Current Research in EEPS: Susan Beck, University of Arizona
A Tale of Two Modern Flat Slabs along the South America Convergent Margin
The Peruvian and Pampean modern flat slab segments along the South American
margin provide insights into the process of flat-slab subduction and are often cited as
modern analogs to past flat-slab subduction in many orogenic belts. Recent seismic
imaging, using a wide range of techniques, has given us unprecedented views of both of
these modern flat slabs’ geometries and processes. Both of these flat slab segments
have near-horizontal subducting segments that correspond to subducting oceanic ridges
with overthickened oceanic crust, gaps in the modern volcanic arc, and inland
deformation. However, the two flat slabs flatten at different depths and displays
different rates of seismicity, suggestive of variable degrees of hydration. The Pampean
flat slab in Argentina is deeper, and the overriding plate is colder, whereas the Peruvian
flat slab has a more complex geometry with a shallower flat slab segment associated
with the Nazca Ridge’s subduction.
Our seismic imaging shows that the subducting Nazca Ridge segment at the
southern end of the large Peruvian flat slab has an oceanic Moho that is horizontal at a
depth of ~70 km. Taking into account the overthickened oceanic crust of the Nazca
Ridge, the top of the slab is likely at a depth of ~50-55 km and in direct contact with the
continental crust. The subducting Nazca Ridge has displaced the South American
lithospheric mantle and part of the lower continental crust beneath the high Andes
suggesting the flat slab segment is strongly coupled to the over-riding plate. Seismic
tomography shows the near-horizontal slab extends ~500 km inland before resubducting
at a near vertical angle into the mantle before flattening out in the lower
mantle below the transition zone.
In the case of the Pampean flat slab where the Juan Fernandez Ridge is
subducting, we observe a high rate of seismicity in both the slab and the over-riding
continental crust. Within the region, we image the horizontal oceanic Moho at ~100-110
km depth before it re-subducts into the mantle at a ~30-40° dip. As the subduction
angle steepen, the oceanic Moho disappears, possibly because the oceanic crust has
transformed to eclogite. The flat slab is at the base of the continental mantle
lithosphere with no evidence of any asthenospheric wedge until the slab re-subducts.
The high rate of seismicity is consistent with oceanic crust dehydration although the fate
of any released fluids is uncertain. The deep crustal earthquakes (~40 km) in the
Precordillera and Sierras Pampeanas imply that the upper plate is relatively cold. Both
flat slabs show signs of tearing although in different locations and scales.
Event will be hosted as a webinar.