- Filabride, Alpujarride and Malaguide complexes (Figure 1).

The two lowest complexes include polymetamorphic units, some of which have undergone high-pressure alpine meta- morphism under eclogite and/or blueschists facies [e.g., Go´mez-Pugnaire and Ferna´ndez-Soler, 1987; Morten et al., 1987; Bakker et al., 1989; Goffe´ et al., 1989; Tubı´a and Gil Ibarguchi, 1991; Azan˜o´n and Goffe´, 1997; Azan˜o´n et al., 1997; Puga et al., 2000; Booth-Rea et al., 2002a, 2003c, 2003d]. Three main tectonic scenarios are proposed for the exhumation of these (HP/LT) rocks: (1) The exhumation of the high-pressure units occurred during Miocene postorogenic extension following a Cretaceous to Paleogene thickening event [Platt and Vissers, 1989;

Vissers et al., 1995; Lonergan and White, 1997; Platt et al., 1998; Argles et al., 1999; Platt and Whitehouse, 1999; Platt et al., 2003a]; (2) there was a more intricate evolution, as part of the exhumation of the HP/LT metamorphic rocks was produced by thrusting in an active orogenic-wedge setting, prior to Miocene postoro- genic extension [Avigad et al., 1997]; (3) alternating extensional and contractive tectonic events occurred, including late underthrusting of low P/T units beneath high-pressure units, after a Paleogene or lower Miocene ductile-thinning extensional event [Azan˜o´n et al., 1997;

Balanya´ et al., 1997, 1998; Azan˜o´n and Crespo-Blanc, 2000].

TECTONICS, VOL. 24, TC2009, doi:10.1029/2004TC001640, 2005

1IFM-GEOMAR Leibniz-Institute fuer Meereswissenschaften, Kiel, Germany.

2Now at Departamento de Geodina´mica, Universidad de Granada, Granada, Spain.

3Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientı´ficas, and Departamento de Geodina´mica, Universi- dad de Granada, Granada, Spain.

4Maison des Geosciences, Universite´ Joseph Fourier LGCA, Grenoble, France.

Copyright 2005 by the American Geophysical Union.

0278-7407/05/2004TC001640

[

] The mechanisms driving the tectonic processes described above include thermal remotion of the dense and thickened mantle portion of the lithosphere in the inner side of the arc [Platt and Vissers, 1989; Vissers et al., 1995; Platt et al., 2003a]; slab roll-back [De Jong, 1991; Royden, 1993;

Lonergan and White, 1997]; slab break-off [Carminati et al., 1998; Zeck, 1999], and delamination of subcontinental lithosphere [Garcı´a-Duen˜as et al., 1992; Martı´nez-Martı´nez and Azan˜o´n, 1997; Calvert et al., 2000]. Tomographic studies show an eastward dipping slab below the Gibraltar Arc and the Alboran Sea, supporting models which suggest slab roll-back, delamination [Blanco and Spakman, 1993;

Calvert et al., 2000; Gutscher et al., 2002] or both, slab roll-back in areas where there was oceanic crust and delamination under continental crust [Faccenna et al., 2004].

[

] To evaluate these different tectonic models, it is fundamental to understand the structural relationships be- tween the stacked units of the ACD and the tectonic evolution undergone by each unit, including their P-T trajectory and its relationship with mineral blastesis and deformation. A large data set exists in the Betics, both of the metamorphic evolution of the individual ACD units and of the tectonic significance of the boundaries between these units, especially in the western and central Betics. However, this is not the case in the southeastern Betics where most of the rocks overlying the HP/LT Nevado-Filabride complex are low-grade high-variance metapelites [Platt, 1986;

Vissers et al., 1995; Lonergan and Platt, 1995], hampering the knowledge of their P-T evolution. Here, we analyze the structure and thermobarometric evolution of low-grade metapelites from tectonic units that crop out above the Nevado-Filabride complex in the southeastern Betics (Figures 1 and 2). We revise the structure of the south- eastern Betics with especial emphasis in the Sierra de Almagro (Figure 2). We used multiequilibrium thermoba- rometry enabling a simultaneous estimation of pressure and temperature from the composition of high-variance mineral parageneses that define several microstructural domains of metapelites [Berman, 1991]. We later relate the penetrative microstructures found in the units to their meta- morphic evolution. These data were integrated with struc- tural field observations from outcrop to map scale. Finally, we link the tectonic evolution of these units with the underlying Nevado-Filabride complex and discuss the possible relationships between them, with a proposal for the tectonic evolution of the ACD.

2. Tectonic Setting

[

] The Gibraltar arc forms the western end of the Mediterranean Alpine chains between the European and African plates. It is a strongly arcuate orogenic system formed by the Betics, Rif and Tell chains, which are connected through the Straits of Gibraltar. The internal part of the arc is occupied by the Alboran Sea (Figure 1). This Figure 1. Geological map of the Betic-Rif orogen, with location of the studied area.

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orogenic system evolved in a convergent context between the African and European plates since the upper Cretaceous [Dewey et al., 1989; Mazzoli and Helman, 1994], with approximately 200 km of N-NNE convergence up to the Tortonian (9 Ma) and with 50 km of further NW-SE convergence between 9 Ma and present day.

[

] The Gibraltar arc formed by collision of several pre- Miocene crustal domains [Balanya´ and Garcı´a-Duen˜as,

1987], in ascending order: the South Iberian and Magrebian domains; Mesozoic-Cenozoic paleomargins respectively of the Iberian and African plates, the Flyschs-Trough units formed by allochthonous sedimentary covers deposited in a deep trough floored by thinned-continental or oceanic basement [Durand-Delga et al., 2000], and the Alboran crustal domain (ACD), thrust over the Flyschs-Trough units and the Maghrebian and South Iberian domains. The ACD Figure 2. Structural map and approximate cross section of the studied area. Analyzed samples are

discussed in the text.

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is formed in ascending order by the Nevado-Filabride, Alpujarride and Malaguide complexes, together with the dorsal and predorsal units between the Flysch Trough and the ACD [Balanya´ and Garcı´a-Duen˜as, 1987].

[

] The Nevado-Filabride complex crops out in the central and eastern Betics forming the core of elongated domes that define the highest mountain ranges of the Internal Betics (Figures 1 and 2). It comprises at least 8 km [Martı´nez-Martı´nez et al., 2002] of metapelites and metaevaporites with metabasite and orthogneiss intercala- tions, metamorphosed under higher-greenschist or amphib- olite facies, which locally include eclogites and blueschists [e.g., Go´mez-Pugnaire and Ferna´ndez-Soler, 1987; Morten et al., 1987; Puga et al., 2000; Martı´nez-Martı´nez et al., 2004]. Three tectonometamorphic units can be differenti- ated, in ascending order the Ragua, the Calar-Alto and the Be´dar-Macael units [Martı´nez-Martı´nez et al., 2002].

