Michel Dubois

Formation of chlorite during thrust fault reactivation. Record of fluid origin and P–T conditions in the Monte Perdido thrust fault (southern Pyrenees)

The chemical and isotopic compositions of clay minerals such as illite and chlorite are commonly used to quantify diagenetic and low-grade metamorphic conditions, an approach that is also used in the present study of the Monte Perdido thrust fault from the South Pyrenean fold-and-thrust belt. The Monte Perdido thrust fault is a shallow thrust juxtaposing upper Cretaceous–Paleocene platform carbonates and Lower Eocene marls and turbidites from the Jaca basin. The core zone of the fault, about 6xa0m thick, consists of intensely deformed clay-bearing rocks bounded by major shear surfaces. Illite and chlorite are the main hydrous minerals in the fault zone. Illite is oriented along cleavage planes while chlorite formed along shear veins (<50xa0μm in thickness). Authigenic chlorite provides essential information about the origin of fluids and their temperature. δ18O and δD values of newly formed chlorite support equilibration with sedimentary interstitial water, directly derived from the local hanging wall and footwall during deformation. Given the absence of large-scale fluid flow, the mineralization observed in the thrust faults records the P–T conditions of thrust activity. Temperatures of chlorite formation of about 240°C are obtained via two independent methods: chlorite compositional thermometers and oxygen isotope fractionation between cogenetic chlorite and quartz. Burial depth conditions of 7xa0km are determined for the Monte Perdido thrust reactivation, coupling calculated temperature and fluid inclusion isochores. The present study demonstrates that both isotopic and thermodynamic methods applied to clay minerals formed in thrust fault are useful to help constrain diagenetic and low-grade metamorphic conditions.

Structural evolution of the Nojima fault (Awaji Island, Japan) revisited from the GSJ drill hole at Hirabayashi

Following the Hyogoken Nanbu earthquake (January 17, 1995, Mw=7.2), three drillholes were sunk through the Nojima Fault (Awaji Island, Japan). Textural and petrographic studies of the Geological Survey of Japan (GSJ) drill cores allow recognition of two deformation episodes. The first one is older than the deposition of the Middle to Late Eocene Kobe Group, corresponds to a left-lateral movement on the Nojima fault and is expressed by pseudotachylytes, kinking of biotite crystals in the low-strain rocks and an intense laumontite hydrothermal alteration. The second one displaces the basal unconformity of the Kobe group, corresponds to a right-lateral reverse displacement and is expressed at least by carbonate-filled hydraulic fractures and thin gouge zones. Different important deformation mechanisms are recorded by the fault rocks, but questions relating to the attribution of deformation and alteration features to one or other deformation episodes remain unresolved.

Estudo de inclusões fluidas do Granito Parapente, Gaspar (SC): implicações para a evolução tectônica da Zona de Cisalhamento Itajaí-Perimbó

Ao colocar rochas do Complexo Metamorfico Brusque em contato com rochas da Bacia do Itajai, a Zona de Cisalhamento Itajai-Perimbo assinala o limite entre os dominios central e externo do Cinturao Dom Feliciano no Escudo Catarinense. A evolucao desta estrutura de escala crustal pode ser investigada por meio do estudo do Granito Parapente, que aflora no Municipio de Gaspar em meio a zona de cisalhamento. Com idade de cristalizacao de 843 Ma, este granito tipo A e um excelente marcador de deformacao relacionada ao funcionamento da zona de cisalhamento. Para estimar condicoes de temperatura, pressao e profundidade relacionadas a deformacao foram analisadas inclusoes fluidas em tres tipos de quartzo do Granito Parapente. As temperaturas minimas de aprisionamento e as pressoes e profundidades medias obtidas pelas inclusoes fluidas sao as seguintes: 220-190oC, 6 kbar e ~23 km, para o quartzo I, da facies menos deformada do granito, considerado mais antigo; 160-130oC, 4 kbar e 14 km, para o quartzo II, de filonito, formado em etapa mais recente da evolucao da zona de cisalhamento; e 260-220oC, 190-170oC, <12 km e <3 kbar para o quartzo III, de veio que corta a foliacao da zona de cisalhamento. A variacao de profundidade evidenciada para a formacao dos quartzos I e II atesta que a zona de cisalhamento funcionou inicialmente como cavalgamento, alcando o Granito Parapente em cerca de 10 km em niveis crustais intermediarios. Nos tres tipos de quartzo foram observadas inclusoes com fluidos aquosos com salinidades baixas, o que evidencia que a Zona de Cisalhamento Itajai-Perimbo foi conduto para fluidos durante um longo periodo.