Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic–Rif arc
, Negro, François, Agard, Philippe, Goffé, Bruno, Saddiqi, Omar
Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP–LT) metamorphism (at c. 7–9 kbar and 330–430 °C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east–west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP–LT Temsamane units: (1) an internal origin related to the subduction and the HP–LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic–Rif region.
Fracture characterisation using geoelectric null-arrays
, Falco, Pierik, Negro, François, Szalai, Sándor, Milnes, Ellen
The term “geoelectric null-array” is used for direct current electrode configurations yielding a potential difference of zero above a homogeneous half-space. This paper presents a comparative study of the behaviour of three null-arrays, midpoint null-array (MAN), Wenner-γ null-array and Schlumberger null-array in response to a fracture, both in profiling and in azimuthal mode. The main objective is to determine which array(s) best localise fractures or best identify their orientation.
Forward modelling of the three null-arrays revealed that the Wenner-γ and Schlumberger null-arrays localize vertical fractures the most accurately, whilst the midpoint null-array combined with the Schlumberger null-array allows accurate orientation of a fracture. Numerical analysis then served as a basis to interpret the field results. Field test measurements were carried out above a quarry in Les Breuleux (Switzerland) with the three null-arrays and classical arrays. The results were cross-validated with quarry-wall geological mapping. In real field circumstances, the Wenner-γ null-array proved to be the most efficient and accurate in localising fractures. The orientations of the fractures according to the numerical results were most efficiently determined with the midpoint null-array, whilst the Schlumberger null-array adds accuracy to the results. This study shows that geoelectrical null-arrays are more suitable than classical arrays for the characterization of fracture geometry.
Thermal structure and metamorphic evolution of the Piemont-Ligurian metasediments in the northern Western Alps
, Negro, François, Bousquet, Romain, Vils, Flurin, Pellet, Clara-Marine, Hänggi-Schaub, Jeanette
In the Western Alps, the Piemont-Ligurian oceanic domain records blueschist to eclogite metamorphic conditions during the Alpine orogeny. This domain is classically divided into two “zones” (Combin and Zermatt-Saas), with contrasting metamorphic evolution, and separated tectonically by the Combin fault. This study presents new metamorphic and temperature (RSCM thermometry) data obtained in Piemont-Ligurian metasediments and proposes a reevaluation of the P–T evolution of this domain. In the upper unit (or “Combin zone”) temperatures are in the range of 420–530 °C, with an increase of temperature from upper to lower structural levels. Petrological evidences show that these temperatures are related to the retrograde path and to deformation at greenschist metamorphic conditions. This highlights heating during exhumation of HP metamorphic rocks. In the lower unit (or “Zermatt-Saas zone”), temperatures are very homogeneous in the range of 500–540 °C. This shows almost continuous downward temperature increase in the Piemont-Ligurian domain. The observed thermal structure is interpreted as the result of the upper and lower unit juxtaposition along shear zones at a temperature of ~500 °C during the Middle Eocene. This juxtaposition probably occurred at shallow crustal levels (~15–20 km) within a subduction channel. We finally propose that the Piemont-Ligurian Domain should not be viewed as two distinct “zones”, but rather as a stack of several tectonic slices.
Stochastic fracture generation accounting for the stratification orientation in a folded environment based on an implicit geological model
2015, Borghi, Andrea, Renard, Philippe, Fournier, L, Negro, François
This paper presents a new approach in generating stochastic discrete fracture networks. The particularity of the approach is that it allows us to simulate the theoretical families of fractures that are expected in a folded environment. The approach produces fractures that are consistent with the local stratigraphic orientation. The fractures are modeled as simple rectangular planar objects. When they are modeled, they are rotated according to the local stratigraphic orientation. As the stratigraphy is modeled using an implicit approach, we use the gradient of this geological potential ﬁeld to retrieve the information about the geological orientation. The fracture number and size are following user-deﬁned probability density functions.
Tectonic evolution of the Betic–Rif arc: New constraints from 40Ar/39Ar dating on white micas in the Temsamane units (External Rif, northern Morocco)
, Negro, François, De Sigoyer Julia, Goffé, Bruno, Saddiqi, Omar, Villa, Igor M
The Betic–Rif orogen, at the western end of the Mediterranean, is a key region to improve our knowledge on the Africa–Eurasia convergence. The Temsamane units, in spite of their external position in the Rif (northern Morocco), underwent medium-pressure low-temperature (MP–LT) conditions (ca. 7–9 kbar; 330–430 °C). We propose a new tectonic and metamorphic evolution scenario for the Rif (southern) branch of the orogen on the basis of first 40Ar/39Ar dating on petrologically and structurally characterized white micas. Three groups of 40Ar/39Ar ages are observed: (1) Chattian or older Si-rich (highest-pressure) mica relics, (2) 15–12.5 Ma corresponding to the micas defining the foliation and (3) Messinian or younger late micas and alteration. We propose that the MP–LT metamorphic event in the External Rif is Oligocene in age, highlighting a subduction event during this period which could be almost contemporaneous with the burial of HP–LT units from the internal Rif (Alboran Domain). The exhumation of these units characterized by an intense E–W stretching and by top-to-the-west shear senses, is Middle to Late Miocene in age. We propose a correlation of tectonic and metamorphic events at the Betic–Rif arc scale. We argue that the exhumation of the external units of the Rif (1) is younger than that of the Alboran Domain (internal) unit of the Rif, and mirrors a different tectonic setting, but (2) strongly resembles to that documented in the lower Alboran Domain units of the Betics. We show that a regional E–W extension is recorded on both sides of the Betic–Rif arc during the Middle Miocene. This extension probably reflects back-arc deformation of an eastward dipping subduction that retreated westward during the Middle to Late Miocene in the Western Mediterranean.
