VOLCANIC EVOLUTION OF THE APOLLINARIS PATERA SUMMIT CALDERA, MARS

Besides their heavily channeled, low sloped flanks, another distinct aspect of Martian highland paterae (HP) are their prominent summit calderas; HP calderas are almost more pronounced than the edifices themselves. However, despite their prominence, processes involved in HP caldera formation remain unconstrained. As part of an ongoing study to constrain the volcanic evolution of HP calderas, we report on the summit caldera of Apollinaris Patera (AP; 8° S, 174°E) – a 200 km diameter solitary volcano located along the boundary between Mars’ northern lowlands and southern highlands. AP’s summit caldera is divisible into two separate calderas – an outer caldera (OC) and an inner caldera (IC). The OC is the larger of the two calderas and its floor is covered by deposits of etched, easily erodible materials that, following previous workers, we interpret as pyroclastic deposits. Along the eastern and southern parts of the OC, concentric graben cut these deposits. In turn, pyroclastic deposits cover the graben in the western part of the OC; some graben have been locally modified and cut by channels. In addition, the OC has been broken into large (>25 km diameter) fault blocks that dip north into the IC where they are embayed by IC deposits that appear to be mechanically stronger than OC deposits, and locally host contractional ridges – observations consistent with the interpretation that IC materials are effusive deposits; in fact, these deposits seem to reflect a lava lake. From cross-cutting relations, we envision the following history for AP’s summit caldera: 1) Two periods of explosive eruptions. As evidenced by graben cutting the pyroclastic deposits, the OC either formed synchronously with these eruptions or post-dated them; it is possible that second period of explosive volcanism resulted in the formation of the OC. 2) Formation of the IC (which occurred at an unknown amount of time after OC formation) with filling by a lava lake; MOLA-derived topographic profiles across the IC indicate that it may have experienced trap door style collapse (a second caldera forming event?) after lava lake formation. Tilting of OC fault blocks possibly occurred during IC formation. 3) Channel formation within AP’s summit caldera and graben modification. This volcanic history is broadly consistent with those observed at other HP.