Rocks from the Cascade Seamount: An Archive of the Unzipping of Gondwana and the Initiation of the ACC?

Abstract:

The Cascade Seamount, on the East Tasman Plateau, is enigmatic with regard to its formation and subsidence history. Understanding the timing and rate of subsidence of the region is an important factor for determining the initiation of the Antarctic Circumpolar Current. Currently situated ~2000m below sea level, the Seamount has archived much of this history. However, our understanding of its evolution is limited by poorly constrained samples recovered from two separate locations. One, from a fishing excursion, recovered rocks from various unknown locations on the seamount while an ODP drill hole collected samples on the East Tasman Rise ~23km from the Seamount (~2000m bsl). The two, limited, sample sets comprise of shallow water sediments but have led to opposing subsidence histories for the area being proposed. Fundamentally, it is the ODP drill hole that has been used as the only constraint for the area in the tectonic modeling that has been conducted (Stickley et al. 2004). The recent RV Investigator voyage to the Cascade Seamount (August, 2016) aimed to elucidate this problem through high-resolution mapping and dredging with the aim to further understand and characterize the Seamount. We present the initial results of the macro- and micro-features of the rocks recovered from the top and flanks of the Seamount. The high-resolution bathymetry data identified a series of terraces on the flanks of the Seamount from which a series of shallow water sedimentary rocks and conglomeratic units were recovered. Here, we describe these rocks, and discuss their formation. These descriptions and interpretations, coupled with the new geophysical data, provide a critical insight into the subsidence history of the Seamount. The ability to understand the history of the Cascade Seamount and the East Tasman Plateau provides a new and important tie-point in understanding the final stages of the regional evolution and initiation of the Antarctic Circumpolar Current.