Patterns and average rates of late Neogene Recent uplift of the Betic Cordillera, SE Spain

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1 Geomorphology 50 (2003) Patterns and average rates of late Neogene Recent uplift of the Betic Cordillera, SE Spain Juan C. Braga a, *, José M. Martín a, Cecilio Quesada b a Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Campus de Fuentenueva s.n Granada, Spain b IGME/Dirección de Geología, Ríos Rosas 23, Madrid, Spain Received 1 September 2000; received in revised form 1 May 2001; accepted 15 July 2002 Abstract The facies distribution in the sedimentary units infilling a series of Neogene basins has been used to reconstruct the relief generation and uplift across the Internal Zone of the Betic Cordillera in southern Spain. Uplift amounts and average rates can be estimated using the current elevation of the outcrops of well-dated deposits indicative of ancient sea-level positions. Coral reefs and coastal conglomerates record the initial development of emergent Betic relief during the Langhian. Continental and marginal marine deposits indicate the existence of a large island centred on the present Sierra Nevada Sierra de los Filabres chain by the end of the Middle Miocene. The precursor of the Sierra Nevada Sierra de los Filabres chain, originally part of this large island, remained emerged whilst the surrounding areas were re-invaded by the sea during the early Tortonian. At the end of the Tortonian the inland basins (Granada and Guadix basins) became continental, while the Sierras de la Contraviesa, Sierra de Gádor and Sierra Alhamilla emerged, separating the Alborán Basin from the Alpujarra, Tabernas and Sorbas basins, which became narrow passages of the Mediterranean Sea. In contrast, the Sierra Cabrera emerged during the late Messinian, suggesting a progressive uplift from west to east of the sierras south of the Sierra Nevada Sierra de los Filabres chain. During the Pliocene, only the low areas closest to the present-day coast remained as marine basins and progressively emerged throughout this stage. The highest average uplift rate recorded is 280 m/ma for the Sierra de Gádor, although the average uplift rates of upper-neogene coastal marine rocks since depositon have maximum values of approximately 200 m/ma. Most of the uplift of the Betic mountains took place before the early Pliocene. The recorded uplift of Neogene rocks was highest at the margins of western Sierra Nevada, where peaks higher than 3000 m occur. The average rates of uplift were lower to the east of this major relief. The main sierras and depressions in the present-day landscape correspond respectively to the emergent land, in which uplift was concentrated, and to the marine basins that existed before the final emergence of the region. The altitude of the sierras reflects the time at which they became emergent, the highest mountains being the first to rise above sea level. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Uplift; Late Neogene; Palaeogeography; Betic Cordillera; SE Spain 1. Introduction * Corresponding author. Fax: addresses: jbraga@ugr.es (J.C. Braga), jmmartin@ugr.es (J.M. Martín), c.quesada@igme.es (C. Quesada). This is a study of the long-term landscape development of the Internal Zone of the Betic Cordillera in X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S X(02)

2 4 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 southern Spain (Fig. 1) as recorded by the sediments which have infilled the Neogene basins that formed and evolved as the Betic mountains were emerging and rising. The modern topographic configuration of southern Spain consists of a series of mountain ranges, with peaks higher than 3000 m in the Sierra Nevada, separated by depressions. The late-neogene uplift history and sedimentary evolution of the region largely pre-determined the present-day landscape, since the major extant ranges (sierras) and depressions are the direct counterparts of the earlier emergent basement highs and the intervening marine basins respectively. The general emergence of the region resulted in a change from marine to continental deposition in the basins and, during the Quaternary, continued uplift caused a switch to the net erosional conditions prevailing in the modern landscape (Harvey, 2001). Within this paper, we describe the palaeogeographic evolution of the area during the late Miocene and Pliocene as deduced from the spatial distribution of coastal marine deposits from successive time slices. We also quantify the amounts and average rates of uplift since their deposition for rocks formed in coastal environments at the basin margins. Previous attempts to quantify relief generation in the Betic mountains are either localised to a specific area (e.g. Weijermars et al., 1985) or focus only on the Plio/Quaternary evolution of the region (Mather, 1991; Viseras, 1991; Stokes, 1997; Garcia, 2001). Several other papers have addressed the timing and amount of the exhumation of the metamorphic com- Fig. 1. Geological schematic map of southeastern Spain. Unless specified, the basins are named after the main town in them.

