Recent tectonic evolution of the Alborán Ridge and Yusuf regions

Resumen   Abstract   Índice   Conclusiones

Pedro Martínez-García

2013-A
Mejor Tesis Doctoral en Geofísica Pura o Aplicada, realizada en Universidades Españolas o de Iberoamérica 

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Resumen

This thesis is organized into eight chapters, which deal with the introductory aspects, data, methodology, interpretation and discussion of results and conclusions on the main findings of this study. Chapters 3 and 4 are made of papers that have been published or have accepted for publication in SCI journals. Chapters 5 and 6 are other manuscripts, which are going to be submitted shortly. The abbreviated content of each chapter of the thesis is as follows:

·         Chapter 1 “Introduction”, concerns a geological overview of the whole study region and explains the scientific problem and the thesis objectives.

·         Chapter 2 “Dataset and Methodology”, presents the comprehensive dataset used in this study, which came from different scientific and commercial cruises. It describes the different geophysical techniques used for data acquisition and the procedure used to digitalize the analogue records of seismic profiles. The bio-stratigraphic and log information from several wells that have been used to tie the interpretation is also presented. Additionally, this chapter contains explanations about the gridding procedure used to generate contour maps and the calculus of interval velocities, depth conversions and sediment accumulation rates.

·         Chapter 3, “Seafloor morphology and recent tectonics”, presents bathymetric data and sub-bottom parametric seismic profiles and includes a description of the main faults and folds with associated Quaternary activity. Sedimentary and erosive features, such as gullies, abrasion platforms and slides, have also been identified and mapped. These results are compared with shallow seismicity data and focal mechanism, in order to assess the position and kinematics of those active fault segments, which may represent a potential seismic hazard. The discussion deals here with the role played by active tectonics, triggering instability processes and erosion. This chapter has been published in Geo-Marine Letters, 2011 (31: 19-36): “Recent structures in the Alboran Ridge and Yusuf fault zones based on swath bathymetry and sub-bottom profiling: evidence of active tectonics”.

·         Chapter 4, “Plio-Quaternary stratigraphy and tectonic evolution”, focuses on the seismic interpretations of the shallow sedimentary sequences and structures of Plio-Quaternary age to record the post-Messinian basin evolution. Subsurface maps, sedimentation rate estimates and reconstructions of basin highs and active faults have been carried out throughout the Plio-Quaternary. With these interpretations the pattern of uplift and subsidence in the basin and how they can be linked to tectonic processes is discussed. The existence of several episodes of shortening since the late Messinian is reconstructed and whether they can be linked to changes in the convergence between the Eurasian and African plates is discussed. This chapter has been accepted for publication in Basin Research with the following title: “Strike-slip tectonics and basin inversion in the Western Mediterranean: the Post-Messinian evolution of the Alboran Sea”.

·         Chapter 5, “Submarine instability processes”, describes several types of mass-transport deposits (MTD) within the Plio-Quaternary sedimentary infill using swath bathymetry data, high-resolution acoustic profiles and single and multichannel seismic reflection profiles. Age estimations are given for most of the MTD by linking seismic interpretation and bio-stratigraphic information. The occurrence of recurrent instability processes in some areas next to the main Plio-Quaternary faults is proved and the role of earthquakes as main triggering factor is discussed. This chapter corresponds with a manuscript in preparation, which is titled “Recurrent slope instability in the Alboran Sea: assessment for changing basin topography from Pliocene to Recent”and will shortly be submitted to an international SCI journal.

·         Chapter 6, “Tectonic inversion and geodynamic implications”, documents pre-Messinian deformations by means of multichannel seismic profiles and compares these observations with the Plio-Quaternary structures. This contribution attempts to decipher how Miocene growth faults were inverted mainly during post-Messinian times. This chapter corresponds with a manuscript that will be submitted to an international SCI journal with the preliminary title: “Tectonic inversions in the Alboran Sea Basin: from Miocene extension to Recent shortening”.

·         Chapter 7 summarizes the conclusions of this thesis.

·         Appendixes contain other papers published in different non-SCI journals together with contributions at international meetings.

 
Abstract

A comprehensive dataset consisting of single and multichannel seismic profiles, bio-stratigraphic and log information from nearby DSDP, ODP and commercial wells, multibeam bathymetry data, high-resolution acoustic profiles (TOPAS) and hypocentre distribution of seismicity and focal mechanisms have been used to constrain the Late Miocene to present-day evolution of the central and south-eastern parts of the Alboran Sea Basin. The evolution from a Miocene extensional basin to complex, compartmentalized sub-basins dominated by strike-slip and reverse faulting during the Plio-Quaternary is described. This study determines that the ongoing deformation of the basin took place heterogeneously both in space and time. Miocene high-angle normal faults trending NE-SW promoted tilting of basement blocks and associated sedimentary wedges with a maximum thickness of 2.5-3 km. According to their trend and geometry a limited amount of NW-SE extension may be inferred.

