Internal-External Zone Boundary Research Articles

Gravitational dismantling of the Miocene mountain front of the Gibraltar Arc system deduced from the analysis of an olistostromic Complex (Western Betics)

A melange complex seals the internal-external zone boundary of the western part of the Gibraltar Arc orogenic belt and constitutes a key element to establish milestones of the Betic-Rif tectonic evolution. The blocks and olistoliths embedded in this melange provide constraints on the geological history of the main tectonic units involved in the Miocene mountain front. We mapped and analysed the blocks and olistoliths included in this melange in order to understand its age and genesis, which have long been a matter of debate. The relationships of this melange La Joya Olistostromic Complex (LaJOC) with the basement units together with the high variability of the block lithologies suggest a sedimentary origin for this melange. Two large-scale olistoliths retain their original structure prior to their emplacement in the LaJOC Basin. The sedimentological and structural analysis allowed us to correlate these olistoliths with the folded and thrust sequence belonging to the Miocene Betic-Rif accretionary prism (Flysch Trough units), and to constrain the age of deposition of the La Joya Olistostromic Complex. The age of the matrix of the melange deposits is poorly known because of the lack of in-place fauna. Indeed, the formation of the inherited fold-and-thrust structure of these olistoliths is well-known in the western Betics. Accordingly, the LaJOC should have been deposited during middle Miocene times and the blocks and olistoliths included within the melange would derive from the gravitational dismantling of the Gibraltar Arc mountain front. The data presented help us to understand the formation of reliefs and basins in the western part of the Gibraltar Arc orogenic system.

Lower Miocene deformation in the hanging wall of the Internal-External Zone boundary of the Betic Cordillera: deformation at the edges of vertical-axis rotation domains in oblique convergent margins

AbstractIn the eastern Betic Cordillera, non-cylindrical doubly plunging folds deform the Subbetic Zone (the hanging wall of the Internal-External Zone boundary). Their hinges define an arc from north-south trends in the east to ENE-WSW in the west. These folds began to form during the Early Burdigalian with a NNE-SSW trend. Middle Burdigalian rocks define progressive unconformities in the cores of the synforms, recording the tightening of the folds and an increase in the plunge of the fold hinges. All these folds experienced verticalaxis rotations during the Early Miocene, acquiring the present-day arcuate pattern. During the Middle Burdigalian, the thrusting of the External Zones over the Internal Zones occurred with a top-to-the-SE sense of movement. Upper Burdigalian deposits seal the tectonic windows eroded in these thrust surfaces. This deformational history records a constrictional deformation with area reduction in the Subbetic Zone during the Early Miocene, which can be correlated with the dextral oblique convergence between the Internal and the External Zones.

Structural evolution of the early Miocene in the eastern Betic internal-external zone boundary (SE Spain)

Abstract The Internal-External Zone boundary (IEZB) in the eastern Betic Cordillera partially coincides with the Cadiz-Alicante Accident, mainly a major transcurrent fault of N060E direction. The study of an area located along the IEZB at a point where it separates from the Cadiz-Alicante accident has provided details concerning the geodynamic evolution of the cordillera at the moment of its structuration. Here the Internal Zone, consists of rocks assigned to the Malaguide Complex, dating its last sedimentation to the Aquitanian, and deposits assigned to the Vinuela Group (early-middle Burdigalian). The nappes of the Internal Zone were emplaced during the latest Aquitanian and the Vinuela Group (here the El Nino Formation) sealed it but was afterwards affected by the collision with the External Zone. On the other side of the boundary, the External Zone comprises two tectonic units: the Penarrubia Unit (late Cretaceous-middle Burdigalian), which is made up mainly of limestones and marls, and the El Frances Chaotic Complex composed by a set of different lithologies, all from the External Zone in a marly matrix that could be interpreted as a collisional melange formed in the early-middle Burdigalian. The contact between the two domains corresponds to a backthrust of the External Zone over the Internal Zone which occurred in the middle Burdigalian. The deposits sealing the IEZB are dated by calcareous nannofossils and planktonic foraminifera as late Burdigalian, and comprise clasts from both domains.

Oligocene to middle Miocene basin development in the Eastern Betic Cordilleras, SE Spain (Vélez Rubio Corridor – Espuña): reflections of West Mediterranean plate‐tectonic reorganizations

The suture between two West Mediterranean crustal blocks once situated several hundreds of kilometres apart can be studied in the Vélez Rubio Corridor – Espuña area of the Eastern Betic Cordilleras. This suture, or Internal–External Zone Boundary, separates the former passive southern margin of Iberia (the External Zone) from a stack of allochthonous nappe complexes (the Internal Zone), of which the highest unit is formed by the weakly or nonmetamorphosed Malaguide Complex. Analysis of the Oligocene to middle Miocene sediments of the Vélez Rubio Corridor and the Espuña, and comparison with coeval deposits elsewhere in the Western Mediterranean shows that (a) up to the middle Miocene, the southern part of the External Zone (Southern Subbetic) was positioned some 100 km more eastward; (b) up to the early Aquitanian, the Malaguide Complex, forming part of the South Sardinian block (the southern section of a West Mediterranean continental segment) was juxtaposed to the North Sardinian block (the northern part of that continental fragment), some 400 km more eastward; (c) West European extensional rifting during the late Oligocene to earliest Aquitanian resulted in deposition of rift valley sediments (Ciudad Granada and Pliego Formations) in the Malaguide realm; (d) during the Aquitanian, the West Mediterranean segment disintegrated and the West Mediterranean oceanic basins opened, resulting in, for example, the south‐westward drift of the Internal Zone, with concomitant thrusting and thinning and deposition of submarine fans (Solana‐Algeciras Formation) along the margin; (e) in the early Burdigalian, the allochthonous Internal Zone collided with the Iberian margin, causing the disruption of the platform‐slope configuration of the External Zone; (f) after the collision a deep basin was formed upon the suture filled in with erosional products from both Internal and External Zones (Espejos–Viñuelas–Millanas Formations); (g) a strong compressive event in the late Burdigalian caused the southward thrusting of the Subbetic over the Espejos Formation, thus double‐sealing the collisional contact; (h) in the latest Burdigalian to Langhian, new strongly subsiding basins were formed in the Western Mediterranean, e.g. along the Internal–External Zone Boundary; (i) dextral strike‐slip faulting in the Serravallian resulted in a westward displacement of over 100 km of the southern Subbetic plus Internal Zone; (j) onset of a new pattern of strike‐slip faulting induced the formation of a new suite of basins in the Tortonian.