3 J.C. Braga et al. / Geomorphology 50 (2003) plexes in the Internal Zone of the Betic Cordillera (i.e. Zeck et al., 1992; Johnson et al., 1997; Lonergan and Johnson, 1998; Platt and Whitehouse, 1999). These papers focus mainly on the tectonometamorphic evolution of the basement rocks following their exhumation during the Early and Middle Miocene. In contrast, we concentrate on the growth of the Betic mountains after the emplacement of the Betic metamorphic complexes to shallow crustal levels. The study area is limited to the central-eastern portion of the Betics, roughly spanning the provinces of Almería and Granada (Fig. 1), in which the stratigraphy, facies distribution and age of the deposits in the Neogene basins are well constrained. The late Neogene uplift history of the Cabo de Gata volcanic province, along the major Carboneras strike-slip fault system, is treated separately in another paper (Martín et al., 2003, although some references to Cabo de Gata and the External Zone are made below. 2. Regional setting 2.1. Basement geology The Betic Cordillera in southern Spain is the westernmost segment of the European Alpine belt. This cordillera has traditionally been subdivided into an External Zone and an Internal Zone (Fig. 1). The External Zone represents the Mesozoic to Middle Miocene southern continental margin of the Iberian Massif, which was divided by rifting into different domains (García-Hernández et al., 1980; Vera 1988). The Prebetic constitutes the external domain where continental and shallow-marine sedimentation prevailed from the Triassic to the Middle Miocene, while the Subbetic, to the south, became a pelagic basin during the Early Jurassic. The Internal Zone consists of three stacked complexes that, in ascending order, are the Nevado Filábride, Alpujárride and Maláguide (Fig. 1). The Nevado Filábride Complex comprises Palaeozoic or older (Gómez-Pugnaire et al., 2000) metamorphic rocks. The Alpujárride tectonic units include a series of Palaeozoic Mesozoic metasediments (Delgado et al., 1981; Martín and Braga, 1987; Tubía et al., 1992). The Maláguide complex consists of a non-metamorphic Mesozoic to Cenozoic cover overlying a pre- Permian basement (Lonergan, 1993; Martín-Martín, 1996). The upper Nevado Filábride tectonic unit (Mulhacén Nappe, Puga, 1976) and the Alpujárride complex were affected by high-pressure metamorphism due to crustal thickening as a result of the convergence of the African and Eurasian plates. The radiometric dates constraining the high-pressure metamorphism in the Nevado Filábride Complex indicate that convergence began at about 51 Ma (Monié et al., 1991), although earlier dates have also been suggested (De Jong, 1991). A sharp decompression in the metamorphic P T path of the Nevado Filábride and Alpujárride complexes (Vissers, 1981; Gómez-Pugnaire and Fernández-Soler, 1987; Bakker et al., 1989; García-Casco and Torres-Roldán, 1996) suggests rock exhumation due to crustal-scale extension during the Early and Middle Miocene (Monié et al., 1991; García-Dueñas et al., 1992; Watts et al., 1993; Comas et al., 1999; Platt and Whitehouse, 1999). Thinning of the previously thickened crust took place (Platt and Vissers, 1989) as a result of the extension. The Maláguide complex did not undergo Alpine metamorphism and, consequently, was probably never subducted (De Jong, 1991). The present contact with the underlying Alpujárride Complex is marked by a thick mylonitic zone cut by normal faults (Aldaya et al., 1991) Neogene basins The Betic Neogene basins developed on both the Internal and the External Zones and underwent deformation and were uplifted as they filled with sediments (Sanz de Galdeano and Vera, 1992). Consequently, the configuration, limits and sedimentary dynamics of each basin changed considerably over time, reflecting both the regional and local tectonic evolution. A major basin, the Guadalquivir Basin, developed at the cordillera front and was open to the Atlantic Ocean. The intermontane basins located on the External Zones are either continental (Prebetic basins) or marginal embayments of the Guadalquivir Basin and as such related to the Atlantic Ocean. The rest of the intermontane basins of the cordillera were connected to the Mediterranean Sea. Two main types of Mediterranean-linked basins can be distinguished. (a) Inner basins (the most distant from the present-day Mediterranean Sea) occur mainly at the contact between the

4 6 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 External and Internal Zones. Although both the Guadix-Baza and Granada basins belong to this type, we have chosen the Granada Basin as the most representative example (Fig. 2). (b) Outer basins (the nearest to the present-day Mediterranean), such as the Vera, Sorbas, Tabernas, and Almería Níjar, are located in the Internal Zone. The Sorbas Basin has been selected as the representative example (Fig. 3). The two types of Mediterranean-linked basins have a similar sedimentary evolution up to the late Tortonian. At the latest Tortonian early Messinian, the inner basins were isolated from the Mediterranean Sea and became continental (Vera, 2000). The outer basins remained connected to the Mediterranean Sea during the rest of the Miocene and, in some cases even during the Pliocene, except for a short time-interval in the Messinian during the so-called Messinian Salinity Crisis (Riding et al., 1998). From the Middle-Miocene to the early Tortonian, the Mediterranean-linked basins evolved under crustal-scale extension, as recorded by normal faults that are well dated in the Alborán Basin (Comas et al., 1992, 1999), while contractive deformation related to wrench tectonics has prevailed since the late Tortonian (Comas et al., 1999). The interaction of strike-slip fault systems determines a complex pattern of transtensive and transpressive local conditions (Keller et al., 1995). Fig. 2. Neogene stratigraphy of the Granada Basin at its eastern margin. This is representative of the Mediterranean-linked inner basins (modified from Braga et al., 1990).

5 J.C. Braga et al. / Geomorphology 50 (2003) Fig. 3. Neogene stratigraphy of the Sorbas Basin. This is representative of the Mediterranean-linked outer basins (modified from Martín and Braga, 1994) Topography A series of mountain ranges, trending roughly E W and separated by basins, determine the topography of the study area. Sierra Nevada (Fig. 1) is the highest mountain range in southern Spain with several peaks over 3000 m (3482 m in the Mulhacén). This, together with the Sierra de los Filabres (2168 m) to the east, is the main outcrop of the lowest Betic nappes. North of Sierra Nevada, the Granada and Guadix basins are separated by the Sierra Arana (1943 m) at the contact between the External and Internal Zones of the cordillera (Fig. 1). Several ranges of Mesozoic rocks of the External Zone extend northwards to the Guadalquivir Basin at the cordillera front. The Alpujarra Corridor, a very narrow, E W trending Neogene basin, separates Sierra Nevada from the Sierra de Lújar Sierra de la Contraviesa Sierra de Gádor chain to the south. The Sierra de Lújar (1824 m) and the Sierra de la Contraviesa (1508 m) con-

