Materia prima requerida

The Sierra Espuna area (Figure 4.1) has been studied by a number o f authors recently in order to date the displacement o f the Internal Zones, including Lonergan (1993) Lonergan and Mange-Rajetzky (1994) and Martin-Martin et al. (1996; 1997; Geel & Roep, 1998).

Internal Zone Malaguide foreland basin sediments crop out on the NW side o f the Sierra Espuna range in the Espuna basin. Oligo-Miocene sediments comprise deep- water pelagic marls interbedded with sandstones, calcarenites, conglomerates and turbidity current and mass gravity flow deposits. In the SE o f the basin the sediments are steeply dipping to overturned on the northern limb o f the Espuna fold (Lonergan,

C h ap ter 4 - Subbetic Deformation

1993; Lonergan & Mange-Rajetzky, 1994) while in the west of the basin the sediments are partly overthrust by and partly onlapping onto the External Subbetic Zone (Lonergan et al., 1994). The Espuna basin is unconformably overlain in the SW by sediments o f the late Neogene-Recent extensional Lorca basin (Figure 4.1), which concealsssss the lEZB as it extends SW to crop out in the Vêlez Rubio Corridor about 4-5 km east of Fuensanta. While there are similarities between coeval sediments in the Vêlez Rubio Corridor and the Espuna basin, however, there are differences in facies during late Oligocene to Aquitanian time (Geel & Roep, 1998). Lonergan (Lonergan et al., 1994; Lonergan & Mange-Rajetzky, 1994) undertook a facies interpretation o f the Tertiary fill o f the Espuna basin, with modifications to the original stratigraphy described by Paquet (1967) and showed that the foreland basin represented deepening facies through time with a deepening carbonate ramp/shelf in Eocene time to eventual basinal pelagic facies in earliest Miocene time.

Compressive deformation o f the Sierra Espuna as a whole took place in the following stages: (1) early -middle Oligocene formation o f N-NW directed imbricate thrusts, (2) late Oligocene progression o f thrusting into the Espuna basin sediments and (3) folding o f the thrust stack into a regional N-NW vergent structure, with associated minor thrusting, after the earliest Miocene (Lonergan, 1993). This compressive deformation in the Espuna area is coeval with proposed extension taking place within the more central parts o f the Internal Zone in late Oligocene to Miocene time (Platt & Vissers, 1989; Lonergan & White, 1997; Lonergan & Platt, 1995).

The relationship o f the sediments on the NW edge of the Sierra Espuna basin with the Subbetic External Zones, along the lEZB, has implications for the timing o f compressive deformation within the External Zones. Owing to the poor preservation and rarity o f planktonic foraminifera, nannoplankton assemblages for the Oligocene- Miocene sediments of the Espuna basin were published by Lonergan et al. (1994) using the biozonations o f Martini (1971) and Okada and Bukry (1980). Lonergan et al. (1994) integrated the biostratigraphic data with kinematic data to assess the timing o f deformation along the lEZB, and drew the following conclusions:

1. SE-directed thrusting affects the Langhian parts o f the Bemabeles Formation, the youngest part o f a continuous (within achievable biostratigraphic resolution (Lonergan et al., 1996)) sedimentary sequence since Late Oligocene time.

2. The regional unconformity o f upper Miocene (Tortonian) rocks on the deformed Internal Zone therefore constrains observable thrusting to middle Miocene time.

Lonergan et al. (1994) recognise that large slumps present in the Bemabeles Formation could have been the product o f active thrusting on the lEZB prior to Langhian time, although the slumps could equally be caused by regional seismicity associated with thrust activity some distance removed from the lEZB.

Revisions to the Martini (1971) nannofossil biostratigraphic zonation scheme have been published (Young, 1998) and permit refinement to the Bemabeles Formation ages based on fauna published by Lonergan et al. (1994). There follows a discussion of the significance of certain key species in this published fauna.

