Integration of K–Ar geochronology and remote sensing: Mapping volcanic rocks and constraining the timing of alteration processes (Al-Lajat Plateau, Syria)

Abstract

The Al-Lajat plateau (southern Syria) is a Quaternary volcanic field hosting numerous historical and archaeological sites. Based on the integration of K–Ar geochronology and remote sensing methods, a new map is presented for Al-Lajat plateau volcanic field, and a chronological model for alteration processes (such as weathering) in relation to Quaternary paleoclimatic changes. Potential use of the geological data in archaeological investigations is discussed. The alteration patterns of ferric, ferrous, and clay minerals were identified by using Landsat Enhanced Thematic Mapper data (ETM+) methods and ratio images of 3/1, 5/4 and 5/7. Principal Components Analysis (PCA), combined with “Crosta technique” interpretation was used to gauge the response of weathered minerals. Using this technique, high concentrations of ferrous minerals appear brightly colored in the third axis of the Principal Components Analysis (‘PC3’), while oxides show the opposite in PC4. Composite images consisting of ratio images (3/1, 4/5, and 5/7), shown in blue, green, and red respectively, appear to be the most useful for mapping the lithologic boundaries of the main lava flows. Based on new K–Ar ages, the timing of the volcanic activity appears to have been between 56 ± 9 ka and 266 ± 6 ka. Three distinct groups of ages can be identified, confirming the sequence of the basaltic units defined by remote sensing: 1) the youngest unit belongs to Tyrrhenian, between 71 ka and 56 ka, 2) the intermediate unit, Pre-Tyrrhenian to Tyrrhenian between 114 ka and 139 ka, and 3) the oldest basaltic unit, surrounding the Al-Lajat plateau, is dated to at least 266 ka. A chrono-alteration model highlights the role of climatic changes in the distribution of the minerals at the surface of the volcanic rocks as a function of the ages. Weathering conditions during the post-glaciation phase are responsible for clay removal in the study area. Integrating dating and remote sensing data are important for mapping the location and timing of past volcanic activity, but can also be used to identify the sources of archaeological tools and architecture in volcanic areas such as the Al-Lajat plateau.