6 8 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 stitute a coastal range while the topography of the Poniente Basin descends gradually from the southern foot of Sierra de Gádor (2236 m) to the Mediterranean (Fig. 1). To the east of the Andarax valley, the Tabernas and Sorbas basins occupy the depression between the Sierra de los Filabres and the Sierra Alhamilla (1387 m) Sierra Cabrera (960 m). The Almería Níjar Basin descends from the southern slope of the latter chain to the Mediterranean Sea delimited at its southeastern margin by the volcanic Sierra de Cabo de Gata (Fig. 1). 3. Methods In this paper, we describe and quantify the regional scale, spatial and temporal pattern of relief generation of the central sector of the Betic mountains during the late Neogene. The palaeogeographic evolution was deduced from the distribution of coastal deposits in the successive upper Neogene sedimentary units filling the basins in the study area. Uplift rates were calculated by using the time-averaged elevation of upper Miocene and Pliocene coastal and marginal marine rocks above modern sea level. These rocks formed at the basin margins and were elevated above sea level as the basement highs and the region in general were uplifted. Two types of rocks have been used to estimate uplift amounts and rates: (a) Coral-reef and beach deposits that allow the (palaeo)position of sea level at the outcrop localities to be reconstructed with a margin of error of only a few metres (less than 10 m). (b) Rocks of shallow-water marine origin deposited on inner platforms in depths of less than 30 m, mostly inner-platform limestones. The identification of the palaeoposition of sea level with these deposits which is affected by a methodological error of F 30 m. The age of these shallow-marine sediments is usually well constrained with precise biostratigraphic or stable-isotope data in time intervals of a few hundred thousand years (100 ka). The elevation of specific shoreline-marker rocks of each time slice analysed gives the amount of rock uplift from sea level since their deposition at each locality. The current elevation, however, has been corrected with available data on global sea-level position (Hardenbol et al., 1998) at the time of formation of the studied rocks (Fig. 4). Average uplift rates were obtained by dividing the corrected elevation by the estimated absolute age of the rocks. Methodological errors arise both from uncertainties in the identification of the ancient shorelines and from the time ranges of available age constraints. The pattern for the amount of uplift of the coastal marine rocks of any studied time slice is indicative of the differential uplift within the region since the deposition of these rocks. The minimum average uplift rate of the highest peaks can be estimated for the Sierra de Gádor, Sierra Alhamilla and Sierra Cabrera as the time of the emergence of these sierras above sea level can be constrained with the stratigraphic record in the nearby basins. The current elevation of the peaks is the result of the interaction between the uplift of the sierras and accompanying erosion. The time-aver- Fig. 4. The present-day elevation of shoreline-marker rocks of each time slice analysed (E) has been corrected with available data on global sealevel position (Hardenbol et al., 1998) at the time of formation of the studied rocks (S) to obtain the rock uplift since deposition (U). S can be negative. Average uplift rates (AUR) can be obtained by dividing the corrected elevation (U) by the estimated absolute age of the rocks (T).

7 J.C. Braga et al. / Geomorphology 50 (2003) aged current elevation of the highest peaks is therefore the minimum average uplift rate since their emergence. 4. Neogene sedimentary record and palaeogeographic evolution of the area The Neogene basins in southern Spain are filled by sedimentary units separated by unconformities. The facies distribution in coeval units from different basins is used here to reconstruct the palaeogeography of the study area in successive time slices. The outcrops of the pre-upper Tortonian units are generally small and disconnected, making an accurate reconstruction of the paleogeography before the late Tortonian difficult. The stratigraphic record and data quality is substantially better for younger deposits, and therefore, the descriptions below focus on the palaeogeography of the area during the deposition of the upper Tortonian, uppermost Tortonian lowermost Messinian, lower Messinian, upper Messinian and lower Pliocene sedimentary units Pre-Tortonian evolution The progressive unroofing and erosion of Betic nappes is recorded by the lithological variety of clasts incorporated in various types of mass-flow deposits within lower Miocene pelagic sediments (Rodríguez- Fernández, 1982). However, the first evidence for emerging Betic basement highs is the occurrence of upper Langhian coral reefs and associated coastal conglomerates in the southern Granada Basin (Braga et al., 1996a) and in the Vera Basin (Barragán, 1997). In the Granada Basin these deposits appear in a single isolated outcrop near Murchas (Fig. 1), indicating the presence to the north of an emerged Betic island, probably the precursor of the Sierra Nevada La Tórtola chain (Fig. 1). The nature of the clasts in the conglomerates suggests that only the uppermost Betic complex in the area (the Alpujárride Complex) was exposed and eroded. The occurrence of Langhian coral reefs in deposits from the Vera Basin points to the existence of an emerged relief in the eastern part of the present-day Sierra de los Filabres as well. Most clasts in the associated conglomerates here come from the uppermost basement complex, the Maláguide Complex, but Alpujárride rocks were eroded as well (Barragán, 1997). To the north, Langhian shallow-water marine limestones were deposited on the platform that developed on the southern margin of the Iberian Massif, corresponding to the Prebetic, i.e., the northern domain of the Betic External Zone. These shallow-water carbonates change southwards to slope and basinal marls and mass-flow deposits (Comas, 1978; Geel et al., 1992), thereby suggesting a complete disconnection between the Betic island(s) and the Iberian mainland by a deep-water trough ( North Betic Straits, Geel et al., 1992). In the southern Granada Basin, the upper-langhian shallow-marine deposits are overlain by a continental unit of red conglomerates, sandstones and silts (Fig. 2) that can be traced around Sierra Nevada (Rodríguez- Fernández, 1982). Flanking the Sierra de los Filabres, red-to-grey conglomerates overlying marine upper? Langhian marls, older Neogene units or the basement has been interpreted as alluvial and fan-delta deposits (Kleverlaan, 1989; Doyle et al., 1996) (Fig. 3). The red conglomerates extend southwards to the Sierras de la Contraviesa and Gádor (Rodríguez-Fernández et al., 1990), Alhamilla and Cabrera (Montenat, 1990; Barragán, 1997), and northwards to the Sierra de las Estancias (Braga and Martín, 1988) and Sierra de Baza (Soria, 1993), underlying later marine deposits in the basins. These continental deposits indicate the existence of a large emerged Betic upland (Fig. 5). In the Granada Basin, micromammal fossils in the red silts point to a Serravallian age (Martín-Suárez et al., 1993), but in general the age of the red conglomeratic units is poorly constrained and a lowermost Tortonian age, suggested by authors such as Montenat (1990) and Doyle et al. (1996) at least for parts of the units, cannot be discarded, even though in most localities they are overlain by lower Tortonian marine sediments. To the south of Sierra Alhamilla and Sierra Cabrera, the Serravallian and lower-tortonian deposits are pelagic marls (Serrano, 1990). Together with the occurrence of deep-water Serravallian deposits in the southern External Zone (Subbetic domain, Soria, 1993), this suggests that the red units formed on an island separated from the Iberian mainland (Fig. 5). The existence of such a large emergent Betic upland by the end of the Serravallian earliest Torto-

