What can magnetism tell us about oceanic tectonics? New insights from scientific drilling
(ECCORD distringuished lecture)


Antony Morris
School of Geography, Earth and Environmental Sciences
Plymouth University, UK


Seafloor spreading and subduction in the oceans are the most significant
geodynamic processes on Earth, and together drive the plate tectonic cycle. It
is widely known that magnetic studies played a key role in understanding how
oceanic lithosphere forms at mid-ocean ridges. Indeed, Vine and Matthews
ground-breaking realization in the 1960s that linear marine magnetic anomalies
result from oceanic crust having acquired magnetizations in different polarities
of the geomagnetic field during accretion at mid-ocean ridges must rank as one
of the great scientific discoveries of the twentieth century. This lecture will
show how magnetic analysis of core samples recovered by scientific ocean
drilling by the International Ocean Discovery Program (IODP) and its
predecessors continues to provide new insights into globally significant
tectonic and magmatic processes in the oceans. A focus will be how our
understanding of slow and ultraslow spreading axes has changed radically
following the discovery of so-called "oceanic core complexes" along ridge
systems. These enormous massifs on the seafloor form by unroofing of lower
oceanic crust and upper mantle rocks by crustal-scale faulting. Palaeomagnetic
data from rocks drilled at several core complexes have quantified this process
and demonstrated that their formation involves large tectonic rotations that
bring rocks formed at depth up to the ocean floor. These rotations provide the
key to recognizing this important tectonic process in ophiolites (fragments of
ancient oceanic crust emplaced tectonically onto the continents), allowing the
record of "detachment-mode" seafloor spreading to be extended back beyond the
age of the oldest crust in the modern oceans. Finally, the lecture will show how
scientific ocean drilling is also providing new information on the nature of the
magnetization of the oceanic crust. Analysis of lower crustal gabbros recovered
in both the Atlantic and Pacific Oceans reveals that these rocks often record
multiple polarities of the Earth's magnetic field within individual samples,
contrasting with the simpler magnetizations of upper crustal rocks responsible
for most of the marine magnetic anomaly signal.