[

] The Alpujarride complex overlies the Nevado- Filabride complex, occupying the largest expanse of the Internal Betics. It is formed by superposed metamorphic units that reached different P-T conditions. The lowest grade rocks of each Alpujarride unit reached conditions ranging between <0.7 GPa-340C and 1.1 GPa-570C during the HP metamorphic peak [Azan˜o´n et al., 1994, 1997; Balanya´ et al., 1997, 1998; Azan˜o´n and Crespo-Blanc, 2000]. Follow- ing the HP-LT/HT metamorphic event, the Alpujarride units underwent isothermal decompression P-T paths [Balanya´ et al., 1993; Garcı´a-Casco and Torres-Rolda´n, 1996; Azan˜o´n et al., 1998; Balanya´ et al., 1997; Argles et al., 1999; Azan˜o´n and Crespo-Blanc, 2000]. In the western Betics the two units at the top of the Alpujarride complex include migmatitic gneisses at their base formed at temperatures above 750C [Loomis, 1976; Torres-Rolda´n, 1981; Tubı´a and Gil Ibarguchi, 1991; Balanya´ et al., 1997; Tubı´a et al., 1997; Argles et al., 1999] during the early Miocene [e.g., Platt and Whitehouse, 1999; Sa´nchez-Rodrı´guez and Gebauer, 2000], recycled from previous Hercynian high- grade rocks [Zeck and Whitehouse, 1999]. In the same area, a slab of subcontinental lithospheric peridotites (Figure 1) is found below granulites of the structurally highest Alpujarride unit [e.g., Tubı´a, 1994; Balanya´ et al., 1997; Tubı´a et al., 1997; Lenoir et al., 2001]. Alpujarride high-grade rocks (sillimanite schists or K-feldspar gneisses) crop out also in the central and eastern Betics [Balanya´ et al., 1998; Azan˜o´n and Crespo-Blanc, 2000;

Azan˜o´n et al., 1998; Zeck et al., 1989] and yield U-Pb zircon ages of 19 – 20 Ma [Platt et al., 2003a].

[

] The Malaguide complex covers the Alpujarride complex. It represents the remnants of a thrust stack that was strongly extended [Tubı´a et al., 1993; Lonergan and Platt, 1995; Booth-Rea et al., 2004a] and intruded by tholeitic dikes between the upper Oligocene and the Miocene [Torres-Rolda´n et al., 1986; Turner et al., 1999].

When complete, each Malaguide thrust sheet comprises a Paleozoic basement, the base of which underwent low-grade metamorphism during the Hercynian orogeny [Chalouan and Michard, 1990] and a Permo-Triassic sedimentary cover. The Permo-Triassic cover reached anchizonal meta- morphic conditions in the structurally lowest Malaguide

thrust sheets [Nieto et al., 1994; Lonergan and Platt, 1995]. Alpine metamorphism in the Malaguide complex followed initial thrusting during collision with the under- lying Alpujarride complex [Lonergan and Platt, 1995;

Balanya´ et al., 1997]. Overlying the structurally highest unmetamorphosed Malaguide thrust sheet there is a kilometer-thick Jurassic to lower Miocene sedimentary cover [Lonergan, 1993; Lonergan and Mangerajetzky, 1994; Martı´n Martı´n et al., 1997].

[

] The boundaries among the complexes were initially considered as thrusts as they coincide with stratigraphic and metamorphic recurrences [Egeler and Simon, 1969;

Kampschuur and Rondeel, 1975; Ma¨kel, 1981; Campos and Simancas, 1989; Simancas and Campos, 1993].

However, many of these contacts have been reinterpreted as extensional plastic and brittle shear zones. These shear zones reworked and faulted the previous thrust contacts extending the ACD in the internal part of the Gibraltar Arc during the lower and middle Miocene, generating the Neogene Alboran basin and other marine basins currently emerged in the Betics [Platt, 1986; Garcı´a-Duen˜as and Martı´nez-Martı´nez, 1988; Galindo-Zaldı´var et al., 1989;

Platt and Vissers, 1989; Aldaya et al., 1991; Garcı´a- Duen˜as et al., 1992; Crespo-Blanc, 1995; Lonergan and Platt, 1995; Martı´nez-Martı´nez and Azan˜o´n, 1997; Comas et al., 1999; Martı´nez-Martı´nez et al., 2002; Booth-Rea et al., 2002b, 2003a, 2004b], for example the Vera and Almanzora basins (Figure 2).

3. Structure of the Alboran Crustal Domain Underlying the Neogene Vera Basin

[

] The Vera basin has a Burdigalian to Quaternary, mostly marine, sedimentary sequence with many uncon- formities [Vo¨lk, 1966; Barraga´n, 1997; Booth-Rea et al., 2003b]. The basement of the basin is formed by the Nevado-Filabride, Alpujarride and Malaguide complexes (Figures 2 and 3). The Alpujarride complex comprises three tectonic units in the southeastern Betics, in ascending order, the Almagro, Almanzora and Variegato units [Booth-Rea et al., 2003d] (Figures 2, 3, 4, and 5). This metamorphic basement crops out mostly in the core of upper Miocene to Quaternary ENE trending antiforms like the Sierra de Filabres, Sierra Cabrera, Sierra Almagrera and Sierra de Almagro [Booth-Rea et al., 2004a] (Figure 2).

3.1. Nevado-Filabride Complex

[

] The lithostratigraphic sequences of the Nevado- Filabride units consist from bottom to top, mostly of black graphitic schist and quartzite of pre-Permian age [Lafuste and Pavillon, 1976]; a sequence of light-colored metapelites and metapsammites; and a marble formation. Permian orthogneisses and late Jurassic metabasites are locally included at different levels of the sequence.

[

] The main foliation in the Nevado-Filabride complex

is an axial-plane crenulation cleavage (S

) associated with

folds with mostly E/W trending hinges. This cleavage grew

under higher greenschist facies in the Ragua unit and in

TC2009 BOOTH-REA ET AL.: EXHUMATION OF ALBORAN HP/LT ROCKS TC2009

most of the Calar-Alto unit, and under amphibolite facies at the base of the Calar-Alto and in the Be´dar-Macael units [Martı´nez-Martı´nez, 1986a; Platt and Behrmann, 1986;

Garcı´a-Duen˜as et al., 1988; De Jong, 1992; Martı´nez- Martı´nez and Azan˜o´n, 1997; Booth-Rea et al., 2004c]. A previous foliation (S

) formed during HP/LT metamorphism is preserved in the lenticular domains of the S

cleavage. In graphite schists of the Ragua and Calar-Alto units the S

foliation equilibrated under conditions of 320– 450C and 1.2 – 1.6 GPa [Booth-Rea et al., 2003c, 2004c]. Peak tem- peratures in the graphite schists and in some metabasites ranging between 500 and 600C were attained after decompression, during the S

development [Go´mez- Pugnaire and Ferna´ndez-Soler, 1987; Booth-Rea et al., 2004c]. In some eclogites [Puga et al., 1999, 2002] and in rare metamorphic ultramafic rocks [Lo´pez Sa´nchez- Vizcaino et al., 2001] peak temperatures of 650 – 700C were reached during the HP peak under pressures of 2.0 – 2.2 GPa. Eclogites occurring in the Nevado-Filabride

complex have been related with the first deformational event [Bakker et al., 1989].

[

] The contacts between the main tectonic units of the Nevado-Filabride complex, the Ragua/Calar-Alto and the Calar-Alto/Be´dar-Macael boundaries, are thick (500 – 600 m) gently dipping ductile-shear zones with a flat geometry [Garcı´a-Duen˜as et al., 1988]. The shear zones have moderate-temperature mylonites (450– 550C) that exhibit an L-S fabric (S

) generated by metamorphic blastesis and dynamic recrystallization, which replaces the S

cleavage. Kinematic indicators have west to WNW sense of shear [Garcı´a-Duen˜as et al., 1988; Soto et al., 1990; Gonza´lez-Casado et al., 1995; Martı´nez-Martı´nez et al., 2002]. These mylonites and the rest of the Nevado- Filabride thrust pile are folded by south to SE vergent, meter-to-hectometer folds with an associated weak perva- sive crenulation cleavage (S

) [Vissers, 1981; Behrmann and Platt, 1982; Martı´nez-Martı´nez, 1986a; De Jong, 1992;

Soto, 1993]. The Nevado-Filabride complex was exhumed Figure 3. Structural map and tectonic units of the Sierra de Almagro anticlinorium. Cross section C-C

is shown in Figure 5. Modified from Booth-Rea et al. [2003d].