During the Messinian salinity crisis, intense erosion and local subsidence resulted in small, isolated, basins with shallow marine and lacustrine sedimentation, comparable to the isolated marginal basins of the Betics. Throughout the Plio-Quaternary, the Alboran Sea Basin underwent complex interplay between tectonics, sediment supply from the surrounding Betic and Rif mountains, residual magmatism and partial inversion of the earlier Miocene structures. During the Plio-Quaternary the prevailing deformation was NW-SE shortening, resulting from convergence between the African and Eurasian plates. This deformation coexisted with thermal subsidence and promoted the inversion of some segments of the Miocene extensional structures that propagated upward in the sedimentary sequences as folds and reverse faults.

Three major phases of shortening affected the basin during the Early Pliocene (~5.33 to 4.57 Ma), the Late Pliocene (~3.28 to 2.45 Ma) and the early Pleistocene (~1.81 to 1.19 Ma). The Alboran Ridge, a linear high trending SW-NE for more than 130 km in the Alboran Sea, accommodated most of the shortening. Its main uplift took place during the Late Pliocene, coinciding with a rotation of the Eurasian/African plate convergence vector from NW-SE to WNW-ESE. The Yusuf fault zone, another major linear feature in the Alboran Sea composed of several WNW-ESE en-echelon fault segments, underwent transtension during these phases of deformation. The right lateral, transtensional slip that occurred in their fault segments is interpreted as having been promoted by their sub-parallel trend in relation to the plate convergence vector at that time.

Partial tectonic inversion took place in the Alboran Sea Basin during the Plio-Quaternary, affecting some of the Miocene NE-SW faults. Those fault segments, oriented perpendicularly to the trend of plate convergence between the African and Eurasian plates, underwent a greater degree of positive inversion. Tectonic inversion of this type took place, for example, along the western Alboran Ridge. Simultaneously other new, south-dipping reverse faults were formed during the Plio-Quaternary along the north-eastern margin of this ridge, where Miocene magmatic intrusions constituted induced anisotropies to focus later deformations.

At present, both the Alboran Ridge and the Yusuf faults deform the seafloor of the Alboran Sea. These two fault zones are connected, constituting a wide and geometrically complex deformation zone tens of kilometres across, including different active fault segments and in-relay folds. Reverse faults and associated folds are SW-NE trending along the Alboran Ridge and veer progressively to have a WNW–ESE trend towards the Yusuf fault zone. Present-day, right-lateral transtensional deformation induces faulting and subsidence in the Yusuf pull-apart basin. The entire band of deformation has scattered associated seismicity including some moderate earthquakes (M_w > 4). A narrow, SSW-NNE oriented fault zone cuts across the western Alboran Ridge and continues northwards deforming the Djibouti High. This fault is called the Al-Idirisi fault zone and consists in narrow folds and reverse south-east-dipping faults. This structure concentrates most of the upper crustal seismicity in the region defining a seismogenic, left-lateral fault zone that connects southwards with the Al Hoceima seismic swarm and represents a potential seismic hazard. There are other active faults in the Djibouti High, which form two conjugate strike-slip fault systems and locate at the south-western termination of the Carboneras fault. One system is NNE-SSW trending and has a dominant left-lateral movement, similarly to the Al-Idrisi fault whilst the other system comprises NW-SE faults with a dominant right-lateral component.

Buried submarine landslides together with more recent ones along the Alboran Ridge and the Yusuf Escarpment are clear signs of submarine instabilities in these seismically active fault zones. Some mass-transport deposits (MTD) are composed internally of several superimosed lobes of chaotic sediments separated by thin layers of draping hemipelagic sediments. These observations collectively suggest that instability processes have been recurrent throughout the Plio-Quaternary, probably triggered by earthquakes occurring during movement along the faults that have been active from at least the Pliocene.

