| Extension of the lithosphere gives birth to a wide range of structures, with characteristic
widths between 10 and 1000 km, which includes continental rifts, passive margins, oceanic
rifts, core complexes, or back-arc basins. Because the rheology of rocks strongly depends on
temperature, this variety of extensional structures falls in two broad categories of extending
lithospheres according to the initial Moho temperature TM. “Cold extending systems”, with
TM < 750∘C and mantle-dominated strength, lead to narrow rifts and, if extension is
maintained long enough, to passive margins and then mantle core complexes. “Hot extending
systems”, with TM > 750∘C and crustal-dominated strength, lead, depending on
strain rate, to either wide rifts or metamorphic core complexes. A much less quoted
product of extension is the exhumation of high-pressure (HP ) metamorphic rocks
occurring in domains of back-arc extension driven by slab rollback (e.g. Aegean;
Appennines-Calabrian) or when the subduction upper plate undergoes extension for plate
kinematics reasons (e.g. Norwegian Caledonides; Papua New Guinea). In these tectonic
environments, well-documented pressure-temperature-time (P − T − t) paths of HP
rocks show a two-stage retrogression path whose the first part corresponds to an
isothermal large pressure drop ΔP proportional to the maximum pressure Pmax
recorded by the rocks. This linear relation between ΔP and Pmax, which likely
results from a stress switch between compression and extension at the onset of
exhumation, is in fact observed in all HP metamorphism provinces worldwide, suggesting
that the exhumation of HP rocks in extension is a general process rather than an
uncommon case. In summary, the modes and products of extension are so diverse that,
taken all together, they constitute a very versatile natural laboratory to decipher the
rheological complexities of the continental lithosphere and their mechanical implications. |