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Figure 4. (a) Geological map of the Sierra de Almagro anticlinorium. (b) Lithologic section and mineral paragenesis of the Alboran Domain in the Sierra de Almagro. (c) Neogene and Quaternary sedimentary cover. (d) Poles to slaty cleavage in the Almagro unit. (e) Poles to the S

and S

cleavages and axes of north vergent asymmetric folds in metapelites of the Almanzora unit. Equal-angle lower-hemisphere stereographic projections. Analyzed samples are discussed in the text. Mineral abbreviations are as follows: Chl, chlorite; phg, phengite; qtz, quartz; alb, albite; Mg-car, Mg-carpholite; prl, pyrophyllite;

Bio, biotite; grt, garnet.

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to the surface between 12 and 8 Ma ago [Johnson et al., 1997] by SW directed brittle and ductile detachments that are the contacts between the Nevado-Filabride and the Alpujarride complexes [Martı´nez-Martı´nez et al., 2002, 2004].

3.2. LP//LT Almagro Unit

[

] The Almagro unit, defined by Simon [1963], crops out in the core of the Sierra de Almagro and in the Sierra de Enmedio, farther north (Figure 2). It comprises two formations; variegated quartzites and slates, at least 600 m thick, with a lower Triassic age [Sanz de Galdeano and Garcı´a-Tortosa, 2002], and a carbonate-gypsum formation with intercalated metabasites and slates (600 m) dated as middle and upper Triassic [Kozur et al., 1985] (Figures 4a, 4b, and 5).

[

] The most penetrative planar fabric in the Almagro unit is a slaty cleavage (S

, Figure 5d) subparallel to the bedding (S

) [De Jong, 1991]. The cleavage is defined by concentration of opaque minerals, truncation of previous mineral grains, and local phyllosilicate growth in pressure shadows (Figure 6a). This slaty cleavage was originated fundamentally by pressure-solution mechanisms. The S

is locally folded by overturned outcrop-scale NNE vergent folds (F

) with an associated spaced axial-plane cleavage (S

) localized in the fold hinges (Figure 7a). The main folds in the Almagro unit are S/SSW vergent F

open to tight folds with a kilometer-scale wave length (geological map, Figure 3, sections A-A

and B-B

, Figures 5, 7b, and 7c). These folds occur in the Sierra de Almagro and also toward the northeast in the Sierras de Enmedio and Carra- scoy [Kampschuur et al., 1973]. They have a similar geometry to the south-to-SE vergent folds that are present in the entire Nevado-Filabride stack [Vissers, 1981;

Behrmann and Platt, 1982; Martı´nez-Martı´nez, 1986a; Soto, 1993; and Martı´nez-Martı´nez and Azan˜o´n, 1997], and in other Alpujarride units in the Sierra de las Estancias [Akkerman et al., 1980]. The spaced Almagro S

crenu- lation cleavage associated with the F

folds is especially penetrative in their overturned limbs, which are flattened and cut by associated reverse faults [De Jong, 1991] (Figure 3;

cross sections A-A

, B-B

in Figure 5; Figure 7d).

[

] Metabasites in the Almagro unit include blue amphiboles that were interpreted as evidence of a high- pressure low-temperature metamorphic event [Simon, 1963]. However, more recent analysis of these amphiboles showed that they are riebeckites, with a large P-T stability Figure 5. Cross sections of the Sierra de Almagro, location in Figures 3 (cross section C-C

) and 4a

(cross sections A-A

and B-B

). Legend in Figure 4c.

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range. Thermobarometric calculations with these iron-rich sodic amphiboles in equilibrium with chlorite indicate that the metabasites reached approximately 0.4 GPa and 300C [Sa´nchez-Vizcaino et al., 1991].

3.3. Almanzora Unit

[

] The Almanzora unit [Simon, 1987] crops out in the northern border of Sierra de Filabres and in Sierra de Almagro (Figures 2 and 3). This unit consists of at least 2 km of quartzites, metapsammites and fine-grained schists with carbonate and gypsum intercalations at the top (Figures 4a, 4b, and 5). The carbonate intercalations yield an upper Ladinian age [Kozur et al., 1985]. Overlying this sequence they are 200 m of gypsum with metapelite, carbonate and metabasite intercalations, and upper Ladi- nian and Carnian carbonates [Simon, 1966; Kozur et al., 1985]. The evaporitic formation of the Almanzora unit is always tectonically detached from the quartz-metapelite formation.

[

] The schistosity (S

) in the fine-grained schists of the Almanzora unit is subparallel to the bedding (S

) and is

defined by quartz + white K mica + chlorite + rutile + tourmaline (Figures 6b and 6c). The S

foliation is crenu- lated by a S

axial-plane cleavage related with tight to isoclinal folds. Linear elements related with the S

cleavage include fold axes, mica lineations and elongated pebbles with E-W to ESE-WNW orientation [De Jong, 1991]

(Figure 4e). Up to 300% extension parallel to the X axis was measured on a pebble from quartzites in the Almanzora unit [De Jong, 1991]. In the most pelitic samples S

is the dominant foliation, defined mostly by white K mica + quartz ± chlorite ± albite ± ilmenite (Figures 6b and 6c).

Both foliations are sealed by postkinematic white K mica and are cut by a spaced cleavage (S

) related to north vergent overturned folds (cross sections A-A

, C-C

in Figure 5; Figure 8c). The north vergent overturned folds are homoaxial to isoclinal folds related with the S

cleavage.

[

] The two differentiated metamorphic fabrics in the Almanzora quartz-metapelites are defined by high-variance mineral assemblages. The P-T path followed by the Alman- zora unit was determined by Bakker et al. [1989], mostly from metabasites included in the metaevaporitic formation.

Figure 6. (a) Metapelite from the Almagro unit. Notice S

slaty cleavage, marked by concentration of opaque minerals, originated by pressure solution. (b) Almanzora metapelite. Notice the S

foliation defined by Chl + WKm + Qtz and the S

cleavage defined by WKm and Qtz. (c) Mineral assemblages and microstructural domains in the Almanzora fine-grained schist, sample Fil.1. (d) Variegato graphite schist. Notice growth of garnet and biotite, and the high penetrability of the S

foliation that has obliterated previous microstructures. Mineral abbreviations are as follows: chl, chlorite; WKm, white K mica; Qtz, quartz; Bio, biotite; Grt, garnet.

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The first segment recorded on their P-T path is an isobaric heating at 0.7 GPa from 400 to 450C, which is followed by a decompression segment with cooling to 0.35 GPa and 425C. Later the Almanzora unit underwent an isobaric heating under pressures of 0.2 – 0.3 GPa, reaching temper- atures slightly above 500C. Finally, these rocks cooled below 400C at 0.1 GPa [Bakker et al., 1989].

