Microstructural and quartz crystal fabric evidence for distributed WNW-NW directed ductile shear during the Caledonian orogeny, Ross of Mull, Scotland

Surface rupturing during the 28 June 1992, Landers, California earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en echelon fault zones with a total length of about 80 km. The offsets were accommodated generally not by faults -- distinct slip surfaces -- but rather by shear zones, tabular bands of localized shearing. In long, straight stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even-narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few m wide) shear zones that accommodate most of the offset of the belt and are portrayed by en echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones, and some of these were faults. Although the narrower shear zones probably are indicators to right-lateral fault segments at depth, the surface rupturing during the earthquake is characterized not by faulting, but by zones of shearing at various scales. Furthermore, understanding of the formation of the shear zones may be critical to understanding of earthquake faulting because, where faulting is associated with the formation of a shear zone, the faulting occurs late in the development of the shear zone. The faulting occurs after a shear zone or a belt of shear zones forms.

Surface rupturing during the 28 June 1992 Landers, California, earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en-echelon fault zones with a total length of 80 km. The offsets were accommodated generally not by faults—distinct slip surfaces—but rather by shear zones, tabular bands of localized shearing. Along simple stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with clearly defined, subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few meters wide) shear zones that accommodate most of the offset of the belt and are portrayed by en-echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left-lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones. Although these probably are guides to right-lateral fault segments below, the surface rupturing during the earthquake is characterized not by faulting, but by the formation of shear zones at various scales.

Sensitivity of shear zones in orogenic wedges to surface processes and strain softening

Synkinematic quartz veins are ubiquitous in the shear zone separating the Veleta unit from the Calar Alto unit in the internal part of the Betic Cordilleras. They have been studied with respect to quartz c-axis fabrics, microstructures and fluid inclusions. Veins were probably generated during syn-metamorphic stacking of the units at P = 500 − 600 MPa and T = 400 − 500°C. Quartz displays two groups of microstructures in the shear zone: (1) older coarse-grained mosaics (CGM) resulting from exaggerated grain growth; and (2) younger fine-grained mosaics (FGM) developed at the expense of the former. The fine-grained mosaics show polygonal granoblastic and elongate mosaic microstructures in general, with ribbon microstructures often found near the boundary of the units. Fluids contained in secondary inclusions vary from high salinity brines to different types of CO2—brine mixtures and low density CO2 fluids. Differences in composition and P-T trapping conditions are indicated for the different types of inclusions. Some fluid inclusions are older than the FGM, whereas others are younger, thus constraining the P- T conditions at which the two microstructural events took place. Fluid inclusion evidence suggests conditions of Pfluid > 170 MPa and T ≧ 370−430°C for the CGM and Pfluid ≧ 20−80 MPa and T > 340°C for the FGM. The quartz c-axis fabrics dealt with here correspond to the second recrystallization event, as little evidence of older fabrics is preserved in the shear zone. C-axis patterns vary across the shear zone from slightly asymmetrical type I crossed girdles in the hanging wall and footwall to more asymmetrical crossed girdles at the boundary of the units. This indicates a correlative increase in the magnitude of the heterogeneous shear strain in the same direction. Most of the deformation is concentrated at the top of the Veleta unit. The sense of movement is top to the west, in agreement with other kinematic markers. The quartz c-axis fabrics resulted from dynamic recrystallization during simple shear. The retrograde P-T path inferred from fluid inclusion analysis, along with other geological and geochronological evidence, indicates that this deformation is coeval with a reduction in the crustal overburden. Geochronological and stratigraphic data show that the proposed Dos Picos extensional detachment, separating the Calar Alto and Veleta units, took place during the early Miocene, synchronous with the intense thinning of the Nevado-Filabride Complex and of the whole continental crust underlying the Alboran Basin.

This study explores the state of finite strain and changes in the mean kinematic vorticity number, grain size, whole-rock chemistry and mineralogy across an upper amphibolite-facies shear zone in a metadiorite, northern Malawi, east-central Africa. P–T conditions during shear-zone formation and deformation were approximately 700–750 °C and 5–7 kbar and are slightly less than P–T conditions for the regional peak of metamorphism. The major rock-forming minerals, plagioclase, hornblende, biotite, and quartz, were deformed by crystal-plastic processes accompanied by, except for hornblende, dynamic recrystallization. The modal abundance of all four major rock-forming minerals shows no systematic change from the country rock into and across the shear zone, indicating that shear-zone development was not associated with retrograde mineral reactions. The grain size of the major rock-forming minerals decreases within the shear zone. Plagioclase and hornblende, which occur as porphyroblasts outside the shear zone, exhibit a bimodal grain-size distribution within the shear zone. Quartz has a unimodal grain-size distribution in the shear zone. Major and trace element chemistry does not change systematically across the shear zone, implying no volume change in the mylonite. Matrix strain data for plagioclase and hornblende by the Fry method and fabric strain as deduced from Rf/φ analysis of plagioclase and quartz grains demonstrate a slightly constrictional strain type (K≈1.5) across the shear zone. The quantitative finite-strain data for the different residual minerals as obtained by unlike methods show no systematic variation, but recrystallized plagioclase grains record higher strain than the residual grains. The mean kinematic vorticity number changes from approximately 0.3 outside to approximately 0.8 within the shear zone, indicating that the bulk deformation path deviated from progressive simple shear. The estimates for finite strain and the degree of noncoaxiality account for approximately 50% of thinning normal to the shear zone.

Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

The Garson Ni–Cu–platinum group element deposit is a deformed, overturned, low Ni tenor contact-type deposit along the contact between the Sudbury Igneous Complex (SIC) and stratigraphically underlying rocks of the Huronian Supergroup in the South Range of the 1.85-Ga Sudbury structure. The ore bodies are coincident with steeply south-dipping, north-over-south D1 shear zones, which imbricated the SIC, its ore zones, and underlying Huronian rocks during mid-amphibolite facies metamorphism. The shear zones were reactivated as south-over-north, reverse shear zones during D2 at mid-greenschist facies metamorphism. Syn-D2 metamorphic titanite yields an age of 1,849 ± 6 Ma, suggesting that D1 and D2 occurred immediately after crystallization of the SIC during the Penokean Orogeny. The ore bodies plunge steeply to the south parallel to colinear L1 and L2 mineral lineations, indicating that the geometry of the ore bodies are strongly controlled by D1 and D2. Sulfide mineralization consists of breccia ores, with minor disseminated sulfides hosted in norite, and syn-D2 quartz–calcite–sulfide veins. Mobilization by ductile plastic flow was the dominant mechanism of sulfide/metal mobilization during D1 and D2, with additional minor hydrothermal mobilization of Cu, Fe, and Ni by hydrothermal fluids during D2. Metamorphic pentlandite overgrows a S1 ferrotschermakite foliation in D1 deformed ore zones. Pentlandite was exsolved from recrystallized polygonal pyrrhotite grains after cessation of D1, which resulted in randomly distributed large pentlandite grains and randomly oriented pentlandite loops along the grain boundaries of polygonal pyrrhotite within the breccia ore. It also overgrows a S2 chlorite foliation in D2 shear zones. Pyrrhotite recrystallized and was flattened during D2 deformation of breccia ore along narrow shear zones. Exsolution of pentlandite loops along the grain boundaries of these flattened grains produced a pyrrhotite–pentlandite layering that is not observed in D1 deformed ore zones. The overprinting of the two foliations by pentlandite and exsolution of pentlandite along the grain boundaries of flattened pyrrhotite grains suggest that the Garson ores reverted to a metamorphic monosulfide solid solution at temperatures ranging between 550 and 600 °C during D1 and continued to deform as a monosulfide solid solution during D2.

The Neoproterozoic Moine Supergroup in the Caledonides of northern Scotland is disposed in a series of thrust nappes. The Skinsdale Thrust in East Sutherland separates Moine rocks of the Loch Coire Migmatite Complex, which underwent anatexis during the Ordovician Grampian phase of the Caledonian orogeny, from the generally unmigmatized Scaraben succession. The Strath Halladale Granite occurs as an east-dipping sheeted complex that cuts discordantly at a low angle across the thrust. The pluton carries a magmatic foliation that was reworked by solid-state deformation at high to moderate temperatures; shear zones within the pluton display top-to-the-NW sense of shear similar to that deduced for the Skinsdale Thrust. Granite sheets are deformed by curvilinear folds that developed during ductile thrusting. Field and petrographic evidence is consistent with a thrust-related mode of emplacement. U–Pb monazite geochronology yields a crystallization age for the granite of 426 ± 2 Ma. This is interpreted to broadly date displacement along the Skinsdale Thrust, which therefore is of similar age to other NW-directed Silurian ductile thrusts within this sector of the Caledonides. These findings substantiate a model involving Ordovician (Grampian) deformation and metamorphism followed by Silurian (Scandian) thrust-related reworking and emplacement of nappes onto the Laurentian foreland.

