R.L.M. Vissers

Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc

Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.

Extensional structures in anisotropic rocks

A distinct class of structures can form as a result of extension along a plane of anisotropy (foliation). The effect of the foliation is to decrease the ductility of the material in this orientation so that brittle fractures or shear-bands develop. Foliation boudinage is caused by brittle failure; extensional fractures cause symmetric boudinage, and shear fractures cause asymmetric boudinage. Extensional crenulation cleavage is defined by sets of small-scale ductile shear-bands along the limbs of very open microfolds in the foliation. The sense of movement on the shear-bands is such as to cause a component of extension along the older foliation. Conjugate cleavage sets indicate coaxial shortening normal to the foliation; the shortening axis bisects the obtuse angle between the sets. A single set indicates oblique or non-coaxial deformation. Extensional crenulation cleavage is microstructurally and genetically distinct from other types of cleavage. It does not occur as an axial plane structure in folds, and has no fixed relationship to the finite strain axes. It is common in mylonite zones, and may be favoured by crystal-plastic and cataclastic deformational mechanisms. These cause grain-size reduction, and hence softening, which favour the development of shear-bands.

Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view

The Betic Cordillera of southern Spain provides a clear example of a collisional orogen that has undergone large-scale extensional collapse while convergent motion of the bounding plates continued. Extension was accommodated by coeval shortening in thin-skinned fold and thrust belts around the periphery of the system, and much of the region has now subsided to form a large marine basin. The thermal and deformational record of these processes is preserved in rocks from the upper mantle, crystalline crust, and sedimentary cover. Upper mantle peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at midlithospheric depths reflects continental convergence, and subsequent heating indicates loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. Crustal rocks in the internal zone of the Betic Cordillera were metamorphosed down to 50 km depth and are now exposed beneath major low-angle normal detachment zones that separate them from heavily faulted low-grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Fault-bounded intramontane basins, developed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal fault systems, followed by a less deformed late Miocene marine succession. All of these phenomena can be explained by convective removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large-scale extension followed by thermal subsidence of the center of the system.

Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation

The western Mediterranean recorded subduction rollback, slab segmentation and separation. Here we address the questions of what caused Oligocene rollback initiation, and how its subsequent evolution split up an originally coherent fore arc into circum-southwest Mediterranean segments. We kinematically reconstruct western Mediterranean geology from subduction initiation to present, using Atlantic plate reconstructions as boundary condition. We test possible reconstructions against remnants of subducted lithosphere imaged by seismic tomography. Transform motion between Africa and Iberia (including the Baleares) between ~120 and 85 Ma was followed by up to 150 km convergence until 30 Ma. Subduction likely initiated along the transform fault that accommodated pre-85 Ma translation. By the ~30 Ma inception of rollback, up to 150 km of convergence had formed a small slab below the Baleares. Iberia was disconnected from Sardinia/Calabria through the North Balearic Transform Zone (NBTZ). Subduction below Sardinia/Calabria was slightly faster than below the Baleares, the difference being accommodated in the Pyrenees. A moving triple junction at the trench-NBTZ intersection formed a subduction transform edge propagator fault between the Baleares and Calabria slab segments. Calabria rolled back eastward, whereas the Baleares slab underwent radial (SW-S-SE) rollback. After Kabylides-Africa collision, the western slab segment retreated toward Gibraltar, here reconstructed as the maximum rollback end-member model, and a Kabylides slab detached from Africa. Opening of a slab window below the NBTZ allowed asthenospheric rise to the base of the fore arc creating high-temperature metamorphism. Western Mediterranean rollback commenced only after sufficient slab-pull was created from 100 to 150 km of slow, forced subduction before ~30 Ma.

Deformation processes in a peridotite shear zone: reaction-softening by an H2O-deficient, continuous net transfer reaction

The Turon de Tecouere peridotite, in the North Pyrenean Zone, is composed of protomylonites grading to a 20–40 m wide zone of ultramylonites within a 0.6 km diameter exposure. The progressive mylonitization is marked by increasing volume fractions of very fine-grained matrix that comprise up to 90% of the ultramylonite. Deformation of the fine-grained matrix took place by grain size sensitive creep, as suggested by a very fine grain size (<10 μm), lack of dislocations in matrix grains, a weak crystallographic preferred orientation, and the alignment of grain boundaries parallel to the foliation. As the percentage of fine-grained matrix increased, weakening and localization resulted from a change in the dominant deformation mechanism from dislocation creep in the porphyroclasts to grain size sensitive creep in the fine-grained matrix. Production of the matrix grains took place by the nucleation of a number of different phases at the margins of porphyroclasts, indicating that the grain size reduction resulted primarily from reaction, and not from dynamic recrystallization. The nucleation of many phases along a single porphyroclast margin can be explained by a syntectonic continuous net transfer reaction associated with the spinel- to plagioclase-lherzolite transition. This continuous net transfer reaction produced new matrix grains with the same mineralogy as the original assemblage (olivine, orthopyroxene, clinopyroxene, spinel), with new compositions, plus plagioclase. Preliminary geothermobarometry indicates that the reaction took place over a range of temperatures and pressures (750–850°C, and possibly as high as 950°C and 0.5–1.1 GPa). The presence of only small amounts of amphibole, the lack of primary fluid inclusions, and no relation between the presence of amphibole and the intensity of mylonitic deformation led Vissers et al. [Tectonophysics 279 (1997) 303–325] to conclude that the deformation took place in an H2O-deficient environment. Reaction-enhanced softening may occur in the upper mantle wherever rocks move in pressure–temperature space and cross-reaction boundaries. Reaction boundaries are often modeled as univariant (lines in pressure–temperature space), yet mantle minerals are solid solutions so that reactions are continuous (multivariant) and take place over a broader region of pressure–temperature space than end-member reactions. It is therefore likely that shear zone deformation in polymineralic rocks will involve reaction-enhanced ductility over much of pressure–temperature space in the lithospheric mantle.

