4:30 PM - 4:45 PM
[SVC30-10] An outcrop-scale, Neogene, basaltic, ring structure: Its formation and magma movements
Keywords:ring complex, volcanic vent, stoping, stress field, joint system, Matsuura Basalts
An outcrop-scale, basaltic, ring structure was found on an abrasion platform in Nagasaki Prefecture. For the reasons below, it was probably one of the volcanic pipes of the Matsuura basalts which remains above the elevation of about 140 m around it. This structure seems to have fill the volcanic conduit. This rock body has been regarded as a ‘bud’ of a dike (Kamada, 1982). We do not deny the possibility that the ring structure developed from a bud, but the occurrence is not a bud.
The ring structure is tear-shaped in plan view, and hosted by gently dipping Oligocene sandstone. The major and minor axes are 14.9 and 10.4 m long, respectively, the former of which has a E-W trend. The ring dike is about 1 m wide and has steep inward dips. Thus, the overall shape resembles the root zone of a diatreme, but there is no evidence of explosion. Bubbles in the dike make clear flow banding roughly parallel to the ring, but there are ‘unconformable’ relations between flow bands, indicating that that the ring dike is a result of the repeated formation and destruction.
The inside of the ring dike has been eroded away except for about 40% of the eastern part. Tongue-shaped intrusions and intrusion breccia make up the remnants. It does not have any tuffaceous materials or blocks of the host rock. Each of the intrusions has columnar joints which wrap a smooth core. The spaces between the togues are filled with intrusive breccia, which sometimes grades from the columnar joints. The breccia also fills the gap between the ring dike and the remnants. The basalt inside the ring dike contains less bubbles but includes mafic xenoliths, but the ring dike does not. Therefore, it is clear that different magmas made the remnants and the dike, but the evidence of their relative timing such as chilled margins was unclear due to the outcrop conditions. The inside the ring dike, the intrusion of tongue-shaped bodies must have pushed up the volcanic plug that filled the pipe. The ring dike was formed in the gap between the plug and the country rock.
The ring dike has a chilled margin along its outer wall, and does not have fault contact with the country rock. The wall was polyhedral in places. This indicates that the volcanic pipe gradually increased its dimension by stoping. The ring structure is tear-shaped, probably because the corner was the part that was stoped out shortly before this volcanic pipe was abandoned.
The following observations indicate that the ring structure was formed under the stress condition with the north-south trending σHmin-axis in the Late Miocene to Pliocene time, during which the Matsuura Basalts were extruded around the ring structure. First, the ring structure itself extends in the same orientation. Second, a vertical dike that is cut by the structure have this trend as well. And, finally, there are systematic joints in this orientation around the structure. A couple of joints curve in the opposite directions to circumvent the structure. These curves can be explained by the stress field in the country rock that acted as an elastic rock body with a circular hole filled with fluid. This means that the volcanic pipe was filled with magma at the time of the extensional stress field that created the systematic joints.
The ring structure is tear-shaped in plan view, and hosted by gently dipping Oligocene sandstone. The major and minor axes are 14.9 and 10.4 m long, respectively, the former of which has a E-W trend. The ring dike is about 1 m wide and has steep inward dips. Thus, the overall shape resembles the root zone of a diatreme, but there is no evidence of explosion. Bubbles in the dike make clear flow banding roughly parallel to the ring, but there are ‘unconformable’ relations between flow bands, indicating that that the ring dike is a result of the repeated formation and destruction.
The inside of the ring dike has been eroded away except for about 40% of the eastern part. Tongue-shaped intrusions and intrusion breccia make up the remnants. It does not have any tuffaceous materials or blocks of the host rock. Each of the intrusions has columnar joints which wrap a smooth core. The spaces between the togues are filled with intrusive breccia, which sometimes grades from the columnar joints. The breccia also fills the gap between the ring dike and the remnants. The basalt inside the ring dike contains less bubbles but includes mafic xenoliths, but the ring dike does not. Therefore, it is clear that different magmas made the remnants and the dike, but the evidence of their relative timing such as chilled margins was unclear due to the outcrop conditions. The inside the ring dike, the intrusion of tongue-shaped bodies must have pushed up the volcanic plug that filled the pipe. The ring dike was formed in the gap between the plug and the country rock.
The ring dike has a chilled margin along its outer wall, and does not have fault contact with the country rock. The wall was polyhedral in places. This indicates that the volcanic pipe gradually increased its dimension by stoping. The ring structure is tear-shaped, probably because the corner was the part that was stoped out shortly before this volcanic pipe was abandoned.
The following observations indicate that the ring structure was formed under the stress condition with the north-south trending σHmin-axis in the Late Miocene to Pliocene time, during which the Matsuura Basalts were extruded around the ring structure. First, the ring structure itself extends in the same orientation. Second, a vertical dike that is cut by the structure have this trend as well. And, finally, there are systematic joints in this orientation around the structure. A couple of joints curve in the opposite directions to circumvent the structure. These curves can be explained by the stress field in the country rock that acted as an elastic rock body with a circular hole filled with fluid. This means that the volcanic pipe was filled with magma at the time of the extensional stress field that created the systematic joints.