Diachronous development of master joints of different orientations in different lithological units within the same forearc-basin deposits, Kyushu, Japan
O. Fabbri et al., Diachronous development of master joints of different orientations in different lithological units within the same forearc-basin deposits, Kyushu, Japan, J STRUC GEO, 23(2-3), 2001, pp. 239-246
The geometrical and chronological characteristics of joints and subordinate
fractures in the Miocene (12-6 Ma) Uchiumigawa Group, Kyushu, Japan, stron
gly depend on the host lithology. In massive sandstone units (11 stations).
joints are normal to bedding and form two suborthogonal sets, a master joi
nt set parallel to the bedding strike and a cross joint set perpendicular t
o the bedding strike. In interbedded sandstone-mudstone units(ll stations),
joints are found only in sandstone layers and have the same characteristic
s 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 j
oint pattern is tentatively explained by a diachronous formation of master
joints which appeared first in massive sandstone units and later in the int
erbedded units, as a result of the superposition in the study area of two d
ifferent deformation episodes independently documented by regional fault-sl
ip data. Transition between the two jointing events took place around 6 Ma.
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