3.4. Variegato Unit

[

] The Variegato unit [Simon, 1963] crops out in all the main mountain ranges of the southeastern Betics [Booth-Rea et al., 2002b, 2003d] where it comprises at least three second-order imbrications. The two lowest of these are formed by fine-grained schists of Permo-Triassic age and by Triassic carbonate rocks. The structurally highest imbrication also includes graphite schist at its base with garnet and biotite [Sanz de Galdeano and Garcı´a- Tortosa, 2002; Booth-Rea et al., 2002b, 2003d]. The graphite schist include staurolite in the NE Sierra de Filabres [De Jong, 1991].

[

] The Variegato metapelites have a main S

foliation, which is a penetrative crenulation cleavage, axial plane of similar folds with ENE-to-E-W trending axes [De Jong, 1991], defined by white K mica + chlorite + tourmaline + quartz in the fine-grained schist and by biotite + garnet + quartz in the graphite schist of the highest thrust sheet

(Figure 6d). A previous foliation (S

) is preserved in the fine-grained schist, in the lenticular domains of the S

crenulation, formed by white K mica + chlorite + quartz. Relic high-pressure mineral assemblages including Mg-carpholite + chlorite + pyrophyllite are preserved in pre-S

quartz veins of the lowest Variegato imbrication [Booth-Rea et al., 2002a]. Locally the S

and S

foliations are cut by an S

axial-plane cleavage related to outcrop- scale overturned folds with mostly NW/SE trending hinges and NE vergence. In the graphite schist the S

foliation is defined by the assemblage chlorite, white K mica, anda- lusite and quartz.

[

] The HP/LT mineral assemblages with carpholite equilibrated at 0.8 – 1.0 GPa and 350 – 410C [Booth-Rea et al., 2002a]. The high-variance assemblage chlorite + white K mica + quartz defining the relic S

foliation in quartz-rich metapelites also equilibrated under HP/LT conditions of 0.8 – 1.1 GPa and approximately 400C [Booth-Rea et al., 2003d].

4. P-T Conditions and P-T Evolution

[

] It is possible to determine equilibrium P-T points for high-variance parageneses with multiequilibrium calcula- tions using the composition of several end members per phase [Berman, 1991]. For example, the P-T conditions for Figure 7. (a) Asymmetric F2Alm anticline in quartzites of the Almagro unit, with associated S

spaced crenulation cleavage. (b) Triassic carbonate (Talmc) and gypsum (Talmg) sequence of the Almagro unit folded by an asymmetric F

SSW vergent syncline. (c) F

SSW vergent fold, refolded by an upper Neogene antiform in the core of the Sierra de Almagro (P-Talmz, Almanzora unit metapelites). (d) S

spaced cleavage and associated reverse fault affecting limestones of the Almagro unit.

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the paragenesis white K mica + Chl + Qtz + Alb + W found in the Almanzora unit can be calculated using ten end members: (water, quartz, Mg-celadonite, muscovite, pyro- phyllite, Mg-amesite, sudoite, clinochlore, albite and para- gonite content in white K mica) in the six-component

chemical system (SiO

, Al

O

, MgO, K

O, Na

O, H

O).

Thirty-two reactions can be written, four of which are independent. The P-T location of these reactions was determined using TWEEQU 1.02 software [Berman, 1991], its associated database JUN92, thermodynamic Figure 8. (a) Thrust bounding the Almagro and Almanzora units. (b) Carbonate mylonite at the base of

the Almanzora unit. (c) Metapelites of the Almanzora unit. Notice the S

and S

cleavages and how the S

spaced cleavage dips toward the north, overturned by the late Neogene Almagro anticlinorium. (d) Hand specimen of carbonate mylonite from the thrust contact between the Almagro and Almanzora units.

(e) Stereographic projection of faults, mylonitic foliation and associated calcite fibers at the base of the Almanzora unit.

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properties for Mg-amesite, Mg-sudoite, Mg-celadonite, and chlorite and mica solid-solution models from Vidal et al.

[1992, 1999, 2001], Vidal and Parra [2000] and Parra et al. [2001].

[

] As discussed by Vidal and Parra [2000], a multi- equilibrium approach is adequate for studying the P-T conditions reached by rocks that include mica-chlorite pairs;

first, because the equilibration of these minerals with varying pressure and temperature is mostly achieved by crystallization/recrystallization processes rather than by changing the composition of older grains by lattice diffusion (especially at the low temperatures of blueschist and greenschist facies metamorphism). Second, the relative growth age of phyllosilicates can be determined using microstructural criteria. Hence, analyzing chlorite-white K mica pairs that define different microstructures of a meta- morphic rock, we can determine the P-T conditions under which each structure grew; defining the P-T path followed by the rock.

[

] Ideally, all the equilibria calculated for a given paragenesis should intersect at a single point. However, in practice some scatter is commonly observed. The scatter results from errors in each reaction that stem from the uncertainties in the thermodynamic properties of end mem- bers and solution models, departure of the analyzed com- positions from equilibrium compositions and analytical uncertainties. The character and magnitude of these uncer- tainties have been discussed by Parra et al. [2001], Vidal et al. [2001], and Trotet et al. [2001]. Following these authors the temperature (sT) and pressure (sP) scatter were calculated using INTERSX software [Berman, 1991] and if sP > 80 MPa or sT > 25C the minerals were considered to be out of equilibrium and the P-T estimates were rejected.

4.1. Local Equilibrium and P-T Paths: Application Example to the Almanzora Metapelites

[

] Several microstructural domains can be differen- tiated in the Almanzora metapelites. The oldest one corresponds with the S

foliation, where chlorite and white K mica coexist with quartz. The mineral assemblage in this domain is formed by WKm + Chl + Qtz + W (Figure 9a). The calculated P-T conditions, in the KMASH system, are constrained by the intersection of 14 equilibria among 8 end members (Al-celadonite, muscovite, pyrophyl- lite, Mg-amesite, clinochlore, sudoite, quartz and water), three of which are independent (Figure 9b). The standard deviation calculated with INTERSX is ±10 MPa and ±3C (Figure 9b). As this scatter is lower than the maximum permissible scatter (±80 MPa and ±25C), the results for this paragenesis (0.86 ± 0.01 GPa and 331 ± 3C) are considered acceptable. This same domain, in the Fil.1 and Fil.4 samples, offers similar P-T results (P-T path in Figure 10).

[

] A younger microstructural domain in the Almanzora metapelites is formed by albite, synkinematic to the main S

foliation, white K mica, chlorite and quartz. The mineral assemblage used in the multiequilibrium calculations was Alb + WKm + Chl + Qtz + W (Figure 9c). The P-T equilibrium conditions were evaluated among ten end

members (water, quartz, Mg-celadonite, muscovite, pyro- phyllite, Mg-amesite, sudoite, clinochlore, albite and para- gonite content in white K mica) in the six-component system NaKMASH. Equilibrium conditions are constrained from the intersection of thirty-two reactions, four of which are independent. An example is shown in Figure 9 where calculations of local equilibrium among the illustrated minerals resulted in P-T conditions of 0.4 ± 0.04 GPa and 424 ± 13C (Figure 9d).

