ABSTRACT:

A set of hydraulic fracturing stress measurements were carried at depths of up to 1400 m below the rig floor at the bottom of the ANDRILL South McMurdo Sound borehole. The measurements were accomplished in open hole through indurated and low permeability glacial diamicts and shales. A 2000-m long wireline hosted straddle packer system was used that allowed for relatively rapid deployment; and seventeen successful measurements were made. Good breakdown pressures were observed in most of the tests; and this allowed constraints to be placed on the magnitude of the greatest horizontal compression. Correspondingly, the vertical stress was calculated by integrating the densities obtained from physical property logging of the nearly continuous core. In all cases, this vertical stress magnitude was intermediate between the horizontal stress values, and consequently indicating a strikeslip stress environment exists at this location.

1. INTRODUCTION

Antarctica remains today somewhat an enigma in terms of understanding the global plate motions[3]. First, the difficulty of access and the absence of commercial exploration severely limit opportunities to make stress measurements from boreholes. Second, Antarctica appears to be tectonically quiescent relative to most other plates; and there is a paucity of earthquake focal mechanisms that can be used to infer stress directions and faulting regimes. This latter situation may change with recent field campaigns that focus on monitoring the seismicity in Antarctica [4] but currently there are vanishingly few points available (Figure 1) in the World Stress Map data base[1, 2]. Broadly, the Antarctic plate is bounded by oceanic ridges that are expected to compress the plate by ridge-push [5] with superposed isostatic readjustments to varying ice loads in the past [6]. In the McMurdo Sound region, the state of stress is further complicated by periods of faulting and current volcanism [7-11].

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