The present-day distribution of deformation in the study area resembles that along the Algerian margin. The entire region comprises different segments under shortening with NE-SW trend and lengths between 125 and 180 km, which accommodate contractional deformation. These are displaced by a number of NW-SE to WNW-ESE right-lateral, strike-slip fault zones that transfer the shortening between the different NE-SW trending segments. All these structures are long-lived fault zones that formed during the Miocene opening of the Algerian and Alboran Sea basins that were later reactivated and locally inverted. The structures and basin geometry inherited from pre-Messinian times has resulted in a partitioning of the subsequent deformation and subsidence and uplift in the Pliocene to Recent basins. Towards the west, some of the inherited Miocene structures, such as the residual normal faults bounding the Djibouti High and the partially reactivated NE-SW structures along the Alboran Ridge, are cut across by the Al-Idrisi fault zone, which is a more recent structure. This fault was formed during the Pliocene and continues being active at the present day. This structure contributes to partition the present-day deformation and its left-lateral strike-slip kinematics promotes the lateral escape of the frontal part of the Gibraltar Arc towards the south-west.

 
Índice

 
Conclusiones

This chapter summarizes the main results of this thesis and discusses briefly how these findings contribute to our knowledge of the stratigraphic and tectonic evolution of the Alboran Sea Basin. The active processes that affect the seafloor in this region should be taken into account in any further detailed investigation into geological risk in the region. The influence of previous geological processes, which are currently inactive but conditioning the present-day location and geometry of active sedimentary and tectonic structures, is also commented upon.

A detailed geomorphological and structural study has been conducted in the central sector of the Alboran Sea, around the Alboran Ridge and Yusuf regions using high-resolution geophysical techniques, such as swath bathymetry and TOPAS profiles, Recent faults and folds deform the seafloor and the most recent Quaternary sediments, control the canyon and channel network and form seafloor escarpments causing seafloor instabilities, which explain most of the submarine slides discovered. In the light of the hypocentre distribution, the magnitude and the focal mechanisms of shallow crustal seismicity (< 11 km), the faults and folds mapped are demonstrated to be active structures. The location and kinematics of active structures in the study area was compared with other surrounding faults to provide a map of active structures for the entire Alboran Sea Basin.

These results confirm that the Alboran Ridge and Yusuf fault zones, previously identified as Plio-Quaternary structures, are currently active. Moreover, the study reveals that these two fault zones are connected forming a wide and continuous band of deformation with scattered, associated seismicity. The orientation of these fault zones varies from west to east, veering progressively from SW-NE to WNW-ESE trend. In the Alboran Ridge, the predominant structures are reverse faults and folds trending SW-NE. They are oriented perpendicular to the NW-SE plate convergence vector inferred by geodetic models and accommodate most of the shortening. Structural mapping and focal mechanisms indicate a predominant reverse component for the Alboran Ridge fault zone. The fault and fold systems along the ridge veer progressively from SW-NE to E-W to connect finally with the Yusuf fault zone, which trends WNW-ESE. There, structures are oblique or sub-parallel to the actual plate convergence vector resulting in considerable strike-slip faulting with a right-lateral component. Transtensional displacements occur between overlapping fault segments.

Additionally, data demonstrate the existence of a previously unknown narrow, NNE-SSW zone with folding and reverse faulting, which cuts obliquely the western Alboran Ridge. This structure, named the Al-Idrisi fault zone, is the focus of most of the upper crustal seismicity in the Alboran Sea region. Fault kinematics deduced from focal mechanisms corresponds to left-lateral, strike-slip to reverse faulting. The Al-Idrisi fault zone connects southwards with a seismic swarm located around Al Hoceima, on the Moroccan shoreline, which is the site of large earthquakes (M_w>6). There is also another seismic swarm next to the northern end of the Al-Idrisi fault zone coinciding with the Adra seismic series. The occurrence of these two seismic swarms at both ends of the fault may indicate active fault propagation along both the northern and southern tip lines.

The Al-Idrisi fault zone is clearly younger than folds and faults along the Alboran Ridge and Yusuf Escarpment, and may make a significant contribution to seismic hazard in the Alboran Sea area. The documented strike-slip movement of this fault leads to the lateral escape of the frontal part of the Gibraltar Arc towards the south-west. The Al-Idrisi fault corresponds to the eastern boundary of this block, which comprises the south-western Betics, the western Alboran Sea and the Rif. The internal clockwise rotation of this block, recognised by recent GPS measurements, may be promoted by the left-lateral kinematics of the Al-Idrisi fault.

The northern part of the Al-Idrisi fault zone deforms the Djibouti High, where other active structures have also been identified. Some of these faults trend NNE-SSW and have a left-lateral, strike-slip movement, similar to that of the Al-Idrisi fault. There are other faults trending NW-SE with a dominant right-lateral, strike-slip component. They correspond to two conjugate strike-slip fault systems that locate at the south-western end of the Carboneras fault.