4.2. Results of Calculation and Comparison With the Previous P-T Estimates

4.2.1. Almagro Unit

[

] Multiequilibrium thermobarometry using a Chl + WKm + Qtz + W assemblage analyzed on large prekine- matic chlorite crystals with white-K-mica intergrowths did not offer any reliable P-T results. Consequently, we consid- ered that these mineral pairs were not in equilibrium. The chlorite crystals are probably of detrital origin [De Jong, 1991]. Other clearly detrital minerals are biotite, quartz and opaques. Thus the Almagro unit has undergone very low- grade metamorphism, below the minimum conditions to equilibrate the mica-chlorite pairs. De Jong [1991] reported rare cloritoid within detrital chlorite, suggesting that the Almagro unit had probably reached 400C, however, con- sidering the lack of mineral crystallization defining meta- morphic fabrics in the Almagro unit, the well preserved sedimentary features in this unit, and the low temperatures determined in the metabasites we suggest that cloritoid was also inherited within the chlorite detrital grains.

4.2.2. Almanzora Unit

[

] We analyzed three samples from the Almanzora fine-grained schist. Sample Alm.8 was collected at the top of the section and had mica-chlorite pairs mostly defining the S

foliation. The TWEEQU P-T results from sample Alm.8 are extreme HP/LT conditions ranging between 1.2 GPa/300C and approx. 0.6 GPa/330C (Figure 10).

Samples Fil.1 and Fil.4 were collected at the base of the fine-grained schist section and have mica-chlorite pairs defining both the S

and S

foliations. In general, P-T results from the later samples indicate higher tempera- ture and lower pressure than in sample Alm.8 (between 0.5 GPa/425 – 475C and 0.35 GPa/425 – 475C). A few results coincide with the HP/LT conditions obtained in sample Alm.8 and others define a small cloud between 300 – 350C at approximately 0.2 GPa (Figure 10). In samples Fil.1 and Fil.4 many mica-chlorite pairs of the S

foliation equilibrated under the same P-T conditions of 0.5 – 0.35 GPa and 425– 475C as the ones growing along the S

cleavage.

[

] The P-T results above indicate that the Almanzora

unit reached higher pressure than previously documented

by Bakker et al. [1989] and De Jong [1991]. Maximum

Si content in phengite reported by these authors was

3.27 apfu, while Si content in white K mica of the above

samples reaches up to 3.45 apfu (Figure 11a), higher values

which would explain this pressure discrepancy. The lower

P/T results obtained in this study for the S

development

(0.6 – 0.7 GPa at 320– 360C) coincides with the higher

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pressure results determined from glaucophane and crossite in metabasites of the Almanzora unit [Bakker et al., 1989;

De Jong, 1991]. Bakker et al. [1989] described a heating event in the Almanzora rocks under 0.2 – 0.3 GPa at 500C. The peak temperature results (425– 475C) occur at higher pressure in this study (0.3 – 0.5 GPa). The highest interlayer-cation content in white K mica in the studied samples, consequently, the highest temperature [Agard et al., 2001] was reached by phengite with a Si content of 3.30 to 3.15 apfu (Figure 11a). Thus pressure during the heating event is expected to be higher than estimated by Bakker et al. [1989] who related the heating event with micas that had Si contents of 3.10 to 3.07 apfu. These pressure discrepancies (0.1 – 0.2 GPa) during the heating event could be related with the fact that most of the data of Bakker et al. [1989] are from the metaevaporitic sequence of the Almanzora unit, found higher up in the section than

the rocks studied here. The stability of kyanite-zoisite- phengite-quartz synkinematic to the S

cleavage in some samples of the Almanzora metapelites suggests conditions around 450C at pressures above 0.6 GPa [De Jong, 1991].

These P-T conditions are higher than the ones obtained using TWEEQU thermobarometry in this study and probably correspond to other rocks of the metapelitic sequence that followed a slightly higher thermal gradient, probably deeper in the Almanzora section.

[

] From the above data we suggest that (1) the Alman- zora fine-grained schist underwent a HP/LT metamorphic event during the growth of the S

foliation at approximately 300C and 1.2 to 0.8 GPa with later heating to approxi- mately 360C at 0.6 – 0.7 GPa; (2) the main S

cleavage equilibrated under P-T conditions of 450– 475C and 0.5 – 0.35 GPa at the base of the fine-grained schist, always within the kyanite stability field (Figure 10); (3) only a few Figure 9. TWEEQU results obtained from the two main microstructural domains of the Almanzora

metapelites. (a) Assemblage quartz + white K mica + chlorite defining the S

foliation, which is preserved in lenticular domains of the S

cleavage. (b) P-T equilibrium conditions for the mineral assemblage analyzed in Figure 9a. (c) Albite synkinematic to the S

cleavage with paragenetic chlorite, white K mica and quartz. (d) TWEEQU P-T result from the mineral assemblage analyzed in Figure 9c.

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micas with Si contents above 3.3 apfu were preserved in the two samples at the base of the Almanzora section, probably indicating that micas in the S

foliation were reequilibrated during the later heating; and (4) after reaching peak tem- perature at intermediate pressure the Almanzora rocks were cooled during further decompression (Figure 10).

4.2.3. Variegato Unit

[

] TWEEQU calculations for fine-grained schist of the Variegato unit only define the HP/LT section of the P-T path undergone by these rocks. This P-T segment describes an isothermal decompression between 1.1 and 0.8 GPa at 400C, always in the carpholite stability field [Booth-Rea et al., 2003d]. The phengite Si/interlayer-content diagram

(Si-IC diagram) for these samples offers some additional information of the P-T path’s overall geometry [Agard et al., 2001], although not offering quantitative results (Figure 11b). The geometry of the Si-IC diagram of the Variegato fine-grained schist is symmetrical to the one obtained from the Almanzora schist (Figure 11); while in the Almanzora schist it indicates cooling and decompression during the later mineral growth, in the Variegato unit it indicates heating and decompression during the growth of white K micas, which replace the HP/LT mineral assemblage in quartz veins.

[

] Graphite garnet schist in the Variegato unit under- went a strong decompression from 0.8 GPa at 500C to 0.2 GPa at 500 to 530C during the growth of the S

main foliation [Booth-Rea et al., 2003d], showing a slight heating at 0.2 GPa (Figure 10). The S

cleavage, axial plane to the late north vergent folds, equilibrated under approximately 450C and 0.2 GPa [Booth-Rea et al., 2003d], similar conditions as those determined from graphite schist of the central Betics [Azan˜o´n and Crespo-Blanc, 2000].

5. Large-Scale Structure of the Alpujarride Units Outcropping in the Sierra de Almagro 5.1. North Directed Plastic-Shear Zones and

North Vergent Folds

[

] The contact between the Almanzora fine-grained schist and the Almagro unit crops out in the southern limb of the Sierra de Almagro antiform. It is a carbonate-gypsum- mylonite shear zone that dips 60– 70S (Figures 8a, 8b, and 8d). This shear zone forms a small angle with the S

foliation and the S

cleavage associated with the north vergent overturned folds in the Almanzora unit, dipping stepper than the S

cleavage and less than the S

cleavage (see southeastern border of the Sierra de Almagro geo- logical maps, Figures 3 and 4, and cross sections A-A

and B-B

in Figure 5). If the Neogene folding of the Figure 10. Thermobarometric results and P-T paths

estimated for the Almanzora metapelites and the Variegato fine-grained and graphite schist, modified from Booth-Rea et al. [2003d]. P-T field for the Almagro unit from Sa´nchez- Vizcaino et al. [1991].

Figure 11. Diagrams of Si and interlayer-cation contents in phengite: (a) phengite from the Almanzora unit and (b) phengite from the Variegato unit. Mica compositions located out of the main shaded trends probably have been altered by submicroscopic illitization [De Jong et al., 2001].

