Intracaldera structure and megabreccias at Dorobu caldera, northeastern Honshu, Japan

In the Pliocene Dorobu caldera, 13 by 6 km in diameter, on northeastern Honshu, Japan, a number of occurrences of a distinctive megabreccia block facies is found within voluminous ash-flow tuffs, the Dorobu ash-flow tuffs A and B. Dorobu tuff A is believed to be the deposit of the initial caldera-forming eruption. An isopach map of tuff B and orientation of narrow dikes indicate that an internal large depression occurred inside the present caldera margin. This and locations of post-caldera conduits and near vent facies of Dorobu tuff C suggest that the caldera has a nested collapse structure, and possible conduits of the tuff B are within the internal depression. Annular collapses that sequentially moved towards the caldera center occurred during eruption of tuffs A and B. The overall structure of Dorobu caldera is nested, and it possibly has step-like floor, as described elsewhere from piecemeal calderas. Megabreccia block facies consists of polyhedral massive blocks composed of Miocene to Pliocene country rocks with large diameters (up to several hundreds meters). The block facies as a whole does not consist of sheets or piles of debris, as described in the San Juan Mountains and Stillwater Range, USA, but as isolated blocks set in the matrix of Dorobu tuffs A and B. In previous studies of caldera development, the mode of emplacement of caldera-collapse breccias has been considered to be by rock fall or rock slide during the caldera forming eruption. A paleo-magnetic study of the block facies shows two significant characteristics. Predominant variations of the magnetic directions of blocks indicate that they were emplaced with rotations, caused by rock slide, rock fall, or a toppling motion. In contrast, magnetized directions of one megabreccia block almost coincide with that of the caldera country rock, suggesting that this block must have kept its original magnetic direction. Our findings suggest that it is not a rock slide deposit derived from caldera walls, but rather a segment of subsided caldera floor. The mode of emplacement of the megabreccia block facies at Dorobu caldera is mostly similar to the other megabreccias, however, additionally, it is necessary to distinguish segments of caldera floor from rolled or toppled megabreccia blocks.