Most o f the samples collected by Lonergan et al. (1994) from the top o f the Bemabeles Formation, contain Sphenolithus heteromorphus, a distinctive form that characterises the major interval from NN4-5, with the base of NN4 marked by the last occurrence (LO) o f S. belemnos (absent from the top Bemabeles Formation samples) and the first occurrence (FO) o f S. heteromorphus. The LO o f & heteromorphus marks the base o f biozone NN6. Helicosphaera perch-nielseniae occurs in one o f the samples, and is a species whose LO is used to subdivide zone NN5, and whose occurrence is rare toward its LO (Young, 1998). The boundary between NN4 and NN5 is defined by the LO o f K ampliaperta, a species that often occurs sporadically toward its LO (Young, 1998) and appears in all 5 samples collected from the top o f the Bemabeles Formation. Subdivision of NN4 requires detailed biostratigraphy over the preceding section to allow relative abundances to be determined, something that is not possible with the published samples, as they do not represent a stratigraphie sequence and were collected along strike from one another. The presence o f some species whose LO is prior to NN4 is likely to be the result o f reworking o f lower Miocene forms. The only reported species in the top Bemabeles samples that is suspect is that o f Calcidiscus macintyrei,

which is reported in three o f the five samples and whose FO is not until NN7 (Young, 1998). The FO o f C d premacintyrei (not reported) on the other hand is a subdividing marker in the Burdigalian part o f NN4, and may be the species identified as Cd. m acintyrei. The above species make a good case for the top o f the Bemabeles Formation being correlated with biozone NN4, which spans upper Burdigalian to lower Langhian time. Since S-directed thmsting o f the Subbetic has affected sediments o f the top-most Bemabeles Formation (see above) it is clear that deformation on the lEZB was

C hap ter 4 - Subbetic Deformation

Still a c tiv e after s o m e p o in t w ith in b io z o n e NN4, and p o s s ib ly after th e FO o f Cd. premacintyrei.

The onlapping o f top Bemabeles Formation onto Subbetic Aquitanian limestone in the more northern part o f the lEZB o f the Pliego valley (Lonergan et al., 1994) is o f increased significance when the detrital component o f the rocks is considered. The Subbetic limestones are free o f detrital metamorphic grains (Lonergan et al., 1994) while the entire Bemabeles Formation stratigraphy (including the Aquitanian parts) contains detrital grains originating in the Intemal Zones that have been used to determine Intemal Zone unroofing histories (Lonergan & Mange-Rajetzky, 1994; Lonergan & Johnson, 1998). This suggests that the Intemal and Extemal Zones were well removed fi-om one another during Aquitanian time, or more specifically, during the deposition o f the Subbetic Aquitanian limestone. Subbetic detritus has been reported from the upper part o f the Bemabeles Formation (Martin-Martin et al., 1996), suggesting that sediments o f the Bemabeles Formation were deposited in a piggyback or foreland basin between the converging Intemal and Extemal Zones. Significant convergence is likely to have taken place prior to biozone NN4 to permit onlapping of Bemabeles sediment onto the Subbetic Zone.

There is disagreement over the nature and timing o f deformation on the lEZB in the Espuna area, in particular Martin-Martin et al. (1996) contend that suturing o f the Intemal and Extemal zones occurred after deposition o f the lower part of the Bemabeles Formation, prior to back-thmsting o f the Subbetic over the Bemabeles Formation, and before deposition o f the upper part o f the Bemabeles Formation, pre-NN4. They found no indication of biozone NN3 fauna, specifically S. belemnos, and concluded that there is an unconformity between the lower and upper parts of the Bemabeles Formation. Observed deformation in the upper part of the Bemabeles Formation was attributed to N- to NW- directed strike-slip faults by Martin-Martin et al. (Martin-Martin et al., 1996) after deposition o f the upper Bemabeles Formation. Lonergan et al. (1996) are o f the opinion that continued convergence between the Intemal and Extemal zones after the deposition o f the uppermost Bemabeles Formation is transferred onto backthrusts within the upper part o f the Bemabeles Formation. Clockwise rotation o f the Malaguide rocks o f the Sierra Espuna between late Oligocene and late Miocene time, and synchronous with Internal-External zone convergence, has been identified through palaeomagnetic work (Allerton et al., 1993). It is invoked by Lonergan et al. (1996) to explain the differences in nature of the lEZB between the northem and southem parts of

the Sierra Espuna basin. Ultimately the disagreement is over the timing o f the termination o f thrusting on the lEZB in the northem part o f the Sierra Espuna basin.

The current age range o f biozone NN4 is between 18.3 and 15.6 Ma, with the FO of Cd. premacintyrei occurring at approximately 17.5 Ma (Young, 1998). This range is from uppermost Burdigalian (approximately N7) to lower Langhian time (before the end of N8). Suturing of the lEZB therefore occurred within this range.