Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece).

Is metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well-preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35 Ma while post-tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro-sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post-tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale

The competition between rates of deformation and solidification in syn-kinematic granitic intrusions: Resolving the pegmatite paradox

While fully-crystallized granites, rich in feldspar, generally serve to strengthen the continental crust, their precursor melts are assumed to be important agents of crustal weakening. Many syn-tectonic granitic pegmatites are deformed within shear zones but ubiquitously preserve undeformed primary magmatic textures, implying that they were largely molten during shearing. Yet the shapes of pegmatite bodies indicate that they deformed with a greater competence than their surroundings. This co-located pair of material behaviours is paradoxical. We interpret field relationships in a typical pegmatite/shear zone association (Torrisdale, NW Scotland) and propose a mechanism by which syn-tectonic granitic melts may, in effect, act as competent bodies while not yet fully crystallized. Competence was rapidly increased by preferential crystallization on intrusion margins that served to encapsulate residual melt inside stiff rinds. Further crystallization may have been pulsed as the concentrations of crystallization-inhibitors (fluxes) increased in residual fluids. Postulating the existence of initial stiff rinds also consistent with modern estimates for rates of feldspar crystallization (cms/yr) from undercooled hydrous silicic magma to form pegmatites. These greatly outpace strain-rate estimates for shear zones. Thus, fully liquid granitic melts may only be present fleetingly and have little opportunity to weaken deforming crust before crystallization begins

Testing thrust tectonic models at mountain fronts: where has all the displacement gone?

The alternative relationships that can exist between a mountain front and the adjacent foreland basin have been recognized for many years. However, seismic reflection data from such areas are commonly of poor quality and therefore structural models may contain large uncertainties. In view of scientific and commercial interest in mountain belts, we have reviewed the methods for discriminating between alternative interpretations using a case study from the Montagna dei Fiori in the central Apennines, Italy. In this area Mesozoic and Tertiary carbonate sediments are juxtaposed with a foredeep basin containing up to 7 km of Messinian and Plio-Pleistocene siliciclastic sediments. A new structural model for this area demonstrates how the structures in this area form a kinematically closed system in which displacement is transferred from the thrust belt to blind structures beneath the present-day foreland. Growth strata show that Pliocene shortening was initially rapid (15 mm a-1) followed by slower rates during the final stages of deformation. Variations in structural elevation indicate a component of basement involvement during thrusting, and this is further supported by magnetic modelling. The results illustrate the interaction of thin- and thick-skinned structures in the central Apennines, and the methods for discriminating between alternative structural models