We document newly recognized Late Ordovician high-pressure (HP) metamorphism in the Scottish Caledonides. Garnet growth at ca. 455−445 Ma has been previously reported from across the Northern Highland Terrane (NHT) and Shetland, yet the metamorphic conditions are unknown and the tectonic drivers associated with this event are controversial. Here we show that garnets dated at ca. 449 Ma within metabasic rocks on the Ross of Mull (southwestern peninsula of the Isle of Mull, Scotland, UK, within the NHT) grew at pressures >0.9 GPa, associated with the formation of kyanite-bearing assemblages in meta-pelites that equilibrated at peak conditions of 1.0−1.3 GPa and 740−780 °C. This requires the burial of rock to ∼40−45 km depth, and the addition of this (now removed) overburden suggests that the crust reached ∼70 km in thickness, enough to support a region of high topography ∼6 km in elevation. The timing of this crustal thickening episode post-dated Late Cambrian−Early Ordovician Grampian arc-continent collision, but predated the final Silurian closure of the Iapetus Ocean, south of the Midland Valley. The presence of similar garnet-bearing amphibolites dated at ca. 455−445 Ma on the Scottish north coast and Shetland suggests that this period of HP metamorphism was a regional feature across the NHT and Shetland. It was followed by Scandian nappe stacking and lower pressure metamorphism at ca. 444−415 Ma, potentially forming a single protracted orogenic phase prior to the final closure of Iapetus. There are several potential drivers for the Late Ordovician event, including (1) subduction flip south of the Midland Valley Terrane to NW-directed subduction followed by collision of cryptic outboard terranes and/or Baltica followed by large magnitude sinistral strike slip along the Great Glen Fault, (2) continued SE-directed subduction and collision of the Midland Valley terrane with Laurentia, or (3) subduction-flip followed by NW-directed flat slab subduction causing protracted accretionary orogenesis without necessarily requiring either collision with Baltica or large-scale strike slip displacements along the Great Glen Fault. Irrespective of the preferred tectonic model, the climax of Caledonian orogenesis in Scotland predated terminal continental collision.

Pure shear-dominated transpression and vertical extrusion in a strike-slip fault splay from the Itapirapuã Shear Zone, Ribeira Belt, Brazil

In the Denbigh Moors area of North Wales some of the Silurian sedimentary rocks are intensely disrupted. Recently the British Geological Survey has confirmed that this is largely the result of extensive slumping of the sediments, which was part of a long sequence of deformation events ranging from what they called ‘penecontemporaneous movements’ and related ‘additional processes’ to at least three periods of regional tectonism. New knowledge of the microstructural behaviour of deforming sediments obtained from laboratory studies allows the prospect of assigning field structures to a position within this long scenario and a better understanding of the complex pre-lithification deformation processes. Experimental deformation of argillaceous sediments has shown that they deform, over a wide range of water content, by intense slip within narrow shear zones rather than by pervasive grain slip. Shear zones of strikingly similar appearance are common in parts of the Denbigh Moors succession, and, being demonstrably pre-diagenetic, can be used as indicators of pre-lithification disturbance. The shear zones give a polished, finely striated, and crenulated appearance to many of the exposure surfaces in the area. The diverse orientations of the zones reflect to some extent different locations within the sliding sedimentary masses. The style of the zones gives an indication of the water content of the sediment and hence the burial depth at the time of deformation. Some beds show features such as truncated upper contacts and are therefore thought to have been disturbed before lithification, yet the rocks lack shear zones. These are interpreted as having been deformed while very near surface, with a water content too high for the shear zone mechanism to have operated. The possibility remains to be explored that some of the shear zones in the area are the result of regional tectonism acting on unlithified sediments.

The development of discrete shear-zones in a granite: stress, strain and changes in deformation mechanisms

A series of tests was conducted in a plane-strain hollow cylindrical testing device. The reinforced sand samples developed wider shear zones than the unreinforced samples and had wider shear zones than similarly reinforced samples tested in direct-shear devices. Shear-zone width increased with increasing reinforcement concentration, reinforcement stiffness, and soil-reinforcement bond strength. Reduction of displacement data to a strain basis reveals that shear, elongational, compressional, and volumetric strains differ in unreinforced and reinforced samples. Dense, dilatant unreinforced sand samples formed narrow shear zones with large strains, whereas the reinforced samples had smaller but more uniform strains distributed throughout the samples. Reorientation of zero-extension shear planes in the reinforced samples limited the range of orientations over which the reinforcements contributed to strength. Overall, the results indicate that reinforced soils deforming in uniform stress fields do not develop narrow shear zones, and, therefore, reinforcement contribution to strength through reorientation and mobilization of bending moments is unlikely under field conditions. To insure that reinforcements are loaded in tension, they should be oriented well within 45° of the minor principal stress.

Synkinematic interplay between felsic dykes and host rock mylonitization: how magmatism assists the formation of ductile narrow shear zones in the Sierra Chica de Córdoba, Argentina

Repeated granitoid intrusions during the Neoproterozoic along the western boundary of the Saharan metacraton, Eastern Hoggar, Tuareg shield, Algeria: An AMS and U–Pb zircon age study