A dense grid of single and multichannel seismic profiles, correlated to nearby DSDP and ODP sites and commercial wells, have been used to record the evolution of the Alboran Sea Basin. Detailed bio-stratigraphic and log information for the Late Miocene to Quaternary sediments has allowed a precise study over the last ~5.33 Ma. Reconstructions of basin structures and sediment accumulation rates reveal how the paleogeography has evolved since the Messinian and indicate that this region has undergone complex interactions between plate convergence, inherited structures, sediment supply from the surrounding Betic and Rif mountains, deformation and subsidence. 

Fault and fold systems along the Alboran Ridge were already connected with the structures along the Yusuf region during the Plio-Quaternary and they moved at different times, causing shortening, uplift and transpression or transtension depending upon the angle between these structures and the prevailing convergence trend at the time in question. Main deformation pulses occurred during the Early Pliocene (~5.33 to 4.57 Ma), the Late Pliocene (~3.28 to 2.45 Ma) and within the early Pleistocene (~1.81 to 1.19 Ma). Detailed structural mapping show that the Alboran Ridge and Carboneras fault zones were not connected during the Plio-Quaternary and therefore do not form a single fault structure cutting across the Alboran Sea Basin from the Betics to the Rif.

The link between the Alboran Ridge and Yusuf fault zones is accommodated by a south-dipping, arcuate reverse fault which has led to the thrusting of the Alboran Ridge over the southern margin of the East Alboran Basin. Localized deformations along the Alboran Ridge and Yusuf Escapment triggered progressive southward migration and tilting of depocentres in the southern basins located on the hanging wall of the reverse fault. Thus, the SAB, Pytheas and Habibas basins became perched basins during the Plio-Quaternary, and experienced a significant reduction in the sediment accumulation rates.

The main uplift of the Alboran Ridge took place during the Late Pliocene, at the same time as the convergence between the African and Eurasian plates veered from NW-SE to WNW-ESE. Along the western part of the Alboran Ridge previous extensional structures trending NE-SW were reactivated and inverted in a similar way to the Xauen Bank. The uplift of these reliefs resulted in the disconnection between the WAB and the SAB, which had been connected during the Miocene. Moreover, the existence of Miocene magmatic intrusions in the eastern Alboran Ridge determined the formation of new north-directed reverse faults and folds, which were active from at least the Early Pliocene. The main phase of shortening and uplift during the Late Pliocene coincided with the first important influx of the Almeria turbidite system into the Alboran Sea Basin. Seafloor uplift favoured erosion and the accumulation of more coarse-grained lithologies during the Late Pliocene and Quaternary and also encouraged abundant gravity-instability processes.

Mass-transport deposits (MTD) have been mainly found along the slopes of the Alboran Ridge and the Yusuf Escarpment, their ages ranging from the Late Pliocene to the present. The superposition of slides, draped by thin layers of stratified sediments, and their location on fault escarpments suggest that they were probably triggered by earthquakes during tectonic pulses of active faults. These processes have occurred since at least the Late Pliocene and alternated with periods of relative tectonic quiescence dominated by hemipelagic sedimentation.

The Djibouti High was not affected by significant deformation during the Pliocene and it remained as a residual high from Miocene rift events. Nevetheless, during the early Pleistocene, coinciding with the most recent phase of shortening, the NNE-SSW strike-slip Al-Idrisi fault zone propagated northwards deforming the Djibouti High. Simultaneously the two conjugated high-angle strike-slip fault systems (NNE-SSW and NW-SE) were formed there.

A complicated tectonic template was inherited from previous Miocene rifting events and this had a strong influence upon the later Plio-Quaternary evolution. High-angle faults, tilted blocks of basement and asymmetric basins filled by syn-rift deposits reveal a predominant NW-SE extension in the study area during the Middle to Late Miocene. It is possible that these SW-NE trending structures had a significant left-lateral, strike-slip component, as they were oblique to the prevailing N-S convergence at this time. In this sector of the Alboran Sea Basin the rifting produced a limited amount of extension as is evidenced by the following features: (1) existence of several contrasting secondary extensional faults trending NW-SE to E-W, in addition to the main SW-NE extensional structures; (2) the scattered distribution of in-relay, short fault segments with both synthetic and antithetic configurations; and (3) Miocene depocentres are smaller than those in the western and northern parts of the basin and achieve a maximum thickness of 2.5-3.5 km.

Overall, the Alboran Sea Basin exhibits a multidirectional extensional system with radial pattern that mimics the curved shape of the entire orogen. Rifting began in the early Miocene in the west and north of the basin. However, syn-rift sequences in the eastern and southern basins (e.g. EAB, SAB) are dominated by Late Miocene sediments indicating that accommodation space from rifting was generated here later. It is also possible that part of the extension occurred while the central and southeastern sectors of the basin were almost at sea level or even raised above it. The end of rifting, which was nearly synchronous across the entire Alboran Basin, took place at about the mid Tortonian (9-7 Ma). Residual normal slip components of faults affecting the Plio-Quaternary sequences were limited for the most recent times and they probably derived from strike-slip structures.