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Almagro anticline is undone, the Almanzora unit’s main foliation (S

) would be a flat lying structure and the shear zone would dip toward the south (see cross sections, Figure 5). There is a metamorphic gap across this shear zone, between the underlying LP/LT Almagro unit [Sa´nchez- Vizcaino et al., 1991] and the Almanzora unit above, which underwent HP/LT metamorphism followed by heating up to 475C. This metamorphic temperature gap of at least 175C is accompanied by the occurrence of different tectonic fabrics in the Almagro and Almanzora units, as discussed above. The mylonitization in this shear zone affects gypsum and carbonate rocks (Figures 8b and 8d) and the mylonites include dolomite and metapelite porphyroclasts, which have calcite symmetric pressure shadows (Figure 8d). The

preferred orientation of the porphyroclasts on the mylonitic foliation produces an ill-defined N-S trending lineation that coincides with the orientation of calcite fibers of the porphyroclast pressure shadows. Asymmetric folds in the mylonites indicate mostly northward shear sense in agree- ment with the general northward vergence of post-S

overturned folds in the hanging wall of the shear zone.

5.2. North Directed Low-Angle Normal Faults (LANFs) [

] The Variegato unit is made up of several imbrica- tions, which have been highly extended by brittle north directed LANFs (Figures 12a, 12b, 12d, 12e, 12f, 12g, and 12h). The LANFs are the contacts between the Figure 12. (a) Two extremely thinned Variegato imbrications, with carbonate lenticular bodies bounded

by low-angle normal faults (LANFs) with north shear sense (locality X, Figure 4). (b) North directed LANFs; present dip must be similar to the original one because this outcrop is in the hinge zone of a Neogene fold. (c) Gypsum mylonitic foliation and fibers measured on a SW directed detachment at the base of the Variegato unit. (d) Line drawing of Figure 12a. Notice sequential character of the LANFs.

(e) Carbonate porphyroclast in a north directed brittle shear located in the southern limb of a Neogene fold. (f ) Clay-rich foliated cataclasites associated to a north directed LANF located in the southern limb of a Neogene fold. (g) Normal faults, associated striae and main foliation measured in Figures 12e and 12f. (h) Same data as in Figure 12g rotated around an horizontal E-W axis to undo the Neogene folding. Notice the original low angle of many of these faults (15– 30). (i) Gypsum mylonitic breccia at the base of the Variegato unit with a penetrative mylonitic lineation and s-type porphyroclasts.

( j) Porphyroclast with asymmetric tails showing SW shear sense. (k) Carbonate and gypsum mylonitic lineation. Equal-angle lower-hemisphere stereographic projections.

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imbrications and formations of the Variegato unit. The cataclasis related to these LANFs has produced a penetrative comminution of the Variegato metapelites, forming clay-rich fault gouge, especially in the contacts with the more com- petent carbonate rocks. Commonly the carbonate formations outcrop as lenticular or brittle-boudin like bodies, which represent isolated extensional horses (Figures 12a and 12d). Kinematic indicators associated with the LANFs, including brittle S-C and S-C

structures, and rotated porphyroclast tails show a mostly northward sense of shear (Figures 12e and 12f). This extensional system is formed by several sets of LANFs. The older LANFs with lower dip angles are tilted by the most recent out-of- sequence ones (Figures 12a and 12d). The sequential character of this extension, with multiple sets of LANFs, is a general characteristic of brittle extension in the Betics [Garcı´a-Duen˜as et al., 1992; Martı´nez-Martı´nez and Azan˜o´n, 1997; Martı´nez-Martı´nez et al., 2002;

Booth-Rea et al., 2002b, 2003a, 2004b].

[

] The LANFs were folded by the Neogene Almagro anticlinorium. Consequently, they dip 50 – 65 toward the south in the southern limb of Sierra de Almagro (see geological sections A-A

and B-B

in Figure 5, and Figures 12e, 12f, and 12g), where they have been folded such that they show thrust kinematics, as observed in other areas of the Betics [e.g., Crespo-Blanc et al., 1994; Tubı´a, 1994; Martı´nez-Martı´nez and Azan˜o´n, 1997]. Their exten- sional nature is clear as they dip less than the S

reference surface of the Variegato unit (Figure 12g), which they cut down-section toward the north when the main foliation is restored to a horizontal attitude (Figure 12h). When the Neogene folding is undone, setting the S

cleavage hori- zontal by rotating along an E/W horizontal axis, the north directed LANFs result in low-angle, (20 – 30), north dipping faults (Figures 12g and 12h).

5.3. SW Directed Low-Angle Normal Faults

[

] Most of the Almanzora unit’s evaporitic formation is a gypsum-mylonite and carbonate-cataclasite and breccia fault zone, which we have called the Almagro detachment (cross sections, Figure 5). This fault zone cuts the Alman- zora north vergent overturned folds and the F

Almagro south vergent folds. Locally it forms a large angle with the internal reference surfaces of both these units, complicating the analysis of the geometrical relationships between the fault zone and the internal geometry of the footwall. This detachment is folded by the upper Neogene anticlinorium of Sierra de Almagro (Figure 3 and cross section A-A

, Figure 5). The fault zone is defined by a flat-lying mylonitic foliation with a NE-SW gypsum lineation in the gypsum- rich lithologies (Figures 12c, 12i, 12j, and 12k), and by carbonate foliated cataclasite and breccia when it affects mostly carbonate rocks. The fault zone reaches a thickness of more than 25 m (mapped as ‘‘g,’’ Figure 4) and includes carbonate and metabasite porphyroclasts and lenticular bodies from outcrop to cartographic scale. The porphyro- clasts have gypsum s-type pressure shadows with SW sense of shear (Figure 12j). SW directed listric LANFs that cut both, the Variegato unit and the north directed LANFs,

detach on this fault zone generating high-extension isolated horses, which are bounded by these SW directed faults at the base and by inactive north directed LANFs at the top.

[

] The Almagro detachment is sealed by early Torto- nian red continental conglomerates (Figure 4), which in- clude the first clasts of the Almagro unit entering the Vera basin [Barraga´n, 1997]. Hence, the SW directed LANFs exhumed the Almagro unit during the latest Serravallian- earliest Tortonian coinciding with the exhumation of the Nevado-Filabride complex in the eastern Betics (12 Ma [Johnson et al., 1997]). The Almagro detachment was probably active during the Serravallian as the Mecina or Filabres extensional detachments described in other more western areas of the Betics [Garcı´a-Duen˜as et al., 1992;

Martı´nez-Martı´nez and Azan˜o´n, 1997; Martı´nez-Martı´nez et al., 2002]. The age of the north directed LANFs, which are tilted by SW directed faults related with the Almagro detachment, would then be pre-Serravallian. Unfortunately, we have not found any sedimentary rocks sealing these faults, so their age in the Sierra de Almagro is undeter- mined. In the central Betics the age of the north directed extensional system is constrained as late Burdigalian and Langhian [Garcı´a-Duen˜as et al., 1992; Crespo-Blanc et al., 1994; Martı´nez-Martı´nez and Azan˜o´n, 1997].