Mantle shear zones and their effect on lithosphere strength during continental breakup

Abstract The Erro-Tobbio lherzolite in the ophiolitic Voltri Massif of northwestern Italy includes several thrusted fragments of lithospheric mantle which were first exhumed to the ocean floor during Jurassic rifting and breakup, and at a later stage became emplaced in the Alpine collisional stack during Tertiary convergence between Africa and Europe. Coherent slices of these mantle rocks contain several sets of major shear zones generated during the Jurassic rift evolution. One such shear zone, several kilometres wide and formed at temperatures between 920 and 1040°C, is transected by up to 200 m wide, ultra-fine-grained hydrated mylonite zones formed at temperatures in the range 990-550°C. All these structures are cut by MORB-type gabbroic and basaltic dykes. The microstructures of the mylonite zones are interpreted to reflect progressive, reaction-related grainsize reduction plus localization of the deformation during the early stages of continental breakup. In view of experimental evidence that wet olivine rocks weaken considerably with decreasing grainsize, in response to a change from grainsize-insensitive dislocation creep to grainsize-sensitive creep mechanisms, it is proposed that shear localization and allied grainsize reduction may have resulted in a drastic decrease in strength of the upper mantle during rifting. In order to obtain an order of magnitude estimate of this rheological effect, we present a layered rheological model of the Piemonte-Ligurian lithosphere, based on the observed microstructures (i.e., grainsizes) and pressure-temperature data, and including appropriate rheological laws for grainsize-sensitive and -insensitive creep in wet olivine. The model calculations suggest strength values for the uppermost mantle up to four orders of magnitude lower than those expected for homogeneous deformation exclusively controlled by dislocation creep of dry olivine.

Uplift and emplacement of upper mantle rocks in the western Mediterranean

The emplacement of mantle peridotites at crustal levels in southern Spain and northern Morocco has traditionally been explained by mantle diapirism in an area now occupied by the Alboran Sea. However, such models are inconsistent with many of the internal features of the peridotite and cannot be related directly to the tectonics of the western Mediterranean. The structure and mineral chemistry of the Ronda peridotite combined with other geological and geophysical data indicate that the peridotite represents lithospheric mantle from the hanging wall of a Late Cretaceous to Paleogene zone of lithosphere-scale underthrusting (subduction), exhumed in the early Neogene in response to detachment of cold, gravitationally unstable lower lithosphere.

Shear localisation in upper mantle peridotites

Upper mantle peridotite bodies at the earths surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle. Deformation which occurred at high temperatures (T>950±50°C) is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres. Below 950±50°C strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites. The microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes. The most important strain softening processes are inferred to be structural and reaction induced softening. Structural softening processes include dynamic recrystallisation and strain-induced transitions from dislocation creep to some form of grain-size-sensitive (GSS) creep. Reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by GSS creep and the formation of weak sheet silicates such as phlogopite, chlorite, talc and antigorite. From experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910±160°C. This temperature range is inferred to be a significant rheological transition in the upper mantle. Below 910±160°C deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct, perhaps weak, rheology. At higher temperatures strain is accommodated in much wider deformation zones.On the scale of the lithosphere the degree of localisation may be different to that determined at the scale of the periodotite massif. An anastomosing network of hundred metre wide mylonitic shear zones forming 0.05–0.3 by volume fraction of the mantle lithosphere atT<950°C could accommodate inhomogeneous or homogeneous bulk deformation depending on the spatial distribution and ordering of the mylonite zones. The higher temperature deformation at deeper levels in the mantle could be markedly inhomogeneous being concentrated in shear zones with widths in the range of 2–20 km, alternatively these zones may widen significantly during deformation, resulting in a decrease in the degree of localisation with increasing bulk strain.

On the role of melt-rock reaction in mantle shear zone formation in the Othris Peridotite Massif (Greece)

A 1-km-wide peridotite mylonite shear zone is exposed in the Othris peridotite massif in central Greece. The mylonites contain lenses of relatively coarse olivine crystals, which are interpreted as remnants of the tectonite microstructure in the adjacent wall rocks. Microstructure and texture analysis using light and SEM microscopy suggests that the dominant deformation mechanism in the tectonites was dislocation creep, whereas the deformation in the mylonites was probably controlled by grain-size sensitive (GSS) creep in fine-grained (<50 μm) bands consisting of a mixture of olivine and orthopyroxene. The development of the fine-grained material in the mylonites can be explained by a melt-present reaction taking place in the tectonite protolith. This reaction led to the replacement of orthopyroxene porphyroclasts by fine-grained olivine and orthopyroxene. Tectonites adjacent to the mylonite zone preserve evidence for this reaction in the form of rims of fine-grained olivine and orthopyroxene around orthopyroxene porphyroclasts. This study illustrates the significance of rheological weakening of oceanic mantle lithosphere as a result of a change from dislocation to GSS creep.

Shear zones in the upper mantle: A case study in an Alpine Iherzolite massif

The Erro-Tobbio upper-mantle lherzolite of northwest Italy, emplaced in the Alpine suture zone during collision of the European and Adriatic plates, has preserved a set of major shear zones formed at temperatures in the range 800-1040 °C during the period of Jurassic rifting and breakup and development of the Piemonte-Ligurian oceanic basin. These structures clearly demonstrate localization of the deformation in the Piemonte-Ligurian upper mantle during lithosphere stretching and breakup.