8 10 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 Fig. 5. Serravallian lowermost Tortonian? palaeogeography in southern Spain. Significant outcrops delineate the minimum extension of the emergent land. nian is in agreement with the timing of cooling to near-surface temperatures of the rocks of the Nevado Filábride Complex, as recorded by fission tracks in zircon and apatite (Johnson et al., 1997). The age of this cooling is about 12 Ma in the Sierra de los Filabres, 11.7 Ma in eastern Sierra Nevada and about 9 Ma in western Sierra Nevada (Johnson et al., 1997). This variation in cooling age from east to west suggests that the unroofing of the Nevado Filábride Complex progressed diachronously from east to west (Johnson et al., 1997). This hypothesis is supported by the provenance of clasts in the Serravallian conglomerates: whereas in the Sierra de los Filabres the Nevado Filábride Complex was already unroofed and contributing clasts (Braga and Martín, 1988), in the western Sierra Nevada only the Alpujárride Complex was exposed and eroded (Rodríguez-Fernández et al., 1990) Early Tortonian The continental/fan-delta Serravallian lowermost- Tortonian red siliciclastic deposits are overlain by lower Tortonian, shallow-marine mixed bioclastic and siliciclastic deposits (Rodríguez-Fernández, 1982; Braga et al., 1990; Rivas et al., 1999) (Figs. 2 and 3). Except for the Granada Basin, the outcrops of these bioclastic rocks are disconnected, making palaeoenvironmental interpretation and dating difficult. Consequently, the palaeogeography of the study area

9 J.C. Braga et al. / Geomorphology 50 (2003) cannot be reconstructed for the early Tortonian with the same confidence as for later intervals. Lower Tortonian carbonate platforms rim the La Tórtola Sierra Nevada Sierra de los Filabres chain (Rodríguez-Fernández, 1982), suggesting that this chain remained emergent during the early Tortonian. Terrigenous material shed from upland fed localised fan deltas around Sierra Nevada. The clasts in these fan deltas indicate that the Alpujárride Complex was still the only basament complex at the surface in the western Sierra Nevada (Rodríguez-Fernández, 1982; Martín and Braga, 1997) (Fig. 6). Lower Tortonian shallow-marine deposits occur south of the La Tórtola Sierra Nevada Sierra de los Filabres chain in scattered outcrops on the southern margins of the Tabernas, Sorbas and Vera basins. The Alpujárride provenance of the clasts suggests they derived from locally emergent reliefs at the location of the modern Sierra Alhamilla and Sierra Cabrera. Coastal volcaniclastic and bioclastic deposits point to the existence of emergent volcanic highs in the Cabo de Gata volcanic province at this time (Braga et al., 1996b; Betzler et al., 1997). North of the La Tórtola Sierra Nevada Sierra de los Filabres chain, shallow-water platforms with carbonate sedimentation extended over both Internal and External Zone substrates. Several Subbetic upland areas were already emerged (Rodríguez-Fernández, Fig. 6. Erosion pulses of western Sierra Nevada and stratigraphic architecture of the corresponding conglomerate units (modified from Martín and Braga, 1997).

10 12 J.C. Braga et al. / Geomorphology 50 (2003) ) and islands rimmed by shallow-water marine platforms replaced the former deep-water trough in the North Betic Straits (Soria, 1993). However, the lack of accurate dating and sedimentological analysis of the shallow-water deposits makes reconstructing the precise palaeogeography of the External Zone at this time a difficult task. The Iberian mainland expanded southwards by the partial emersion of the Prebetic domain, on which shallow-marine sedimentation is restricted to the southernmost areas Late Tortonian Upper Tortonian deposits in the Neogene basins in the study area comprise proximal deltaic siliciclastic sediments and carbonates that include coral reefs. These proximal deposits pass laterally to distal marls, silty marls and turbidite sandstones and conglomerates (Martín et al., 1989; Braga et al., 1990; Martín and Braga, 1994) (Figs. 2 and 3). The distribution of these upper Tortonian deposits indicates that a narrow platform with coral reefs and localised fan deltas rimmed the southern margin of the Sierra Nevada Sierra de los Filabres chain (Martín and Braga, 1996), but to the south of this main chain deep-water marine basins developed in areas that had been emergent during the early Tortonian and at the end of the Middle Miocene (Fig. 7). Upper Tortonian marls and turbidites were deposited on top of the continental conglomeratic and shallow-marine bioclastic units in the Alpujarra, Tabernas, Sorbas and Vera basins (Rodríguez-Fernández et al., 1990; Weijermars et al., 1985; Kleverlaan, 1989), which were laterally connected at that time. Pelagic sediments encroach the sides of present-day sierras south of these basins, such as the Contraviesa, Gádor, Alhamilla and Cabrera. These sierras have no shallow-water deposits around them, indicating that they were not emergent (Fig. 7). Nevertheless, current direction and sedimentary body geometries suggest the existence of submarine swells at the modern location of the sierras (Haughton, 1994, 2000). Deep-water marine sedimentation extended southwards to the Almería Níjar (Serrano, 1990) and Alborán basins (Comas et al., 1996). The Nevado Filábride Complex was unroofed and subjected to erosion, providing clasts incorporated in fan-delta conglomerates deposited around the Sierra Nevada for the first time, especially at its western end (Martín and Braga, 1997; Fig. 6). Coral reefs developed on these fan deltas and on the shelves rimming the emergent uplands, thus allowing the upper Tortonian palaeogeography to be accurately traced (Esteban et al., 1996). La Tórtola remained as an island (Braga et al., 1990) and the main relief was the Sierra Nevada Sierra de los Filabres chain, which merged with the extensively emergent External Zone to the north (Rodríguez-Fernández, 1982; Soria, 1993; Soria et al., 1999). The northern coasts of the marine Granada and Guadix basins were rimmed by coral reefs as well (Fig. 7). By the end of the late Tortonian, most of the Subbetic and Prebetic areas formed a continuous mainland with the Iberian Massif (Soria et al., 1999; Fig. 7). The Guadalquivir basin was still open to the Atlantic and the Guadix basin was connected to the east with the main Mediterranean by the Almanzora corridor. Some fan deltas developed in this corridor, probably reflecting a pulse in the uplift of the Sierra de los Filabres. The northern coast of the reeffringed Almanzora corridor (Martín et al., 1989) was formed by an Internal Zone high (Sierra de las Estancias), probably continuous with the above-mentioned emergent External Zone Latest Tortonian earliest Messinian Uppermost Tortonian lowermost Messinian rocks in the Granada and Guadix basins consist of fluviatile conglomerates, sandstones and silts with lacustrine clays and evaporites (Dabrio et al., 1982; Martín et al., 1984; Soria et al., 1999; García-Aguilar and Martín, 2000; Fig. 2). In the basins south of the Sierra Nevada Sierra de los Filabres, proximal deposits of this age comprise bioclastic carbonates with various proportions of siliciclastic grains (Fig. 3) and local fan-delta conglomerates (Rodríguez-Fernández et al. 1990; Martín and Braga, 1994), except for the Tabernas Basin in which only siliciclastic fan deltas occur (Kleverlaan, 1989). All these shallow-water materials pass laterally to distal marls and turbidite conglomerates and sandstones (Kleverlaan, 1989; Haughton, 1994; Braga et al., 2001). The geographical distribution of the deposits from this time interval indicates that at the end of the Tortonian, the Granada, Guadix and Almanzora basins were uplifted and became continental basins with fluviatile and lacustrine sedimentation. The Sierra de la Contraviesa, Sierra de

