Séminaire de l'IPGS
Intervenante : Karin Sigloch , Université de Munich.
Titre : Terrane stations: Seismic tomography explains where the North American Cordillera came from
Lieu : EOST, 5 rue René Descartes, amphi Rothé.
Résumé :
The western quarter of North America consists of accreted terranes,
crustal blocks that were added to the margin in a series of collisions
over the past 200 million years - but why? The most widely accepted
explanation posits a scenario analogous to Andean subduction, with these
terranes conveyed to the continental margin while the oceanic Farallon
plate subducted under it. Yet purely Andean-style subduction under North
America is questionable as a terrane delivery mechanism, since no
comparable accretion sequence took place along the South American
margin, and since North American terranes are of very varied provenance.
I consider this geological question directly related to a geodynamical
one: Why has it been so difficult to reconcile - even on the largest
scale - the geometries and locations of slabs in the lower mantle, as
imaged by seismic tomography, with Cretaceous plate reconstructions of
the North American west coast (unless anomalous mantle rheology or ad
hoc shifts of absolute reference frame are invoked)? This problem was
recognized soon after the discovery of the massive, lower-mantle
"Farallon slabs" by Grand (1994), but has recently been aggravated.
Thanks to a worldwide unequaled coverage of the continent by seismic
broadband stations (USArray), and their exploitation in finite-frequency
waveform inversion, we discovered more westerly slabs in the lower
mantle. Contrary to the widely held assumption, not all of these slabs
can be Farallon, unless very non-vertical and/or uneven slab sinking
behavior is allowed for.
As a joint solution, I offer a radical reinterpretation of
paleogeography and test it quantitatively: The seas west of Cretaceous
North America must have resembled today's western Pacific. The Farallon
and two more plates subducted into the intra-oceanic trenches of a vast
archipelago in the eastern Panthalassa (proto-Pacific) ocean, both from
the east and the west. The trenches remained stationary throughout much
of Jurassic and Cretaceous times, depositing the massive, near-vertical
slab walls imaged in the lower mantle today. On their overriding plates,
island arcs and subduction complexes nucleated, and assembled with
exotic fragments.
The archipelago was gradually overridden by North America on its
westward journey away from Pangaea. Episodes of crustal accretion and
Cordilleran mountain building (Sevier, Canadian Rocky Mountains,
Laramide) occurred when the continental margin collided with various
parts of the archipelago. Geodynamically, this scenario is simpler than
previous models in that it is consistent with purely vertical slab
sinking. Sinking rates can be quantified from slab and plate geometries,
and range between 9 and 12 mm/yr.