Analysis of present-day continental deformation in the Iranian plateau using strain tensor extracted from permanent and campaign GPS observations

Document Type : Research Article

Author

Department of Surveying, Faculty of Civil Engineering, University of Tabriz, Tabriz, Iran. E-mail: arastbood@tabrizu.ac.ir

Abstract

Regional GPS networks are now becoming sufficiently dense that one can, with increasing confidence, calculate the full two-dimensional velocity gradient tensor rather than rely on one-dimensional transects. The two-dimensional tensor provides additional insight by allowing one to calculate the vertical axis rotation and dilatation rate tensors. Furthermore, the principal horizontal strain rate axes are not always obvious from examination of the velocity vectors alone. In this study, the Iranian plateau at the oblique collision zone of the Arabia-Eurasia tectonic plates has been selected as the study area. Deformation measured by regional GPS networks in the Iranian plateau reflects the geologic and tectonic variability of the region. Using GPS observations, the velocity gradient tensor can be obtained and by calculating the scalar quantities extracted from this tensor, we can study the continental deformation and interpret the tectonics of the region.
For the Iranian collisional plateau, the regional strain and rotation rate are analyzed by inverting GPS velocity vectors to calculate the two-dimensional velocity gradient tensor. In the general case, estimated deformations using GPS data show characteristics of regional deformation. Principal shortening and extension rate axes, vertical axis rotation, and two-dimensional volume strain (dilatation) are very consistent with long-term geological features over large areas, indicating that the GPS velocity fields reflect processes responsible for the recent geologic evolution of the Iranian plateau. Differences between geological and GPS descriptions of deformation can be attributed either to GPS networks that are too sparse to capture local interseismic deformation, or to permanent deformation that accurs during strong earthquakes.
Vertical axis rotation amplitude and sign changes are due to distributed deformation throughout the plateau. The presence of large regions with almost constant low amplitude rotation rates indicates a quasi-rigid bodies rotation inside the Iranian collisional plateau bounded by faults. The estimated compressive axis confirms the direction of the Arabia-Eurasia tectonic plates collision. The internal amplitude of shortening is lower than the boundaries of the plateau and has a negative two-dimensional dilation almost everywhere. A negative dilation rate is associated with vertical crustal uplift.
The maximum value of negative volumetric strain was obtained in the southern part of Eastern Alborz. It was calculated as 8.1×10-3 and 4.6×10-3 per year, respectively, using the nearest neighbor and weighted distances methods.
We show the superiority of the gridding method over the Delaunay triangulation method. On the other hand, to study the strain locally in regions with active deformation, if the number of GPS stations in there is high, the value of α in the distance-weighted method or the number of neighborhoods in the nearest neighbor method should be chosen less. Current two-dimensional GPS networks are adequate to resolve first-order regional-scale instantaneous strain variations. However, the resolution of some of the issues raised here must await the deployment of dense, continuous, and high-rate GPS networks. Such networks, scaled to the dimensions of the problems one wishes to study, will provide more regular temporal sampling allowing one to calculate more reliably near fault interseismic strain. More importantly, they will bring a critical third dimension of velocity measurement, allowing one to calculate the full three- dimensional velocity gradient tensor. Furthermore, the deployment of dense networks of continuous GPS stations can be used in conjunction with differential radar interferometry to provide a more spatially and temporally complete snapshot of tectonic deformation.

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References

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Journal of the Earth and Space Physics
Volume 49, Issue 1
online publication date: 14 June 2023
June 2023
Pages 97-117

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