Diachronous development of master joints of different orientations in different lithological units within the same forearc-basin deposits, Kyushu, Japan
Résumé
The geometrical and chronological characteristics of joints and subordinate fractures in the Miocene (12-6 Ma) Uchiumigawa Group, Kyushu, Japan, strongly depend on the host lithology. In massive sandstone units (11 stations), joints are normal to bedding and form two suborthogonal sets, a master joint set parallel to the bedding strike and a cross joint set perpendicular to the bedding strike. In interbedded sandstone-mudstone units (11 stations), joints are found only in sandstone layers and have the same characteristics as the joints in the thick sandstone units, except that the chronology is opposite: in nine stations, master joints are perpendicular to the bedding strike and cross joints are parallel to the bedding strike. The observed joint pattern is tentatively explained by a diachronous formation of master joints which appeared first in massive sandstone units and later in the interbedded units, as a result of the superposition in the study area of two different deformation episodes independently documented by regional fault-slip data. Transition between the two jointing events took place around 6 Ma.