Russian Journal of Earth Sciences
Papers in press
Crustal shortening during mountain building: A case study in the Pamir–Tien
Shan and Altay–Mongolia Region
F. L. Yakovlev and S. L. Yunga
Abstract
Unraveling the underlying causes of mountain building is one pivotal issue of
geotectonics. The most popular model invokes crustal thickening through
horizontal, across mountain ranges, contraction of the area occupied by a
mountainous country with its ensuing isostatic uplift. Another model calls for
crustal underplating due to either underthrusting or to melts derived from mantle
plumes, also resulting, through isostatic mechanisms, in mountain building. The
objective of this paper is to establish relations between these two likely processes
using case studies from the Pamir–Tien Shan region, as well as Altay and Mongolia.
In tackling this problem, two methods are employed to define the amount of
crustal shortening. Method 1 draws on crustal thickness as a starting value, and
shortening is calculated as a function of its increase relative to the initial "cratonic"
thickness. In contrast to well-known approaches, geometric characteristics of
magnitudes of neotectonic movements used in this algorithm enable one to establish
the direction of maximum shortening and the relation between maximum and
minimum shortenings. A data base on Moho depths and neotectonic movement
magnitudes from Tien Shan was used. The N–S crustal shortening was found to be
between 12 and 25% for Tien Shan. Method 2 is a modification of routine
calculations based on folding and faulting deformations of peneplanation surfaces.
Shortening values thus obtained were 4–12% on average for Tien Shan, 35 to 60%
for the Afghan–Tajik basin, and less than 1% for Pamir, whereas Altay, Sayan, and
Mongolia yielded both extension and shortening of 0.1% to 1.2%. Comparison of
crustal shortening values obtained for Tien Shan by the two different methods
shows a match of results obtained from maximum shortening directions and from
spatial distribution of shortening maxima. A good correlation of these values was
established, which is interpreted to show the model of crustal shortening and
thickening (Method 1) to be in keeping with the natural process. Analyzing the
regression of paired shortening values shows that lack of shortening from
peneplanation surface deformations corresponds to a crustal thickening of 7 km on
average across Tien Shan. This is interpreted to suggest that, in Tien Shan, the two
mountain building processes, horizontal shortening and crustal underplating, are in
operation concomitantly.
Therefore, one cause of mountain building in Tien Shan is pressure exerted
by Pamir and Tarim and transmitted 300–400 km northward from their
boundary. In Mongolia, Sayan, and Altay alike, mountain building might be
driven by the same causes related to rifting. Crustal shortening data rule out
any impact from the Indian subcontinent pressing northward.