11 J.C. Braga et al. / Geomorphology 50 (2003) Fig. 7. (A) Upper Tortonian palaeogeography of southern Spain. (B) Enlarged schematic map of the study area showing the outcrops of reefs and other coastal deposits of this age (modified from Esteban et al., 1996). Gádor and Sierra Alhamilla were also uplifted and emerged, restricting the marine basins to the south of the Sierra Nevada Sierra de los Filabres as a narrow E W corridor open to the Alborán Basin through the Andarax Corridor and to the Mediterranean through the Vera and Almería Níjar basins (Fig. 8A and B). Re-folding of the Betic basement units by crustal shortening and isostatic uplift has been suggested as the most likely mechanism to explain the emergence of Sierra Alhamilla at the end of the Tortonian

12 14 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 Fig. 8. (A) Uppermost Tortonian lowermost Messinian palaeogeography of southern Spain. (B) Enlarged schematic map of the study area. (C) Lower Messinian palaeogeography of SE Spain. (D) Upper Messinian palaeogeography of SE Spain. Note the progressive emergence of the sierras and surrounding areas in the region from west to east during the Messinian (B C). (Weijermars et al., 1985). In the Cabo de Gata area several volcanic highs, including recently formed volcanic domes, were also emergent (Martín et al., 1996; Betzler et al., 2000) Early Messinian (pre-evaporitic) Lower Messinian marine deposits are restricted to the SE Betic Neogene basins. In the study area, they

13 J.C. Braga et al. / Geomorphology 50 (2003) occur in the Tabernas, Sorbas, Vera, Poniente and Almería Níjar basins. Proximal sediments of this age consist of two successive reef units characterised respectively by bioherms and fringing reefs with associated calcarenites and calcirudites (Riding et al., 1991; Martín and Braga, 1994; Fig. 3). Bodies of deltaic siliciclastic deposits occur locally among these carbonates (Braga and Martín, 1996). Both reef units change laterally to silty marls, marls and turbidite conglomerates and sandstones (Fig. 3). As before, siliciclastic deposits prevailed in the Tabernas Basin (Kleverlaan, 1989; Haughton, 2000). The location of lower Messinian reef outcrops suggests a landward displacement of the palaeocoast around the Sierra Nevada Sierra de los Filabres that could be the result of the global eustatic sea-level rise recorded during the early Messinian (Haq et al., 1987). Uplift nonetheless continued in the Sierra de Gádor and Sierra Alhamilla, both of which expanded to displace the palaeocoast radially from the sierra axes. The western part of the Alpujarra corridor was also uplifted, restricting the marine basin to its easternmost part as the emergent areas in the Sierra de la Contraviesa Sierra de Gádor chain merged with the Sierra Nevada (Fig. 8C). In the Cabo de Gata, a re-arrangement of the volcanic highs produced changes in the palaeogeography of the area (Braga et al., 1996b) Late Messinian After the desiccation of the Mediterranean related to the Messinian Salinity Crisis and before the end of the Messinian, the sea re-invaded the Tabernas, Sorbas, Vera and Almería Níjar basins (Riding et al., 1998), but the area of marine sedimentation was more restricted compared to previous periods. Evaporites, mainly gypsum, formed in the first phases of sea-level recovery. As the sea level rose, conglomerates, sandstones, oolites, stromatolites and coral patch-reefs formed on top of the previous lower Messinian reef platforms (Fig. 3; Dabrio et al., 1985; Martín et al., 1993). These proximal deposits change basinwards to silts and silty marls with turbidite intercalations (Martín et al., 1993; Aguirre and Sánchez-Almazo, 2000). During this period, a N S high separated the Tabernas and Sorbas basins. The Sierra Cabrera remained as a submerged swell during the early Messinian with no evidence of shallow-water sedimentation around it (Braga et al., 2001) and emerged in the late Messinian to separate the open-marine Vera Basin from the restricted evaporitic Almería Níjar Basin to the south (Riding et al., 1998; Fig. 7D) Early Pliocene During the early Pliocene, marine sedimentation took place only in the basins closest to the presentday Mediterranean: the Poniente, Almería Níjar and Vera basins, except for a brief sea invasion of the already continental Sorbas Basin (Mather, 1991). The lower Pliocene proximal deposits in these basins include mostly conglomerates and sandstones, with variable proportions of bioclastic material and calcarenites locally. These coarse-grained sediments change laterally and prograde over distal silts and silty marls (Fortuin et al., 1995; Stokes, 1997; Aguirre, 1998). The emergence of the eastern portion of the Alpujarra corridor and the eastern part of the Fig. 9. Lower Pliocene palaeogeography of southeastern Spain (modified from Aguirre, 1998).

