Mapping Lithologic Components of Ophiolitic Mélanges Based on ASTER Spectral Analysis: A Case Study from the Bangong-Nujiang Suture Zone (Tibet, China)

Ruisi Zhang,Min Zeng

doi:10.3390/ijgi7010034

Abstract

ASTER (Advanced Spaceborne Thermal Emission and Reflection) satellite imagery is useful in assisting lithologic mapping and, however, its effectiveness is yet to be evaluated for lithologic complex such as tectonic mélange. The Mugagangri Group (MG), the signature unit of the Bangong-Nujiang suture zone (BNSZ), Tibet and consisting of ophiolitic mélanges, was previously mapped as a single unit due to its poorly-described internal structures and an informative map with refined lithologic subdivision is needed for future petrologic and tectonic studies. In this paper, based on a combination of field work and ASTER data analysis, the MG is mapped as five subunits according to our newly-proposed lithologic subdivision scheme. In particular, we apply a data-processing sequence to first analyze the TIR band ratios to reveal approximate distribution of carbonates and silicate-dominated lithologies and then the VNIR/SWIR band ratios and false color images to differentiate the lithologic units and delineate their boundaries. The generalized procedures of ASTER data processing and lithologic mapping are applicable for future studies in not only the BNSZ but also other Tibetan ranges. Moreover, the mapping result is consistent with that the MG represents an accretionary complex accreted to the south Qiangtang margin as a result of northward-subduction of the Bangong-Nujiang oceanic crust.

Highlights

Geologic mapping provides essential knowledge for regional geologic studies and remotely-sensed images have been effectively applied assisting geologic mapping for decades [1]
Along its east-west stretch, the Bangong-Nujiang suture zone (BNSZ) is represented by a lithotectonic unit, the Mugagangri Group (MG), which consists of coherent deep-water deposits and mélanges [16,17]
In this paper, based on a combination of field study and ASTER imagery analysis, we mapped out a traverse across the Bangong-Nujiang suture zone (BNSZ) in Gaize, Tibet

Summary

Introduction

Geologic mapping provides essential knowledge for regional geologic studies and remotely-sensed images have been effectively applied assisting geologic mapping for decades [1]. The Himalayan and Tibetan ranges are mostly poorly-vegetated with good rock exposure. Providing such an essential condition for remote-sensing application, ASTER-assisted geologic mapping has rarely been applied in the areas [4] and, its effectiveness needs to be evaluated. The Bangong-Nujiang suture zone (BNSZ) in Tibet (Figure 1) represents the relicts of the Bangong-Nujiang Tethyan Ocean (BNO) and records the collisional orogeny between the Qiangtang and Lhasa terranes (e.g., [7,8,9,10,11,12,13,14,15]). Along its east-west stretch, the BNSZ is represented by a lithotectonic unit, the Mugagangri Group (MG), which consists of coherent deep-water deposits and ophiolitic

Methods

Results

Discussion

Conclusion