EVIDENCE FOR A GIANT LANDSLIDE ON THE NORTHEASTERN FLANK OF HALEAKALA VOLCANO, ISLAND OF MAUI, HAWAII

Haleakala is a dormant, late-stage volcano comprising the eastern two-thirds of Maui Island. Significant geomorphic features on Haleakala are three broad, transversely flat-floored valleys developed along the volcano’s summit ridge and separated by narrow, sharp ridges. From west to east, the valleys are Haleakala “crater” (HC), Kipahulu Valley (KV), and Waihoi Valley (WV). Also important are north-flowing Keanae Valley, which drains HC on the northwest and south-flowing Kaupo Valley, which drains HC from the southeast. All five of these valleys have been flooded by recent lava flows of the Hana series. Some early geological investigators suggested various of these valleys were formed by volcanic renting or block faulting. Later, Stearns (1942) carefully noted the lack of evidence for a structural origin to any of the valleys and proposed that they were created solely from erosion. Since then, Stearn’s interpretation has been the accepted paradigm. Apparently overlooked, however, is evidence for faulting along the axes of Keanae, Waihoi, and Kipahulu Valleys. The evidence consists of non-alignment of the relatively un-incised volcanic slopes on opposite sides of each valley. Across the upper two-thirds of Keanae Valley, the eastern volcanic slope is 75 to 150 m higher than the western slope measured parallel to contours. Across Waihoi Valley, the south volcanic slope is approximately 200 m higher than the north slope measured parallel to contours. It is hypothesized here that the faulting is related to large-scale landsliding of Haleakala’s north flank. The head of the proposed “slump”-style landslide is spatially coincident with HC, KV, and WV. The fault underlying Keanae Valley is the western lateral boundary. Finally, the location of the landslide toe and eastern lateral boundary are uncertain, but likely lie offshore. The present geomorphic expression of HC, KV, and WV may reflect graben development at the landslide head or the valleys may have been mostly created by erosion that exploited rock weakened from landslide faulting. In either case, Hana lavas flooring HC, KV, and WV post-date landslide movement and bury whatever landslide structures would otherwise be exposed within the valleys. Based on the age boundary between the Kula and Hana series basalts the landslide likely moved between 150 and 120 ka.