6. Discussion and Conclusions

[

] The metamorphic and structural data presented above indicate different P-T conditions and tectonic evolu- tions for the three tectonic units outcropping in the Sierra de Almagro. Differences can be appreciated not only in the degree of metamorphism, but also in the geometry of the P-T paths. These differences in metamorphic evolution are coupled with the different microstructures of the Almagro, Almanzora and Variegato units. Hence, these units under- went different tectonic evolutions prior to their present superposition, being exhumed by different tectonic pro- cesses acting in collisional environments, for example:

corner flow and underplating during active collision in an orogenic wedge (with uncoupling of the overriding plate) [Shreve and Cloos, 1986; Platt, 1986, 1993; Burov et al., 2001], synorogenic extension [Platt, 1986; Jolivet et al., 1996], or back arc postorogenic extension [Dewey, 1988; Platt and Vissers, 1989]. Exhumation produced by extension is a well-documented process in the Betics [e.g., Platt, 1986; Garcı´a-Duen˜as and Martı´nez-Martı´nez, 1988;

Galindo-Zaldı´var et al., 1989; Platt and Vissers, 1989;

Aldaya et al., 1991; Garcı´a-Duen˜as et al., 1992; Crespo- Blanc, 1995; Lonergan and Platt, 1995; Martı´nez-Martı´nez and Azan˜o´n, 1997; Comas et al., 1999; Martı´nez-Martı´nez et al., 2002; Booth-Rea et al., 2002b, 2003a, 2004b].

However, extensional shear zones in the Betics developed at shallow crustal levels, detaching at maximum depths of 15 – 20 km [Martı´nez-Martı´nez et al., 2002, 2004]. Conse- quently, tectonic processes other than extension are neces- sary to explain the exhumation of HP-LT/HT metamorphic rocks in the Betics from depths of 40 to 70 km.

[

] Corner flow, positive buoyancy forcing and material

dragging by the overriding plate are reasonable processes

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for exhuming HP-LT/HT rocks along isothermal P-T paths, by crustal circulation within orogenic wedges, and probably have contributed to the exhumation of HP-LT/HT rocks subducted to depths of 30 – 70 km in the Mediterranean Alpine belts, [Bousquet et al., 1997; Trotet et al., 2001;

Parra et al., 2002]. Buoyancy-driven exhumation within an orogenic wedge is predicted by thermomechanical models of Alpine-type collision belts [e.g., Burov et al., 2001;

Boutelier et al., 2004]. Below, we will discuss which processes contributed to the exhumation of the rocks in the southeastern Betics, proposing a model for their tectonic evolution and analyzing the implications of this model for the general evolution of the Betic orogen, under the light of other recent data published about this area.

6.1. HP/LT Metamorphism and Initial Crustal Thickening

[

] The S

foliation and coeval HP/LT metamorphism in the Almanzora unit must have developed during a crustal collision event. To explain the maintenance of such low temperature (300C) it is necessary to have other low- temperature rocks underplating the Almanzora unit during its HP/LT P-T segment. This evolution could be explained by crustal circulation in an orogenic wedge by corner flow mechanisms, where the Almanzora unit reached depths of approximately 40 km and later was exhumed to midcrustal depths (15 – 20 km). However, it is necessary to find underplated rocks that registered a similar cold high- pressure metamorphic event.

[

] The Variegato unit reached higher temperature dur- ing the HP/LT metamorphic event (400C); these conditions could also be attained in the higher chamber of a crustal collision orogenic wedge [Burov et al., 2001] at depths of approximately 36 km (Figure 13a). The thickening event responsible for HP-LT metamorphism in the Alpujarride complex has been attributed to collision of the Alpujarride below the Malaguide complex during the Cretaceous- Paleogene [Lonergan, 1991; Lonergan and Platt, 1995;

Balanya´ et al., 1997, 1998]. Remnants of this orogenic wedge are found in the northeastern Betics, where several tectonic units from the Malaguide and Alpujarride com- plexes crop out, metamorphosed between diagenetic and greenschist conditions [Lonergan, 1991; Nieto et al., 1994;

Lonergan and Platt, 1995; Booth-Rea et al., 2004b]. This initial orogenic wedge probably had SE seaward vergence, after correcting the NW directed thrust kinematics observed in the Malaguide complex for 200 degrees of paleomagnetic rotation [Lonergan, 1991].

[

] The questions here are what relationships were there between the thickening events in the Almanzora and Vari- egato units? Was the thickening accomplished in the same orogenic wedge? Did it occur at different periods in which thermal gradients had changed, which would explain the different P/T ratios during HP/LT metamorphism in both units? The Almanzora unit followed a unique P-T path that is not typical of Alpujarride units which are commonly characterized by having registered isothermal decompres- sion during the growth of their main foliation (S

), reaching

low P/T ratios in the andalusite stability field, at least in the high-grade Alpujarride rocks [Balanya´ et al., 1993; Monie´

et al., 1994; Garcı´a-Casco and Torres-Roldan, 1996;

Azan˜o´n et al., 1997; Balanya´ et al., 1997; Argles et al., 1999; Azan˜o´n and Crespo-Blanc, 2000]. Bakker et al.

[1989] noticed strong stratigraphical similarities between the Almanzora and the Nevado-Filabride Permo-Triassic series. Initial HP/LT conditions estimated for the Be´dar- Macael unit, the structurally highest Nevado-Filabride unit, located below the Almanzora unit, are also in the same range of 300C and 1.0 – 1.1 GPa [Bakker et al., 1989]. This is also the case of the underlying Ragua unit that enjoyed a HP/LT metamorphic event under 1.2 – 1.4 GPa at 300– 400C [Booth-Rea et al., 2003c]. The Be´dar- Macael unit underwent a prograde P-T path to approxi- mately 500C and 1.0 GPa [Bakker et al., 1989]. This heating is of a similar magnitude to the one observed in the Almanzora unit at lower pressure (425– 475C and 0.5 GPa). The Be´dar-Macael unit was probably seated at deeper crustal depths during heating to attain thermal equilibrium of the overthickened wedge (Figure 13b).

We propose that the Almanzora unit subducted in the same tectonic scenario as the Nevado-Filabride complex, as previously suggested by Bakker et al. [1989].

[

] Where sections of the highest Alpujarride unit are best preserved (in the western Betics), they include gran- ulites at their base, and locally subcontinental peridotites.

Hence, the highest Alpujarride unit, with the Malaguide complex above, represents a continuous lithospheric section [Tubı´a et al., 1993], including rocks from diagenetic and anchizone conditions at the top [Nieto et al., 1994;

Lonergan and Platt, 1995], in the Malaguide units, to granulite and subcontinental peridotites at the base [Balanya´ et al., 1997; Argles et al., 1999; Lenoir et al., 2001]. This lithospheric section remained at peak meta- morphic temperature during the lower Miocene, until 19 Ma [Platt and Whitehouse, 1999; Platt et al., 2003a; Whitehouse and Platt, 2003]. Consequently, the Nevado-Filabride complex, which mostly underwent temperatures below 600C [Martı´nez-Martı´nez, 1986a; Go´mez-Pugnaire and Ferna´ndez-Soler, 1987; Bakker et al., 1989; De Jong, 1992;

Soto, 1993; Puga et al., 2000; Booth-Rea et al., 2003c, 2004c] and locally probably 650 – 700C [Muentener et al., 1997; Lo´pez Sa´nchez-Vizcaino et al., 2001; Puga et al., 2002] could not have been structurally below the higher temperature Alpujarride complex before 21 to 19 Ma. This implies that if the higher Alpujarride units constituted the crustal overburden of the Nevado-Filabride complex, HP/LT metamorphism in the later would be of upper Burdigalian or a younger age. A 15 Ma age was recently proposed [Lo´pez Sa´nchez-Vizcaino et al., 2001]. However, this middle Miocene age conflicts with the Cretaceous to Paleogene geochronological ages (91 – 53 Ma) determined by other authors in the Nevado-Filabride complex [Portugal Ferreira et al., 1988; Andriessen et al., 1991; De Jong et al., 1992;

Nieto et al., 1997]. It also conflicts with other sound geological evidence that indicates extensional attenuation of the Nevado-Filabride complex, coeval with the develop- ment of its latest ductile fabrics at 16 Ma [Monie´ et al., 1991;

TC2009 BOOTH-REA ET AL.: EXHUMATION OF ALBORAN HP/LT ROCKS TC2009

Figure 13. Schematic evolution of the Alboran orogenic wedge (see discussion in the text). X axis not to scale and with variable orientation; with SE-NW to SW-NE orientation in Figure 13a [Lonergan, 1991; Faccenna et al., 2004] and approximately WNW-ESE orientation in Figure 13c [Platt et al., 2003b].