At the end of the Miocene, during the Messinian salinity crisis, the region underwent deep desiccation as consequence of the closure of marine passages between the Atlantic Ocean and the Mediterranean Sea. Nevertheless, a number of residual shallow marine or lacustrine basins existed in the Alboran Sea region at this time, comparable to other marginal basins in the onshore Betics. During the late Messinian the SAB and the Alboran Channel were still connectd at the western end of the Alboran Ridge.

After rifting ceased during the Late Miocene, the Alboran Sea Basin underwent uniform thermal subsidence, which resulted in the accumulation of a thin, rather tabular layer of Plio-Quaternary sediments (< 500 m thick) on top of most of the inherited Miocene intrabasin highs. However, this residual thermal subsidence coexisted with shortening and some of these residual highs were reactivated and uplifted during the Plio-Quaternary. In these cases the Plio-Quaternary sequences on top of highs are folded and show onlapping of growth strata onto the crest of folds and thickening of sedimentary sequences away from these structures.

The reactivation and shortening of some of the old extensional-related Miocene structures document a partial tectonic inversion process in the Alboran Sea Basin. Seismic profiles and structural maps reveal that several NE-SW reverse faults and associated folds have undergone inversions of various magnitudes at different times. These structures occur mainly along the Alboran Ridge and Habibas Basin. In most cases, tectonic inversion was partial and shortening was unable to compensate the larger amount of earlier Miocene extension. Maximum extension of up to 8 km has been recorded in the SAB, EAB and the easternmost part of the WAB and it was mainly caused by Miocene rifting events. In contrast, the calculated Recent shortening is considerably less reaching maximum values of about 2 km since the Late Pliocene and being mainly accommodated in the Alboran Ridge region. This fact explains that the location of Plio-Quaternary depocentres coincides roughly with those realms that underwent considerable Miocene extension, where there is still residual accommodation space favouring recent subsidence and compaction.

In addition to the structures deriving from positive tectonic inversion, the Alboran Sea Basin comprises some other examples of partial reactivation of Miocene normal faults in the form of strike-slip structures during the Plio-Quaternary. The WNW-ESE, Yusuf fault zone is the clearest example of this type of structures. Reactivation has occurred mainly along the eastern sector of the Yusuf fault zone, where the Plio-Quaternary right-lateral strike-slip fault planes with small vertical-slip component are rooted on high-angle conjugate-convergent normal faults, which limits a thick Miocene sediment accumulation. Nevertheless, strike-slip faults along the western part of this fault zone, i.e., along the Yusuf Escarpment, are not linked to previous extensional structures, indicating that strike-slip reactivation of Miocene faults was also partial.

Plio-Quaternary tectonic inversion and fault reactivation in the Alboran Sea Basin only affected some of the structures or individual fault segments that form the Miocene extensional system. At least two main factors controlled this selective reactivation: (1) the orientation of previous structures in relation to the renewed NW-SE convergence vector between the Eurasian and African plates; (2) the existence of previous volcanic intrusions, which are aligned with some of the reactivated fault segments and constitute rheological anisotropies within the crustal basement.

The distribution and orientation of Plio-Quaternary to active structures in the central and south-eastern part of the Alboran Sea Basin resembles those along the Algerian margin to the east. The entire region comprises different NE-SW trending segments about 125-180 km in length, which run roughly perpendicular to the present-day vector of convergence between the Eurasian and African plates and accommodate most of shortening by means of reverse faulting and folding. These segments are displaced by a number of NW-SE to WNW-ESE right-lateral, strike-slip fault zones that act as transfer faults connecting the different reverse NE-SW segments. The whole system resulted from the inversion and reactivation of previous extensional-related structures that were formed during the Miocene phase of basin opening in the westernmost Mediterranean.

At present-day, continent-young ocean plate convergence occurs along the foot of the continental slope of the Algerian margin. Compressional and strike-slip structures along this region connect towards the west with the wide band of deformation that comprises the Alboran Ridge and Yusuf fault zones in the Alboran Sea Basin. A stronger coupling within the Alboran Sea Basin may be surmised, as crustal basement is continental at both sides of the collisional band. In this case the buoyancy of continental lithosphere obstructs the onset of a new subduction or obduction and deformation is widely distributed affecting to both blocks under collision and extending northwards and southwards across the Betics and Rif.