Abstract:

The tectonic veins, as the field crop markers of tectonic fluid, record the source, environment and dynamic characteristics of geo fluid related to tectonic movement. Three periods of calcite veins (V1, V2 and V3) related to pre tectonics, syn tectonics and posttectonics respectively were identified in the Dabashan foreland. The formation environment conditions and source of fluid represented by V3 veins and developed within fractures under stretching environment were studied based on field survey and measurement of the occurrence of V3 veins together with the statistic and geometric features and wall rock mechanical properties of V3 veins，as well as the analysis of paleostress and isotope geochemistry of V3 veins. It is indicated that V3 veins are trending mainly in NE SW with steep dip almost upright. The directions of principal stress axes of σ1, σ2 and σ3are 67°∠76°, 200°∠13° and 293°∠10°, respectively. The paleostress field of V3 veins is of NW SE tension. The relationship of fluid pressure of V3 veins with principal stress axes is pf<σ2<σ1. Fluid overpressure is about 92167 MPa and the maximum depth is 1.87.2 km. Results of δ13C δ18O analysis show that V3 calcite veins formed from the decarbonization of carbonate of wall rocks. δDδ18O isotope of fluid inclusions contained in V3 calcite veins indicated that the water was mostly metamorphic water formed within basin and migrated to shallow place and mixed with surface water. Because abnormal high pressure fluid existed widely in Dabashan foreland, deep fluid with abnormal high pressure broke through the wall rocks and migrated along fractures towards shallow place and mixed with surface water to precipitate to form V3 veins under extensional tectonics in the Cretaceous Paleogene.

Key words: tectonic fluid, veins, stress field, abnormal pressure, Dabashan structure belt, intra continent orogen