Illite crystallinity patterns in the Anti-Atlas of Morocco
2008, Ruiz, Geoffrey M. H., Helg, Urs, Negro, François, Adatte, Thierry, Burkhard, Martin
The low-grade metamorphism of the sedimentary cover of the Moroccan Anti-Atlas is investigated using Illite crystallinity (IC) method. More than 200 samples from three key areas (southwestern, central and eastern Anti-Atlas) have been taken from a maximum of different stratigraphic levels and have been analysed. The metamorphism is of low to very low degree throughout the southern flank of the Anti-Atlas. It increases from northeast to southwest. Whereas in the eastern Anti-Atlas diagenetic and anchizonal IC-values are predominant, in the western and central Anti-Atlas also epizonal IC-values are found. In every respective area the IC improves with stratigraphic age. At the scale of the entire Palaeozoic Anti-Atlas basin the IC correlates best with estimated paleo-overburden. However, burial metamorphism cannot be the cause even though considering missing sedimentary pile of Late Carboniferous age. The ‘abnormal’ paleo-geothermal gradient of 43–35 °C/km we evidenced for the Carboniferous is a true one, and has to be related to a basement sequence enriched in heat producing elements such as series of the West African Craton.
High-pressure metamorphism in Taiwan: from oceanic subduction to arc-continent collision?
, Beyssac, Olivier, Negro, François, Simoes, Martine, Chan, Yu-Chen, Chen, Yue-Gau
The Taiwan orogen has been the focus of a number of models of mountain building processes, but little attention has been paid to high-pressure (HP) metamorphic rocks that are found as exotic blocks intermingled within the deepest units of the mountain belt. In this study, we re-appraise from updated petrological and thermodynamic databases the physical conditions of HP metamorphism in Taiwan, and we combine our findings with available geochronological data to estimate the thermal history of these rocks. Our results indicate that peak metamorphic conditions of 550 C and 10–12 kbar have been followed by a rapid isothermal decompression, with exhumation possibly as rapid as burial. These units have subsequently been stored at a pressure of 3 kbar for 4–5 Myr, before their final exhumation, probably facilitated by the accretion of passive margin sequences during the Late Cenozoic collision. Therefore, HP units in Taiwan maintain a record of processes at depth from the early stages of oceanic subduction to the present arc-continent collision.
Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material
, Negro, François, Beyssac, Olivier, Goffé, B, Saddiqi, O, Bouybaouène, L
In the Rif (northern Morocco) and the Western Betics (southern Spain), the Alboran Domain forms a complex stack of metamorphic nappes including mantle peridotites (Beni Bousera and Ronda). We present in this paper new temperature data obtained in the Alboran Domain based on Raman spectroscopy of carbonaceous material (RSCM thermometry). In the lower metamorphic nappes of the Alboran Domain (lower Sebtides–Alpujárrides) temperature ranges from > 640 °C at the base of the metapelitic sequence to 500 °C at the top. The relationships between field isotherms and nappe structure show that peak temperatures were reached during strong ductile thinning of these nappes whereas they partly postdate this main episode in the Rif. In the upper nappes of the Alboran Domain (Ghomarides–Maláguides), generally supposed to be only weakly metamorphosed, temperatures range from ~500 °C at their base down to < 330 °C at the top. This temperature gradient is consistent with progressive Cenozoic resetting of K–Ar and 40Ar–39Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides–Alpujárrides show that this metamorphism is related to the metamorphic evolution of the whole Alboran Domain during the Late Oligocene–Early Miocene. Such thermal structure and metamorphic evolution can be explained by generalized extension in the whole Alboran Domain crustal sequence. At a larger scale, the present thermal structure of the Alboran Domain is roughly spatially consistent around the Beni Bousera peridotites in the Rif, but much more affected by late brittle tectonics around the Ronda peridotites in the Western Betics. Therefore, on the basis of the observed thermal structure, the metamorphic evolution of the Alboran Domain can be interpreted as the result of the ascent of hot mantle units contemporaneous with thinning of the whole lithosphere during an Oligo-Miocene extensional event. The resulting structure has however been dismembered by late brittle tectonics in the Western Betics.