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Vitrinite reflectance and shear-induced graphitization in orogenic belts: A case study from the Kandersteg area, Helvetic Alps, Switzerland

1997; Elsevier BV; Volume: 34; Issue: 1-2 Linguagem: Inglês

10.1016/s0166-5162(97)00018-9

1872-7840

Václav Suchý, Martin Frey, M. Wolf,

Hydrocarbon exploration and reservoir analysis

Vitrinite reflectance (VR) is an important indicator of incipient metamorphism and it is generally assumed that temperature and, to a lesser degree, time, are the two main variables. The role of stress in VR is controversial and will be addressed in this study. In the Kandersteg area, north of Lake Oeschinen, four different Helvetic tectonic units are present, comprising mainly limestones, marls and greywackes of Mesozoic and Lower Tertiary age. VR (Rr, Rmax, Rmin) and illite crystallinity (IC) were determined from a very steep section at elevations between 1700 and 2900 m a.s.l. In general,Rr, Rmax and IC increase from tectonically higher to lower units, i.e. from upper diagenetic (2–3 to 4–5% Rr) to upper anchizonal conditions (3–4 to 5–6%Rmax). Several samples contain, in addition to coaly fragments, abundant semigraphitized and graphitized particles that are believed to be of authigenic origin. 'Transitional matter' showsRmax values ranging from 7 to about 11% and Rmin of 1.3–3.4%; 'optical graphite' showsRmax of about 15–16% and Rmin of 1.6–1.8%. There is good textural evidence that tectonic deformation has played a major role in the development of these strongly coalified particles. The first tiny needle-like crystals of graphitic material occur within narrow shear zones transecting the rock matrix. The beginning of graphite formation parallels the onset of plastic deformation and dynamic recrystallization of quartz. Elevated vitrinite reflectance (Rmax) and bireflectance values are found near thrust planes, where simple optical observation on quartz grain morphology indicates an increase in strain intensity. It appears that frictional heating associated with thin shear zones may have been responsible for the formation of authigenic graphite in the Kandersteg samples.