14 16 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 Fig. 10. Altitude in metres of the outcrops of lower Tortonian inner-platform deposits in the study area. Contour interval, 100 m. A few contours have been suppressed for clarity. Fig. 11. Altitude in metres of the outcrops of upper Tortonian reefs and other coastal deposits in the study area. Contour interval, 100 m. A few contours have been suppressed for clarity.

15 J.C. Braga et al. / Geomorphology 50 (2003) Tabernas basin concentrated the southern siliciclastic discharge from the Sierra Nevada Sierra de los Filabres in a delta located between the Sierra de Gádor and Sierra Alhamilla and open to the Almería Níjar Basin (Postma, 1983) (Fig. 9). The volcanic relief of the Cabo de Gata was almost completely emerged except for small embayments of the Mediterranean Sea, such as the Carboneras and Agua Amarga basins (Aguirre, 1998). The coarse-grained clastics produced by their erosion accumulated in a delta discharging into the Almería Níjar basin (Boorsma, 1992, 1993) (Fig. 9). At the end of the early Pliocene, a regression in all the marine basins in the area resulted in a progressive withdrawal of the sea. The palaeogeography during the late Pliocene, probably after an uplift pulse (Aguirre, 1998), was quite similar to the present day and only the southernmost areas of the Almería Níjar and Poniente basins were still covered by the sea, forming a shallow bay (Aguirre, 1998). As mentioned above, the Granada and Guadix basins have been continental since the latest Tortonian. The source of the conglomerate clasts and the locations of alluvial fans suggest a widening of the Sierra Nevada relief during the early Pliocene (Martín and Braga, 1997) (Fig. 6). During the late Pliocene and Pleistocene, a later denudation phase associated with the eastern part of Sierra Nevada is recorded in the Guadix Basin (García-Aguilar and Martín, 2000) and the emergent Tabernas Basin (Kleverlaan, 1989). 5. Uplift amounts and average rates We report here the amount and average rate of uplift of shoreline-marker rocks of the analysed time Fig. 12. Altitude in metres of the outcrops of uppermost Tortonian lowermost Messinian inner-platform deposits in the study area. Contour interval, 100 m. A few contours have been suppressed for clarity.

16 18 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 slices since the early Tortonian. The current elevation corrected with the known values of global sea-level position at the time of deposition (amount of rock uplift) of outcrops of rocks of the succesive units is plotted in Figs The contours indicate points of estimated similar uplift since the formation of the rocks. The absolute age range of the rocks of each analysed time slice is discussed and then used to obtain the average uplift rate since deposition. In the case of the Sierra Nevada Sierra de los Filabres chain, uplift rates can be estimated only for the surrounding basin margins, since the altitude that had been reached by the major Middle Miocene upland area when the lower Tortonian shallow-marine sedimentation started is unknown. The subsequent uplift history of the spine of the sierras is also poorly constrained Lower Tortonian rocks Outcrops of lower Tortonian inner-platform deposits reach their maximum elevations in the western and northwestern margins of the Sierra Nevada and in the Sierra Arana, with altitudes of up to 1830 m (Sanz de Galdeano and López-Garrido, 1999), which can be considered the minimum uplift value of Sierra Nevada since the early Tortonian (Fig. 10). According to Brachert et al. (1996), these platform deposits probably formed during the global sea-level lowstand separating cycles TB3.1 and TB3.2 of Haq et al. (1987). The estimated global sea level at that lowstand was about 10 m lower than at the present-day (Hardenbol et al., 1998). This difference in sea level is lower than our methodological error in estimating sea-level position by inner-platform facies distribution and is a negligible percentage of the present-day altitude of the lower-tortonian outcrops in the study area. The elevation values for the lower Tortonian innerplatform deposits decrease in very steep gradients towards the Granada and Valle de Lecrín depressions (Fig. 10). Lower Tortonian shallow-water deposits in Sierra de la Tórtola crop out up to 1380 m, defining a highly elevated area separated from Sierra Nevada (Fig. 10). In the eastern part of the Sierra Nevada Sierra de los Filabres chain, the maximum height of lower Tortonian inner-platform deposits always remains below 870 m, suggesting a general eastward decrease of the Sierra Nevada Sierra de los Filabres average uplift since the early Tortonian. Fig. 13. Altitude in metres of the outcrops of lower Messinian reefs in the study area. Contour interval, 100 m. A few contours have been suppressed for clarity.