TC2009 BOOTH-REA ET AL.: EXHUMATION OF ALBORAN HP/LT ROCKS TC2009

Gonza´lez-Casado et al., 1995; Martı´nez-Martı´nez et al., 2004]. Furthermore, the dated zircons grew surrounding the eclogite assemblage [Lo´pez Sa´nchez-Vizcaino et al., 2001]; consequently, they could have formed during a later heating event. The maximum cation interlayer content in white K micas from metapelite samples analyzed in all the Nevado-Filabride units, and consequently the higher tem- peratures [Agard et al., 2001], occur in micas with low Si content (Si = 3.2 – 3.1 apfu) formed at pressures around 0.3 – 0.4 GPa [Booth-Rea et al., 2004c]. This LP-HT peak, also determined from oligoclase rims around albite in the Be´dar-Macael unit [Bakker et al., 1989], supports the possibility that zircons grew in a late stage after strong decompression in the Nevado-Filabride complex, probably favored by garnet breakdown like in garnet granulites of the Alpujarride complex [Whitehouse and Platt, 2003]. A Tertiary age for the HP/LT metamorphism is possible, because the older geochronological ages in the Nevado- Filabride rocks, based on Ar isotopes, may have been strongly affected by excess atmospheric-Ar incorporation related with 15 to 14 Ma submicroscopic illitization and fluid ingress [De Jong et al., 2001]. However, with the existing data we consider the age of the HP-LT peak in the Nevado-Filabride complex is older than 15 Ma and still remains unconstrained.

[

] Summing up, if the HP/LT metamorphism in the Nevado-Filabride is older than 19 Ma then the Alpujarride and Nevado-Filabride units subducted in different verticals, probably in different stages of the orogenic wedge evolution (Figures 13a and 13b). The crustal overburden producing the Nevado-Filabride HP/LT metamorphism would have to be found in other rocks that have not followed the typical Alpujarride tectonic evolution, for example the Almanzora and Almagro units, lower pressure sections of the Nevado- Filabride complex or/and other Alpujarride units that did not undergo the early Miocene high temperature.

[

] There is no evidence of HP/LT metamorphism or differentiated metamorphic fabrics in the Almagro unit.

Consequently, this unit must have occupied a shallow crustal position during the HP/LT metamorphism that char- acterizes the overlying units and the Nevado-Filabride complex (Figure 13b).

6.2. Tectonic Exhumation Processes During Intermediate-to Low-P/T Gradient Metamorphism

[

] The S

crenulation cleavage grew under higher temperature than the S

foliation in the Almanzora unit.

Tectonic quiescence after the S

formation would have permitted thermal equilibration of these rocks explaining part of the heating (475C at 0.5 GPa). However, the P-T conditions reached during the S

development, after initial isothermal decompression (475C at 0.3 GPa C), indicate an evolution toward a high geothermal gradient (36C/km).

This high gradient is also observed in the Nevado-Filabride complex [Bakker et al., 1989; Gonza´lez-Casado et al., 1995] and could be indicative of an extensional context, during which the Almanzora and Be´dar-Macael units under- went ductile flattening. Further decompression during the development of the S

fabric was accompanied by cooling.

Cooling during decompression has been related to fluid circulation during extensional shearing [e.g., Ruppel et al., 1988; Morris and Anderson, 1998; Trotet et al., 2001].

This cooling could also represent a period of slower extensional exhumation that would have permitted the relaxation of the high thermal gradient reached after the initial decompression.

[

] The main foliation in the Alpujarride rocks is a flat- lying fabric related with strong vertical isograde condensa- tion developed during coaxial ductile crustal extension [Balanya´ et al., 1993; Garcı´a-Casco and Torres-Rolda´n, 1996; Azan˜o´n et al., 1997; Balanya´ et al., 1997; Argles et al., 1999; Azan˜o´n and Crespo-Blanc, 2000]. In the higher- grade rocks, the Alpujarride main foliation completely obliterates previous planar fabrics and is cut by C

shear planes. Thermal gradients reached after development of the S

foliation are typical of greatly extended terrains, like the substrate below the Alboran Sea [Zeck et al., 1992; Monie et al., 1994; Sosson et al., 1998; Argles et al., 1999; Comas et al., 1999; Platt and Whitehouse, 1999; Booth-Rea, 2004].

Final postorogenic character of this extension would explain tholeitic volcanism in the Malaguide complex [Torre´s- Rolda´n et al., 1986; Turner et al., 1999] and heating in a very late stage of the metamorphic evolution (Figure 13b), as observed in the phengites of the Variegato fine-grained schists or in other Alpujarride units [Argles et al., 1999;

Platt et al., 1998; Platt and Whitehouse, 1999; Platt et al., 2003a].

6.3. Late to Postmetamorphic Underthrusting of LP//LT Rocks

[

] If the Alpujarride isothermal decompression repre- sents an extensional event after the main HP/LT thickening as proposed by most authors [Balanya´ et al., 1993; Garcı´a- Casco and Torres-Roldan, 1996; Azan˜o´n et al., 1997;

Balanya´ et al., 1997; Argles et al., 1999; Platt and Whitehouse, 1999; Azan˜o´n and Crespo-Blanc, 2000], related with the formation of the Alboran basin according to the following authors [Argles et al., 1999; Comas et al., 1999;

Platt and Whitehouse, 1999; Platt et al., 2003a], why are higher-grade metamorphic units found above lower-grade ones at the scale of the whole Betic orogen? Because, although this study focuses on the structure of the Sierra de Almagro, representative of the eastern Betics, the same situation is also found in the central and western Betics where this has been intensively discussed [Tubı´a et al., 1992; Simancas and Campos., 1993; Azan˜o´n et al., 1994;

Avigad et al., 1997; Azan˜o´n et al., 1997; Balanya´ et al., 1997; Tubı´a et al., 1997; Azan˜o´n et al., 1998; Balanya´ et al., 1998; Platt, 1998; Azan˜o´n and Crespo-Blanc, 2000].

[

] We defend the following hypothesis: after collision

of the Alpujarride complex below the Malaguide complex

(Figure 13a), and initial HP/LT metamorphism, instabilities

in the thickened wedge caused orogenic collapse of the

Alpujarride-Malaguide orogen, active extension, which

probably coincided with sublithospheric thermal erosion,

as evidenced in the Ronda peridotites [Lenoir et al., 2001],

and with Oligocene to lower Miocene volcanism in the

Malaguide Complex [Turner et al., 1999] (Figure 13b). The

TC2009 BOOTH-REA ET AL.: EXHUMATION OF ALBORAN HP/LT ROCKS TC2009