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Migmatitic granulites and arc-related felsic intrusives of PanAfrican age form the bedrock in the Rio de Janeiro area, SE Brazil. These rocks preserve a partial record of three parageneses. The earliest assemblage (M1) grew during fabric formation in the rocks (D1) and is characterized by the mineral assemblage Pl þ Bt þ Sil þ Kfs þ Qtz. Peak metamorphic conditions (M2) are characterized by the assemblage Bt þ Crd þ Kfs þ Pl þ Grt þ liq þ Qtz and are inferred to have developed during D2 folding of the rocks at T 1⁄4 750–800 C and P1⁄4 7 kbar.M3 reaction textures overprint theM2 assemblage and comprise symplectitic intergrowth of cordierite(II) and quartz that formed after garnet, whereas secondary biotite formed as a result of reactions between garnet and K-feldspar. By comparing the observed modal abundances with modal contours of garnet, cordierite and quartz on the relevant pseudosection a post M2 P– T vector indicating contemporaneous cooling and decompression can be deduced. The inferred equilibrium assemblage and reaction textures are interpreted to reflect a clockwise P–T path involving heating followed by post-peak decompression and associated cooling. We infer that metamorphism occurred in response to advective heating by the abundant syn-collisional (arc-related) I-type granitoids in the region, consistent with the unusually high peak T/P ratio.

A subduction complex composed of ocean floor material mixed with arc-derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120–80 Ma and 58–47 Ma, respectively. Seven metamorphic zones (I–VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite–actinolite facies, through the crossite–epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnet–amphibole and garnet–biotite pairs yields temperatures of about 350 °C in zone III to about 525 °C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al2O3 content in alkali amphibole, NaM4/AlIV in sodic-calcic and calcic amphibole, AlVI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6–7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350 °C. Zoned minerals and other textural indications locally enable inference of P–T –t trajectories, all with a clockwise evolution. A reconstruction in space and time of these P–T –t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D1 & D2) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D1 represents the subduction movements expressed by the first vector of the clockwise P–T–t path, D2 reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D3 corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure.

Around the southern margins of the Sao Francisco Craton, there is a zone of tectonic interference between the Brasilia belt to the west and the younger Ribeira belt to the east. U-Pb monazite and Ar/Ar cooling age determinations carried out in the area reveal the cooling histories of these belts and the timing of tectonic overprint, unraveling the final stages of Brasiliano Orogeny in SE Brazil. The U-Pb monazite data from migmatized paragneisses and late-stage pegmatites in the Socorro-Guaxupe Nappe System of the southern Brasilia belt show that migmatization peaked between ca. 613±1 and 607±3 Ma. Ar/Ar biotite and muscovite ages of paragneisses and schists in this area indicate that the northern high-grade core of the Nappe System (Guaxupe Domain) was uplifted and cooled through the 350°C isotherm between 599±1 and 587±1 Ma. In contrast, samples from the southern high-grade core of the Nappe System, the Socorro Domain, south of the Jacutinga shear zone, yields a broader and younger spectrum of Ar/Ar biotite ages between 571±1 and 562±1 Ma, attributed to a later uplift and cooling of the crust. The cooling ages can be assigned to local resetting of the Ar/Ar system during transpressive tectonic overprint due to reactivation as a result of collision of the Ribeira belt. A younger group of Ar/Ar mica ages (537±1 to 521±1Ma) in schists of the Socorro Domain, are associated with transpressional structures of the Ribeira belt. Rock samples from the Jacutinga and Tres Coracoes shear zones, yield Ar/Ar biotite-muscovite ages around 520 Ma. These are typical cooling ages of the Ribeira belt, and are interpreted to mark the western limit of the Ribeira belt transpressional regime within the Brasilia belt. The youngest biotite-muscovite cooling ages in schists of the Socorro Domain, between 510±2 and 491±1 Ma, mark the final cooling and exhumation of that part of the Brasilia belt.

Many deformed rocks contain sites with a deviant mineralogy and fabric, interpreted as an effect of rearrangement of material by local dilatation and precipitation during deformation. Such ‘dilatation sites’ can be isolated and elongate (veins), flanking rigid objects (strain shadows) or occur in the neck of boudinaged layers or elongate crystals (Fig. 6.1). Strain shadows are also referred to in the literature as pressure shadows. Most veins and many strain shadows and boudin necks have sharp contacts with the wall rock and may form by precipitation of material from an aqueous solution in a fracture, as outlined below. Such sites are usually filled with polycrystalline material which may be massive, but commonly consists of rod-shaped crystals known as fibres (Figs. 6.1–6.3). Fibrous veins and strain shadows (the latter also known as strain fringes; Fig. 6.3) are some of the most complex microstructures to be found in rocks, and contain much information about deformation and deformation history (Figs. 6.1–6.3; Zwart and Oele 1966; Choukroune 1971; Durney and Ramsay 1973; Beutner and Diegel 1985; Etchecopar and Malavieille 1987). Strain fringes are also known as pressure fringes. Since the shape of the aggregates gives primarily information on strain distribution around an object, and not on forces, we advocate the use of the term strain fringe. The same applies to pressure shadow where we prefer the use of strain shadow. Some veins and strain shadows have fuzzy boundaries (Fig. 6.1). They may form by local alteration of the wall rock along a fracture (replacement veins) or rigid object, or by deformation and recrystallisation of veins with sharp boundaries (Chap. 6.5, 7.5 and 7.6.8).

A hunter who investigates tracks in muddy ground near a waterhole may be able to reconstruct which animals arrived last, but older tracks will be partly erased or modified. A geologist faces similar problems to reconstruct the changes in shape that a volume of rock underwent in the course of geological time, since the end products, the rocks that are visible in outcrop, are the only direct data source. In many cases it is nevertheless possible to reconstruct at least part of the tectonic history of a rock from this final fabric. This chapter treats the change in shape of rocks and the methods that can be used to investigate and describe this change in shape. This is the field of kinematics, the study of the motion of particles in a material without regard to forces causing the motion. This approach is useful in geology, where usually very little information can be obtained concerning forces responsible for deformation. In order to keep the discussion simple, the treatment is centred on flow and deformation in two dimensions.