17 J.C. Braga et al. / Geomorphology 50 (2003) Fig. 14. Altitude in metres of the outcrops of lower Pliocene inner-platform deposits in the study area. Contour interval, 100 m. A few contours have been suppressed for clarity. The lower Tortonian mixed siliciclastic and bioclastic materials considered here formed after the appearance of N. acostaensis (Rivas et al., 1999) dated at 10.9 Ma (Berggren et al., 1995) and before the first occurrence of N. humerosa (Martín-Pérez, 1997) at 8.5 Ma (Berggren et al., 1995). The most accurate estimate for the age of these deposits is the Sr isotopic age of approximately 9.2 Ma determined in Sierra Alhamilla by Hodgson (2002). This age value implies maximum average uplift rates since the early Tortonian (for the deposits at the western Sierra Nevada margin) of up to 200 m/ma (Fig. 15A) Upper Tortonian rocks The first occurrence of N. humerosa (8.5 Ma, Berggren et al., 1995) is in the fine-grained materials at the bottom of the upper Tortonian reef unit in the Granada Basin and in the Almanzora Corridor (Guerra-Merchán and Serrano, 1993; Martín-Pérez, 1997). The Planktonic Foraminifer Event 1 of Sierro et al. (1993), dated at 7.5 Ma (Krijgsman et al., 1997), is recorded almost at the top of this unit in the Sorbas Basin (Sánchez-Almazo, 1999). Upper Tortonian reefs, therefore, formed approximately in the time interval from 8.5 to 7.5 Ma. According to Esteban et al. (1996) and Brachert et al. (1996), this upper Tortonian reef unit can be correlated with the highstand of TB3.2 cycle of Haq et al. (1987), which reached a sea level about 30 m above the present-day one (Hardenbol et al., 1998). The maximum altitudes of outcrops of upper Tortonian reef and other coastal sediments are concentrated at the western and northwestern margins of Sierra Nevada and southeastern Sierra Arana, with a rapid decrease in values towards the Granada depression away from the Sierra Nevada (Fig. 11). La Tórtola stands out as an area of highly elevated upper Tortonian reef outcrops isolated from the Sierra Nevada. As in the underlying sedimentary unit, maximum outcrop elevation generally decreases towards the eastern Sierra Nevada Sierra de los Filabres

18 20 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 Fig. 15. (A) Maximum average uplift rates since formation of the coastal rocks from the lower Tortonian to the lower Pliocene in SE Spain. (B) Minimum average uplift rates of the highest peaks in Sierra de Gádor, Sierra Alhamilla and Sierra Cabrera. Rates correspond to m/ma. Shaded segments in the time scale correspond to the constrained interval of formation for the studied units. chain. In some areas, such as to the north and northwest of Granada, the similarity in altitude of the lower and upper Tortonian deposits indicates that almost no uplift took place at those basin margins between the formation of the two sedimentary units, except perhaps for the several tens of metres needed to compensate for global sea-level rise between the formation of the lower and upper units. These areas were, therefore, mostly uplifted during the last 7.5 Ma. In other regions, such as the western Sierra Nevada margin and La Tórtola, significant uplifts of one to several hundred metres are recorded from 9 to 7.5 Ma. The maximum average rate of uplift estimated for the upper Tortonian coastal sediments since their formation is approximately 150 F 10 m/ma (Fig. 15A). This value for the reefs in La Tórtola and the western Sierra Nevada and Sierra Arana margins contrasts strongly with the much lower rates of 60 m/ma estimated for the outcrops in the Alpujarra and the eastern Almanzora corridors.

19 J.C. Braga et al. / Geomorphology 50 (2003) Uppermost Tortonian lowermost Messinian rocks The uppermost Tortonian lowermost Messinian bioclastic carbonates were deposited after 7.5 Ma (datum recorded in the underlying unit). These shallow-water carbonates change laterally and upwards to marls and silty marls in which the first occurrence of Globorotalia mediterranea (dated at 7.2 Ma, Krijgsman et al., 1997) has been recorded (Sierro et al., 1993). An absolute age of 7.2 F 0.2 Ma is considered the best estimate for this sedimentary unit, which probably formed during the lowstand separating cycles TB3.2 and TB3.3 of Haq et al. (1987) (Martín and Braga, 1994; Brachert et al., 1996). The global sea level was only 5 m lower than today (Hardenbol et al., 1998), which makes a negligible difference to the uplift-rate estimates for this sedimentary unit. The uppermost Tortonian earliest Messinian shallow-water carbonates formed in the southeastern Betic basins in areas which had yet to emerge by the late Tortonian. The maximum altitudes of the inner-platform carbonate outcrops of this age can be found in the Sierra de Gádor at ca m (Fig. 12). The elevation of the outcrops decreases radially outwards from the centre of the sierra. Outcrop altitude likewise decreases away from the margins of Sierra de la Contraviesa, Sierra Alhamilla and Sierra de los Filabres to the surrounding depressions. In addition, a general decrease in elevation of the inner-platform outcrops of this unit from the Sierra de Gádor to the east can also be recognised. The lowest outcrops in the study area occur in the Vera Basin at the eastern end of Sierra de Filabres (Fig. 12). The Sierra de Gádor was not emergent during the deposition of the upper-tortonian reef unit (Fig. 7). The highest peak of the sierra (2126 m) has therefore been uplifted over the last 7.5 Ma with a minimum average rate of 280 m/ma (Fig. 15B). The highest outcrops of uppermost Tortonian lowermost Messinian carbonates in the sierra rose at an average rate of 220 F 5 m/ma over the last 7.2 F 0.2 Ma (Fig. 14A). Likewise, the Sierra Alhamilla emerged after the deposition of the upper-tortonian reef unit (Figs. 7 and 8A,B) and its highest peak (1387 m) was uplifted at a minimum average rate of 180 m/ma (Fig. 15B). The areas with outcrops of uppermost Tortonian lowermost Messinian carbonates at the eastern margin of Sierra Alhamilla have uplifted at much lower rates (from 100 to 40 m/ma) Lower Messinian rocks The Messinian reefs were coeval with the Planktonic Foraminifer Event 4 of Sierro et al. (1993, dated at 6.36 Ma by Krijgsman et al., 1997) recorded in the lower reef unit (Braga and Martín, 1996), continuing until the end of the pre-evaporitic Messinian marine sedimentation dated at 5.9 Ma in the Sorbas Basin (Gautier et al., 1994; Krijgsman et al., 1999). The preevaporitic Messinian reefs in the basins of southeastern Spain therefore grew from approximately 6.4 to 5.9 Ma, during the highstand of Cycle TB3.3 of Haq et al. (1987) (Martín and Braga, 1994; Brachert et al., 1996). The highest sea level in this highstand was about 40 m above present-day sea level (Hardenbol et al., 1998). Sea-level oscillations have been recorded within the Messinian reef units (Goldstein and Franseen, 1995; Braga and Martín, 1996), but reefs at the highest altitudes in each outcrop area probably formed at the peak of the global highstand. The Messinian reefs crop out several tens of metres higher than the previous bioclastic-carbonate unit at the Sierra de los Filabres margin (Fig. 13). This increase in elevation is mostly the result of a global sea-level rise between the formation of the two units, implying that the margin of Sierra de los Filabres was essentially stable from 7.2 to 5.9 Ma and no substantial uplift of the area took place during that time interval. In contrast, the northeastern margin of the Sierra Alhamilla was probably uplifted some tens of metres since the uppermost Tortonian lowermost Messinian bioclastic carbonates crop out several tens of metres higher than the Messinian reefs. Discounting the additional 40 m in the global sea level compared to the present day, maximum average uplift rates of the reef outcrops have been 110 F 5 m/ma (Fig. 15A). Present-day Messinian reef altitudes (Fig. 13) and the corresponding uplift rates decrease towards the eastern Sierra de Filabres and towards the depression of the Almería Níjar Basin away from Sierra de Gádor and Sierra Alhamilla Upper Messinian After the desiccation of the Mediterranean related to the Messinian Salinity Crisis and evaporite

