DRILLING STRATEGY
LIPs are enormous constructions that present considerable challenges for
adequate sampling to address the major questions outlined in the
preceding sections. Our knowledge of oceanic Cretaceous LIPs is
rudimentary, similar, perhaps, to that of mid-ocean ridges before the
general acceptance of the plate tectonics paradigm in the late 1960s.
Geophysical surveys and a grid of shallow (100 to 200 m) basement drill
holes are necessary to address Cretaceous mantle dynamics, the physical
and chemical processes involved in construction of these LIPs, and their
environmental consequences. Understanding the temporal and
compositional history of the Kerguelen Plateau and Broken Ridge requires
a multifaceted drilling strategy (Fig. 14), including (1) transects of
shallow basement holes across the surface of the LIP; (2) offset drilling
in tectonic windows that expose deeper levels of the LIP that are
otherwise inaccessible; (3) intermediate (1000-2000 m) and deep (>2000
m) basement holes at carefully chosen locations; and (4) reference holes
on older adjacent oceanic crust. Leg 183, complemented by Legs 119 and
120, is part of the fundamental and necessary reconnaissance phase of
sampling. To obtain a comprehensive database of eruption ages and lava
compositions for the entire LIP, we sampled igneous basement to depths
of ~100 to 150 m at as many morphologically and tectonically distinct
regions of the Kerguelen Plateau-Broken Ridge LIP as possible during one
drilling leg (Table 1; Figs. 3,
4, 5, 6). In addition, the sedimentary section
immediately overlying the basement provides estimates of minimum ages
for extrusive basement, important information regarding eruption and
weathering in a subaerial vs. submarine environment, and evidence for
tectonic events in the plateau's history. Neogene to Cretaceous sediments
overlying basement also provided significant paleoceanographic
information for high southern latitudes. At some
sites, tephra horizons of various ages provide information on explosive
eruptions at nearby islands (McDonald, Heard, and Kerguelen Archipelago)
and perhaps more distant volcanoes.
Leg 183 Principal Results - Sites 1135/1136
Leg 183 Table of Contents