Magnetic Surveys in parts of Eastern Ghats Mobile Belt,India

Regional magnetic surveys were conducted in three different areas of Eastern Ghat Mobile Belt (EGMB) comprising coastal districts of Andhra Pradesh. Out of these three areas, area I is about 15,000 sq.km belongs to NE coastal districts of Andhra Pradesh, India and is bounded by latitudes 17030’-18045’N and longitudes 82030’ - 84030’E. Area II covers EGMB in the northern half and a part of the petroliferous Krishna-Godavari (KG) basin in the southern half measuring 8000 sq.km and bounded by latitudes 16030’-17030’N and longitudes 81045’- 82045’E. Area III covers parts of Eastern Ghats Mobile Belt and south west Krishna basin occupying an area of 5800 sq.km lying between 150-160N latitudes and 79045’-80045’E longitudes. Interpretation of magnetic anomalies obtained in the above three areas revealed that the basement is mainly Charnockitic in nature in all the three areas. The hypothesis of Narayana Swamy (1975) that the Khondalitic rocks of the EGMB are underlain by the Charnockitic basement was proved for the area I. The investigation in area II clearly established that the Charnocktic rocks extend into the KG basin also, and is probably involved in faulting and folding simultaneously with the Khondalitic group of rocks during different stages of EGMB tectonics as well as sedimentation in the KG basin. The study in area III, shows that the Charnockites extend south of Ongole also, and attributes the seismicity of the region to the faulted Charnockitic basement. The inferred basement was faulted and folded due the tectonic activity that had shaped the east coast of India, EGMB. The continued seismic activity in the Ongole region can therefore be clearly related to adjustments of various faulted basement blocks and reactivation of some of the faults within the Krishna trough. Similar low-magnitude earthquake activity reported around Vizianagaram in the northern part of the EGMB, was also traced to the adjustments of faulted Charnockitic basement blocks. Locations of the study areas