Negative Bouguer Gravity Anomalies Research Articles

World rift system

The midocean ridge system is a continuous tectonic feature over 60,000 km in length which covers an area equal to that of all the continents (Figure 1). It is the longest single geological structure on the surface of the Earth.Associated with the center of the ridge over much of its length is a fracture or rift which is the locus for many shallow earthquakes. Although the ridge system is generally restricted to the ocean basins, it crosses the continental margins at several points. One point is off the west coast of Mexico, where it extends through the Gulf of Baja California and into the fracture zones of the western United States; another point, located in the Indian Ocean, extends through the Gulf of Aden into the Red Sea. Here it bifurcates, one branch extending north to the Gulfs of Suez and Aqaba, the Dead Sea, and the Jordan valley, the other branch extending south to the great east African rift‐valleys. Topographic profiles across the system (Figure 2) illustrate the changes occurring as one goes from continent to ocean. The Gulf of Aqaba is a narrow rift marked by negative Bouguer gravity anomalies which, like the Gulf of Suez, bears a close resemblance to the east African rifts. The rift zone is much wider in the Red Sea, and it is marked by positive Bouguer gravity anomalies and, in the southern part especially, by very strong magnetic anomalies in the central portion. Seismic refraction measurements indicate that the shelves of the Red Sea are founded upon downdropped continental crustal blocks (5.8 km/sec), whereas the axial trough is based on high‐velocity (7+ km/sec) basic material (Figure 3). Similar high velocities are found in the Gulf of Aden, where the soundings indicate a rift to the west grading into a low ridge with a median rift as one moves toward the Indian Ocean. In the Indian Ocean a true ocean ridge system is found with an axial rift marked by numerous earthquake epicenters and frequently by a distinctive magnetic anomaly. Although there are few seismic refraction measurements in the Indian Ocean, the structure must be similar to other parts of the ridge system where velocities of 7+ km/sec are found at relatively shallow depths. Heat flow measurements from the Atlantic, Pacific, and Indian oceans show systematically higher values in the vicinity of the ridge, although the amount of increase varies (Figures 4 and 5). Heat flow measurements in the marginal Gulf of Aden and in the Gulf of Lower California also reveal higher than normal values. Several wells drilled in the Imperial valley just north of the Gulf of Lower California have encountered bottom hole temperatures in excess of 550°C and an unusually high concentration of minerals in the brine. Of a few measurements made recently in Lake Nyasa, the most reliable indicated high values due to a fairly local heat source.