GNGTS – Atti del 18° Convegno Nazionale / 05.03
G. Musacchio (1), D. J. White (2), I. Asudeh (2) and C. Thomson (3)
(1)
IRRS-CNR, Milano, Italy
Geological Survey of Canada, Ottawa, ON, Canada
(3) Queen's University, Kingston, ON, Canada
(2)
THE LITHOSPHERE P-WAVE VELOCITY STRUCTURE IN THE
WESTERN SUPERIOR ARCHEAN CRATON
The evolution of the early crust is an outstanding unraveled issue of
geoscience. Much of the preserved continental crust formed during the first 2 billion
years, the Archean eon, of the planet’s lifetime; thus Archean processes of
lithosphere formation are crucial to understanding crustal evolution. The Superior
Province of the Canadian Shield, being the world’s largest preserved Archean
Craton, represents an ideal natural laboratory for investigating early crust evolution. It
is in the western portion of the province that the LITHOPROBE Western Superior
Geo-Transect is currently carried out. The transect’s geophysical component
includes Reflection, Refraction/Wide-Angle Reflection (R/WAR) and Teleseismic
surveys, in common corridors (Fig. 1).
Ancient continental
core (Cc)
Volcanic rocks erupted
duringlater rifting of
the continent. (V)
Deep crustal granitoid
terrane (GB)
Composite terranes of
ocean floor, continent
fragments & island arcs
(GGB)
Oceanic sedimentary
rocks eroded from
other terranes (MB)
Seismic reflection line
Teleseismic corridor
Shot points for the
R/WAR line
Fig. 1
Fig. 1 – Structural map and collected data.
In this note we discuss the Lithosphere structure as derived from ray-based Pwave travel-time inversion (Zelt and Smith, 1992) of the R/WAR data collected in
1996. 550 portable seismographs (PRS-1 and PRS-4; Reftek; SGR) were deployed
at 1 to 2 km spacing, along two 600 (500) km long high-resolution seismic lines. The
shots (1000-3000 kg of explosive), spaced 50-km apart, were 12 for the 600-km-long
east-west line and 11 for the north-south line, allowing a penetration down to
Lithosphere depths (12 sec of reduced travel-time (Vred=8.25 km/s). The lines run
parallel (Line1) and perpendicular (Line2) to east-west trending metaplutonic,
GNGTS – Atti del 18° Convegno Nazionale / 05.03
metasedimentary and volcanoplutonic subprovinces that dominate the regional
structure of the Province, in its western portion (Fig. 1).
Data allow to pick refracted phases through the upper, Pg, middle, Pmc, and
lower, Pc, crust as well as through the upper mantle, P n, Pn1, Pn2 . Reflected phases
yield constraints on the depths of the bottom of the upper, P iP, middle, PmcP, lower,
PmP crust and the first layer just below the Moho, P nP. P-waves apparent velocity for
upper mantle phases show azimuth dependence.
Fig. 2 – Vertical cross-sections.
The Western Superior Province P-wave crustal velocity structure (Fig. 2) highly
resembles Proterozoic and Phanerozoic terranes and supports the idea that Earth’s
crust forming processes have been similar through age. Higher crustal P-wave
velocity in the northern and eastern part of the lines well agrees with the high crustal
average Vp/Vs ratios (1.78-1.8) found along the same lines (Kay et. al., 1999).
The image of the Moho derived here agrees with what found with other
techniques applied to the same data set (i.e., NMO wide-angle stacks in Kay et al.,
1999), and with what shown by brute stacks of the reflection data along Line1 (see
map in Fig. 1). Although intracrustal and upper mantle boundaries display lateral
topography, the Moho shows only long wavelength undulations. It overlies a layer
where VP (7.9 to 8.0 km/s) may indicate crust-mantle interaction. High amplitudes
phases reflected from a boundary 10-to-5 km beneath the Moho might resemble, in
some of the shot gathers, PmP phases, therefore arising the problem of how PmP are
defined. In the upper mantle, P-waves show an east-west trending fast propagation
direction. Such seismic velocity azimuth dependence is in agreement with waves fast
polarization direction of SKS (Kay et. al., 1999). This and the P-wave lithosphere
velocity structure allow speculating that the Western Superior Province has formed
with a northwestward accretion, possibly in modern plate tectonic environment.
GNGTS – Atti del 18° Convegno Nazionale / 05.03
REFERENCES
Kay I., Musacchio G. et al.; 1999: Imaging the Moho and V p/Vs ratio in the Western Superior Archean
craton with Wide Angle Reflections. G.R.L., 26,16,2585-2588.
Kay I., Sol S. et al.; 1999: Shear Wave splitting observation in the Archean Craton of Western
Superior. G.R.L., 26, 17, 2669-2672.
Zelt C. and R. B. Smith; 1992: Seismic traveltime inversion for 2-D crustal velocity structure. Geophys.
J. Int., 108, 16-34.