Diachronous development of master joints of different orientations in different lithological units within the same forearc-basin deposits, Kyushu, Japan

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.