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Tectonic and crustal structure of the Eastern Pontides using Bouguer gravity data

Elmas Ali

Acta Geophysica

2021

Vol. 69, no. 5

1637--1650

To study the crustal structure of the Eastern Pontides, gravity data obtained from the World Gravity Map (WGM2012) were analyzed using total horizontal derivative (THD), Parker–Oldenburg (PO) inversion, and normalized full gradient (NFG) techniques. The THD method was applied to the data after bandpass fltering of the Bouguer gravity data in order to image the discontinuities in the basement levels. The maximum amplitude values of the THD were used to reveal the discontinuities caused by the density diference in the Eastern Pontides and its vicinity. In addition, the basement upper surface topography of the region was calculated and mapped with the inverse solution, and the presence of uplift areas in the Eastern Pontides was determined. As a result of the inverse solution, it was found that the depth of the basement in the Eastern Pontides belt reached approximately 5.5 km. An average basement upper surface depth of 3 km was calculated for the area under the Black Sea coastline. Finally, depth values were calculated using the NFG method under the determined profles, and the depth values found by the inverse solution were tested. For the basement upper surface topography, the results obtained from the two depth calculations were found to be compatible, within error limits of ±0.2 km.

Interpretation of gravity anomaly to delineate thrust faults locations at the northeastern part of India and its adjacent areas using source edge detection technique, tilt derivative and Cos(θ) analysis

Ghosh Gopal K.

Acta Geophysica

2019

Vol. 67, no. 5

1277--1295

The Northeast India and its adjacent areas converge among the three diferent plates, viz. Eurasia, India and Sunda plates. The tectonic interaction of Northeast India and underlying dynamics of the Himalayas as well as the Indo-Burma Ranges might cause the Assam Syntaxis. The area of study is located between latitude 23°–28°N and longitude 88°–96°E and situated in one of the most seismically active tectonic provinces in the world with seismic zone-V. This area had demonstrated several thrust faults activities and tectonic evident accomplishments during the recent past. The complicated geotectonic setups inspirits various smaller magnitude earthquakes, and the current seismicity shows seismic activities are still enduring in the Shillong Plateau, Arakan-Yoma fold belt, Bengal Basin, Naga Hills, Mikir Hills, Upper–Lower Brahmaputra Valley and Mismi Hills of Himalayan foothills. It is imperative to obtain wide-ranging learning tectonic confguration, thrust faults delineation for improved geoscientifc study. Parts of the areas are extremely unreachable, and very limited thrust faults were marked by studying GIS map received from the various agencies and feld geological study. During the past studies, most of the prominent lineaments/thrusts are marked; however, many active and hidden thrust faults are still unidentifed. Seismic data can provide better information about the thrust faults locations, but due to small number of seismic data, the information is not adequate. In this paper, attempt has been made to study and reinterpret the available ground gravity data of northeastern parts of India for understanding thrust fault locations using various applications of gravity derivatives like analytical signal, horizontal gravity gradient, tilt derivative, horizontal tilt angle derivative and Cos(θ) analysis. Source edge detection technique has also been premeditated to categorize thrust fault locations. It is understandable that the low gravity is observed at Assam Valley which contributed sediment accumulations and higher gravity anomaly observed at Shillong Plateau and Bengal Basin containing denser formations. Bouguer gravity data is used after isostatic correction assuming Airy’s isostasy root depth model and frst-order trend removal using least square technique. The derived thrust fault locations from the present study are superimposed with the existing thrust-fault locations for correlation. Some additional thrust faults are narrated which are not previously mapped. It is also suggested that Brahmaputra Thrust, Dauki Fault, Naga Thrust, Disang Thrust and Kopili Fault have key responsibility for high seismicity and tectonic movement causing upliftment and depression that encouraged some anticlockwise rotation in the area.