20 22 J.C. Braga et al. / Geomorphology 50 (2003) 3 26 formation, sea level recovered and the remaining marine basins in SE Spain were re-flooded at the end of the Messinian (Riding et al., 1998) (Fig 8D). The first sedimentary evidence of the emergence of the Sierra Cabrera is coeval with gypsum deposition in the Sorbas and Almería Níjar basins, which took place approximately 5.5 Ma ago (Riding et al., 1998, 1999). The highest peak in Sierra Cabrera (961 m) has been uplifted at a minimum average rate of 170 m/ma since then (Fig. 15B) Lower Pliocene rocks The marine sedimentation in the basins of southeastern Almería continued during the early Pliocene and shallow-water platform deposits from this age can be used to estimate the uplift of the basin margins since then. The lower Pliocene deposits in the Almería basin formed from 5.2 to 3.6 Ma (Aguirre, 1998), which includes the highstand of the TB3.4 cycle of Haq et al. (1987) with a highest sea level some 90 m above the present-day level (Hardenbol et al., 1998). The highest elevations of lower Pliocene innerplatform deposits are found in the Tabernas Basin (620 m), decreasing southwards along the modern Andarax Valley depression (Fig. 14). High altitudes of up to 540 m are concentrated around Sorbas while altitudes of up to 410 m are recorded in the Almería Níjar basin, where outcrop elevation decreases southwards away from Sierra Alhamilla and Sierra Cabrera. This was the last episode of marine sedimentation in the Tabernas and Sorbas basins as they emerged above sea level soon afterwards. Taking into account that global sea-level was up to 90 m higher during the early Pliocene than today, average uplift rates range between 70 and 100 F 20 m/ma for the Sorbas Basin outcrops and up to 120 F 20 m/ma for the Tabernas Basin (Fig. 15A). If we accept the above-mentioned figure of a lower Pliocene sea level 90 m higher than today, the Poniente area has subsided during the last 3.6 Ma, since all the lower Pliocene outcrops in the area are below 80 m. Nonetheless, uncertainties introduced by the methodological error of estimating palaeobathymetry ( F 30 m in the case of inner-platform deposits), together with the difficulties of establishing the accurate timing of sediment formation in the Poniente Basin in relation to the sea-level oscillations inside the lower Pliocene sea-level cycles, prevent any confident conclusion. 6. Concluding remarks The palaeogeographic evolution of the Internal Zone of the Betic Cordillera from the end of the Middle Miocene to the Pliocene can be summarised as follows: There was a large island at the end of the Middle Miocene, which records the initial uplift of the present-day highest peaks in the Betic Cordillera, the Sierra Nevada Sierra de los Filábres chain (Fig. 5). The spine of this chain remained emergent for the rest of the Cenozoic but during the early Tortonian the sea invaded the previously emergent southern part of the Middle Miocene island on which the Alpujarra, Tabernas, Sorbas and Vera basins started to develop as marginal basins connected to the Alborán Basin (Fig. 7). These basins reached their maximum water depth during the late Tortonian and became narrow corridors at the end of this interval, when the Sierra de la Contraviesa, Sierra de Gádor and Sierra Alhamilla emerged (Fig. 8A,B). Major uplift of the External Zone connected the Betic islands to the Iberian mainland during the Tortonian. In the latemost Tortonian, the inland Granada and Guadix basins, together with the Almanzora Corridor, became continental. The Sierra Cabrera emerged in the Late Messinian, suggesting a progressive uplift from west to east of the southern sierras (Sierra de la Contraviesa, Sierra de Gádor, Sierra Alhamilla and Sierra Cabrera) (Fig. 8A D). During the Pliocene only the basins closest to the modern Mediterranean remained marine and progressively emerged throughout this period (Fig. 9). The average rock uplift rates of upper Neogene coastal and shallow water marine deposits in the study area since their formation have maximum values of approximately 200 m/ma. The maximum average uplift rate calculated for a particular sierra since its emergence is 280 m/ma in the case of Sierra de Gádor. All these values lie well below the exhumation rates estimated for the of the Nevado Filábride Complex after metamorphism ( m/ma, Weijermars et