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A comparative approach on TIR and VNIR-SWIR datasets of ASTER instrument for lithological mapping in Neyriz ophiolite zone, SW Iran
Majid H. Tangestani
Department of Earth Sciences,
Shiraz University, 71454 Shiraz, Iran
Tel: +98-711-2284572
Fax: +98-711-2280926
tangestani@susc.ac.ir
ABSTRACT
The Neyriz ophiolite occurs along the Zagros suture zone, SW Iran, and is part of a 3000-km obduction belt that was thrust over the edge of the Arabian continent during the Late Cretaceous. This complex lithologically consists of ultramafic unit, layered and massive gabbros, sheeted dykes, and pillow lavas, surrounded by radiolarites and limestone. The TIR and VNIR+SWIR datasets of Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) instrument were used for enhancing the rock units. The VNIR-SWIR spectra of field samples were measured and implemented as the potential spectral end-members, as well as the published spectra of the USGS library. ASTER matched-filter and spectral angle mapper (SAM) algorithms as well as minimum noise fraction (MNF) and principal components analysis (PCA) processing were used for lithologic mapping. Output images were compared and evaluated with the field evidences in order to evaluate the capability of each dataset in mapping the lithological units. Results showed that TIR dataset enhances the ophiolite units more efficiently and discriminates the ultramafic, pillow lavas and sheeted dykes as well as the radiolarites and marbles; while the VNIR-SWIR dataset enhances different Cretaceous rock units, northeastern Zagros suture zone.
1. INTRODUCTION
The Neyriz ophiolite, found in a semi-arid environment along the Zargos Thrust Zone, SW Iran, is a well-preserved part of the oceanic lithosphere (Sarkarinejad, 2005). The main ophiolite body outcrops northern the Bakhtegan Lake and predominantly consists of mafic and ultramafic rocks. Adjacent rock units are mainly limestones, exposed at the northeastern and southern parts of the body.
The Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) (Fujisada, 1995; Iwasaki et al., 2002) instrument is included on the Earth Observing System (EOS) TERRA platform, and records radiation from the Earth in 14 spectral bands. This instrument, providing enhanced capabilities for geological mapping and mineral exploration, measures reflected radiation in visible-near-infrared region-VNIR, in three bands (between 0.520 and 0.860 µm) and in short-wave infrared region-SWIR, in six bands (from 1.00 to 2.43 µm), with 15-m and 30-m spatial resolution, respectively (Fujisada, 1995). Stereoscopic images can be acquired at 15-m resolution by imaging with the back-looking telescope as well as the nadir-viewing system. Emitted radiation is also measured in five bands in the 8.125- to 11.650-µm wavelength region (thermal-infrared region-TIR) at 90-m resolution. The swath width is 60 km, but the pointing capability to 232 km (Fujisada, 1995).
The purpose of this paper is evaluating and comparing the VNIR-SWIR and TIR data of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) for mapping the lithological units exposed at the Neyriz ophiolite zone. This paper implements the laboratory spectral features of rock units exposed at the study area as well as spectral curves of the same rock units available from USGS spectral library. In order to determine the effectiveness of ASTER data for mapping the lithologic units outcropped at the Neyriz ophiolite zone several image processing procedures such as spectral angle mapping (SAM) and matched filtering (MF) were explored.
2. GEOLOGY
The Neyriz ophiolite is part of a 3000 km obduction belt that was thrust over the edge of the Arabian continent during the Late Cretaceous. This ophiolite occurs along the Zagros suture zone, southwestern the Sanandaj-Sirjan metamorphic belt, where the Arabian and Eurasian plates have collided, and is considered to be an allochthonous fragment of Tethyan oceanic crust and mantle (Alavi, 1994).
A geological map at scale of 1:100,000 is compiled and published by Geological Survey of Iran (1996). The same geological map as well as field observations was used for comparing the output images to the rock units. The rock units of the study area (53° 52' 00" – 54° 17' 30" E, and 29° 07' 11" – 29° 40' 05" N) occur at five geological zones: 1) Zagros folded zone; 2) Zone of Pichakan radiolarite; 3) Ophiolitic zone; 4) Tertiary flysch zone; 5) Sanandaj-Sirjan zone. The following paragraphs summarize general characteristics of geological Formations and rock units.
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