Oligocene to Middle Miocene basin development in the Velez Rubio Corridor - Espuna (Internal-External Zone Boundary; Eastern Betic Cordilleras, SE Spain).

The Velez Rubio Corridor and the area northwest of the Sierra Espuna are located on the Internal-External Zone Boundary. The External Zone is represented by the Southern Subbetic, the most basinward part of the former passive margin of Iberia, the Internal Zone by its unmetamorphosed highest unit, the Malaguide Complex, tectonically underlain by the metamorphosed Alpujarride Complex. During the Oligocene and Aquitanian, the Southern Subbetic was the locus of slope deposition with northeastern provenance of detritus. In the Malaguides of the Espuna, the detritus of Lower Oligocene transgressive conglomerates and Middle Oligocene fan deltas indicates Sardinian proximity. The Upper Oligocene to lower Aquitanian Ciudad Granada and Pliego formations of the Malaguide Complex, carrying exclusively Malaguide detritus, were deposited in grabens within the Malaguide realm during an extensional rifting phase. The Malaguides, still far removed from the Subbetic, underwent major thrusting during the Aquitanian. Of the sedimentary units found between the Internal and External Zones, the oldest unit (the allochthonous Aquitanian Solana formation) was deposited in submarine fans outside the Subbetic or Malaguide realms proper, but in close connection with the latter. The Internal Zone collided with the External Zone in the early Burdigalian with concomitant disruption of the Southern Subbetic slope. On the suture, a deep basin was formed and filled in by the Burdigalian Espejos formation carrying detritus from the Subbetic and from the Malaguide and Alpujarride Complexes. In the late Burdigalian, the Subbetic was thrust southward over the Espejos formation, thus double-sealing the collisional contact. During the latest Burdigalian to Langhian, new basins were formed along the Internal-External Zone Boundary and within the Southern Subbetic. The onset of strike-slip faulting caused shoaling and uplift of these basins. Onset of a new pattern of strike-slip faulting induced the formation of a new suite of basins during the Tortonian, e.g. the Lorca Basin.

The Internal-External zone boundary in the eastern Betic Cordillera, SE Spain: Reply

Abstract The Internal-External Zone Boundary (IEZB) in the Betic Cordillera of southern Spain separates the Internal Zone, which was deformed and in part metamorphosed before the early Miocene, from the External Zone, which consists of the cover rocks of the Iberian margin shortened to form a thin-skinned fold and thrust belt in early to middle Miocene times. Over much of its length the IEZB is roughly linear, trending approximately 070°. It has been referred to as a major dextral strike-slip zone, and has even been considered as the northern boundary of a westward moving Alboran microplate. Field and kinematic data from the eastern Betic Cordillera show that the IEZB crops out over a 60 km distance as a gently-dipping thrust with displacement to the southeast or south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeontological studies of calcareous nannoplankton indicate that the Oligo-Miocene basin along the boundary was the site of continuous deposition until the beginning of the middle Miocene. The thrust at the IEZB cuts early Miocene rocks, and is overlapped by the middle Miocene. Thrusting therefore occurred in this time interval. The IEZB is therefore unlikely to have been the dextral boundary of a westward-moving Alboran microplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Discussion

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Discussion

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Reply

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Reply

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Discussion

The Internal-External Zone Boundary in the eastern Betic Cordillera, SE Spain: Discussion

The internal-external zone boundary in the eastern Betic Cordillera, SE Spain

The Internal-External Zone Boundary (IEZB) in the Betic Cordillera of southern Spain separates the Internal Zone, which was deformed and in part metamorphosed before the early Miocene, from the External Zone, which consists of the cover rocks of the Iberian margin shortened to form a thin-skinned fold and thrust belt in early to middle Miocene times. Over much of its length the IEZB is roughly linear, trending approximately 070°. It has been referred to as a major dextral strike-slip zone, and has even been considered as the northern boundary of a westward moving Alboran microplate. Field and kinematic data from the eastern Betic Cordillera show that the IEZB crops out over a 60 km distance as a gently-dipping thrust with displacement to the southeast or south-southeast, oblique to its regional trend. There is no evidence of dextral strike-slip movement along the boundary. New micropalaeontological studies of calcareous nannoplankton indicate that the Oligo-Miocene basin along the boundary was the site of continuous deposition until the beginning of the middle Miocene. The thrust at the IEZB cuts early Miocene rocks, and is overlapped by the middle Miocene. Thrusting therefore occurred in this time interval. The IEZB is therefore unlikely to have been the dextral boundary of a westward-moving Alboran microplate at this time, but was more likely to have been the locus of NW-directed dextrally oblique convergence.