Friday, 26 May, 11:00 - EARTH SYSTEM PHYSICS SEMINAR

[Earth System Physics Section]

EARTH SYSTEM PHYSICS SEMINAR
Friday, 26 May - 11:00 hrs
Leonardo Building, Luigi Stasi Seminar Room

LITHOSPHERIC STRUCTURE OF THE NORTH AMERICAN MODEL

Magdala Tesauro
University of Trieste


Lithospheric structure of the North American continent
Seismic data on the North American continent are unevenly distributed 
and in some cases discrepancies exist between published models. In order 
to construct a consistent 3D crustal model with three layers in the 
crystalline crust, the following sequence of steps have been 
implemented: 1. Definition of the geometry of the main tectonic 
provinces of North America; 2. Selection and evaluation of the 
reliability of seismic crustal models in the database; 3. Estimation of 
the P-wave seismic velocity and thickness of the upper, middle and lower 
crust for each tectonic province; 4. Estimation of the interpolated Pn 
velocity distribution. In comparison with the global crustal model CRUST 
1.0, the new crustal model, NACr14, is more heterogeneous, showing a 
larger spatial variability of the thickness and average velocities of 
the crustal layers. The largest velocities of the crystalline crust 
(>6.6 km/s) reflect the presence of a 7.x layer (>7.0 km/s) in the 
lowermost part of the crust.
Using NACr2014, a regional (NA07) and a global (SL201sv) seismic 
tomography model, and gravity data, an iterative technique, which 
jointly interprets seismic tomography and gravity data, to estimate 
temperature and compositional variations in the NA upper mantle has been 
applied. The results obtained demonstrate that temperature of the 
cratonic mantle is up to 150°C higher than when using a uniform 
compositional model. The differences between the two tomography models 
influence the results more strongly than possible changes of the depth 
distribution of compositional variations. Strong negative compositional 
density anomalies, corresponding to Mg # >92, characterize the upper 
mantle of the northwestern part of the Superior craton and the central 
part of the Slave and Churchill craton. The Proterozoic upper mantle of 
the western and more deformed part of the NA cratons, appears weakly 
depleted (Mg# ~ 91) when NA07 is used, in agreement with the results 
based on the interpretation of xenolith data. When SL2013sv is used, the 
same areas are locally characterized by high density bodies, which might 
be interpreted as the effect due to fragments of subducted slabs, as 
those close to the suture of the Appalachians and Grenville province.
The two thermal models have been used to estimate the integrated 
strength and effective elastic thickness (Te) of the lithosphere. In the 
peripheral parts of the cratons, as the Proterozoic Canadian Platform 
and Grenville, the integrated strength for model NA07 is ten times 
larger than in model SL2013sv, due to a model-dependent temperature 
difference of >200°C in the uppermost mantle. In both models, 
Proterozoic regions reactivated by Meso- Cenozoic tectonics (e.g., Rocky 
Mountains and the Mississippi Embayment) show a weak lithosphere due to 
the absence of the mechanically strong part of the mantle lithospheric 
layer. Intraplate earthquakes are distributed along the edges of the 
cratons, characterized by a weak lithosphere or pronounced variations in 
integrated lithospheric strength and Te. In addition, the sum of the 
seismic moments shows that most of the energy is released by the weak 
lithosphere. These results suggest that the edges of the cratons are 
more prone to accumulation of tectonic stress and subsequent release by 
earthquakes, in comparison with the stable cratonic regions which resist 
deformation.**