『Abstract
We assess the results of a number of deep seismic soundings performed
in China over the last few decades, and study the variations in
crustal structure in 18 tectonic units comprised of three platforms
and 15 fold systems. Thickness data on 344 Mesozoic-Cenozoic sedimentary
basins, as well as data on Moho depth are collected in order to
discuss the relationship between the thickness of the basins and
the average thickness of the consolidated crust in each tectonic
unit. The degree of mirror-image symmetry between Mesozoic-Cenozoic
sedimentary basins and the uplifting topmost parts of the mantle
is herein analyzed using deep geophysical data on sedimentary
basins. By applying standard methods of least-squares analysis
to both datasets, we have obtained both the average thickness
of the consolidated crust and the mirror-image symmetry factor
for every platform and tectonic fold system, thereby allowing
us to explore the correlation between the depths of the bottom
of the sedimentary basins and the top of the uplifting mantle.
The thickness of the consolidated crust in China is found to be
between 20 and 63 km, following a pattern of gradual thickening
from east to west. Expressed in terms of spatial seismicity and
the concentration of seismic energy, and according to the sharing-out
of earthquake hypocenters in the top 80 km of the earth, the rheology
of the area does not appear to suit the widely accepted “jelly-sandwich”
model for the continents, which does not seem to be entirely valid
in China. The findings on the mirror-image symmetry factor show
that this parameter varies mainly in the range -0.5 to -1.8 compared
with each other tectonic unit. Most of the tectonic systems in
China appear to be isostatically compensated. Tibet is an exception,
in that the symmetry factor is positive for the Gangdise-Nyainqentanglha
(1.0) and Himalayan (0.1) fold systems, implying that these tectonic
zones are far from being in the isostatic equilibrium of the other
regions. We have also analyzed the zoning characteristics through
the logarithmic relationship in R = ln (h/|a|), using the average
thickness of the consolidated crust and the absolute value of
the symmetry factor. The key finding is that regardless of geographical
location, all the values of R = h/|a| for those sedimentary basins
in which oil/gas reservoirs have to date been found, fall into
the narrow range of values between 19.38 and 37.40. There is some
appeal in a possible relationship involving the ratio of crustal
thickness to symmetry factor, more so when the results obtained
appear to suggest a prognostic tool for exploratory practice in
relation to oil/gas reservoirs.
Keywords: Moho depth; crustal rheology property; mirror-image
symmetry; isostasy; oil/gas reservoirs; China』
Contents
1. Introduction
2. Tectonic partition
3. Deep seismic soundings at a glance
4. Characteristics of the crust
4.1. Moho depth
4.2. Crustal rheology
4.3. Sediment power and thickness of the crust
5. Symmetry signature of the crust
5.1. Concepts
5.2. Method and symmetry features
5.3. Data uncertainty and estimation of errors
6. Discussion
6.1. Isostatic compensation
6.2. On the ratio of crust thickness to symmetry factor
7. Conclusions
Acknowledgements
References