The continental lithosphere

Lithospheric structure, composition, and thermal regime of the East European craton:
Implications for the subsidence of the Russian Platform.

This study seeks to explain subsidence of the Russian platform by density
variations in the lithospheric mantle, here hypothesized to be related to
Proterozoic and Paleozoic rifting. The thermal regime and density
structure of the lithospheric mantle of the EEC are estimated to show that
the Precambrian and Phanerozoic rifting had different effects on the
thermal structure and lithospheric composition, and consequently on the
topography of the basement rocks.

1. Proterozoic rifting resulted in modification of the entire crustal column
of the Russian platform. Regions with a thick (>20 km) upper crust (5.8
<Vp<6.4 km/s) and an almost complete absence of the middle crust (6.4
<Vp<6.8 km/s) are spatially correlated with the craton-scale Central
Russia rift system.

2. Moho temperatures calculated from the surface heat flow data vary
from 350-600 oC in Archean – early Proterozoic terranes to 550-700 oC
in the southern parts of the Russian platform and to 600-850 oC in middle-
late Proterozoic regions of the Baltic Shield. Proterozoic rifting is not
reflected in the present thermal regime of the Russian platform, where
lithospheric temperature variations are not sufficiently pronounced.

Thermal lithosphere of the Russian platform is ~180-210 km thick, with
only slightly smaller values (160-180 km) along the Central Russia rift
system. Paleozoic rifting, however, resulted in a pronounced lithosphere
thinning (to 120-140 km) in the southern parts of the Russian platform.

Fig. 1. Major tectonic units of the East European
Craton and their tectono-thermal ages.
Thickness of sediments deposited in different parts of
the Russian platform in Proterozoic to Cenozoic varies
from ca. 1 km to 20+ km. A peak in sedimentation at
375-215 Ma, synchronous over the entire Russian
platform, was related to the Uralian orogeny. Southern
parts of the platform affected by Paleozoic rifting
continue to subside even in the post-Uralian time.

3. Buoyancy-based estimates suggest lithospheric density deficit
of ~1.4±0.2% for the Archean-early Proterozoic Finnish Bay -
Kola-Karelian Province. A strong low-density anomaly in the
Kola-Karelian Province (centered over the White Sea) correlates with
seismic velocity anomalies at depths of 200-250 km; a strong density
anomaly in the Finnish Bay (Baltic Sea) correlates with seismic velocity
and attenuation anomalies only down to 100-150 km depth, suggesting
its shallow origin.

Lithospheric density deficit decreases southwards from the Baltic
Shield and is 0.8±0.2 % in most of the Russian platform. In the southern
parts of the Russian platform, that were rifted in the Paleozoic,
lithospheric density is similar to the lithospheric mantle of western
Europe. Regions of less than ca. 0.4-0.6% density anomaly coincide
with the area of post-Uralian subsidence, implying that compositional
modification of the cratonic lithosphere caused post-Uralian subsidence
of the platform.

4. Rifting of the Russian platform has resulted in a decrease of
lithospheric depletion (manifested by an increase of average
lithospheric density), probably due to metasomatism. Proterozoic
thermo-magmatic events might have led to formation of a two-layer
lithosphere due to Fe-enrichment of its lower part. The boundary
between a highly depleted upper and a more fertile lower layers can be
at ca. 90-150 km depth and can produce a seismic pattern similar to the
top of a seismic low-velocity layer.

Paleozoic rifting had a more severe impact on the lithospheric
structure of the Russian platform, leading to compositional modification
and/or detachment of the entire lithospheric column, its further
replacement by younger fertile material, and the consequent, on-going,
subsidence of the southern Russian platform.

      Cartoon showing reworking of the cratonic crust and
the subcrustal lithosphere of the EEC during Proterozoic,
presumably plume-related rifting, accompanied by intensive
magmatism and metasomatic reworking of the lithosphere (a).

      Thermal cooling at the post-rifting stage (b) leads to thermal
subsidence and basin formation. Magmatic intrusions
accumulate at the crustal base and at the mid-crustal level,
increasing thickness of the upper crust. Ductile middle crust can
be squeezed sidewards. The lower part of the lithospheric mantle
(from the base up to a 100-150 km depth) is metasomatised and
may be removed by thermal erosion and/or by delamination of a
dense lowermost lithosphere.

      Further subsidence (c) is a result of fertilization of cratonic
lithosphere during mantle-plume interaction and involves a much
larger area than affected by rifting. This process is accompanied
by an accretion of a new basal part of the lithosphere, with the
fertile composition typical for Phanerozoic regions.
      If the whole density deficit in the subcrustal lithosphere of
the Russian platform (which is ca. 50% of a typical density
deficit of the Archean-early Proterozoic lithosphere) (Fig. 4a) is
concentrated in its upper part, presumably unmodified by
Proterozoic metasomatism, the lower half of the cratonic root
has been replaced since the Proterozoic. In this case one may
expect a sharp compositional boundary at a depth of ca. 90-150
km from depleted to Fe-enriched composition. This provides an
alternative explanation for the 8o seismic discontinuity (the top of
a low-velocity zone observed in several cratons at ca. 100 km
depth [Thybo et al., 1996) as the base of ancient depleted
lithosphere.

      LAB = lithosphere-asthenosphere boundary.