688 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The island of Barbados lies on a ridge that flanks the convex side of the 

 trough and that plunges northward into deep water. Southward from 

 Barbados, the ridge continues to Tobago, where it merges with a broad 

 shelf off Venezuela. 



Gravity Anomalies 



Since Vening Meinesz's ( 1930) discovery of the belt of high deficiencies 

 in gravity around the islands of the West Indies, the U.S. Navy has taken 

 numerous gravity readings, under the direction of several scientists, and 

 has demonstrated there a strip or belt of great negative anomalies. Its 

 position is shown on the map of Fig. 42.9, which has been compiled by 

 Lyons (personal communication, 1956) from all available sources. The 

 anomaly values along the negative strip commonly reach — 150 milligals, 

 with the largest over the Puerto Rico trough north of Puerto Rico of — 183 

 milligals. Here the axis of the negative strip is practically coincident with 

 the axis of the trough. The negative axis extends over the Barbados ridge, 

 however, as it is traced southwards, and over land in Trinidad and adja- 

 cent Venezuela where negative values of over —200 milligals are re- 

 corded. Another axial strip of high negative anomalies lies just north of 

 the Dutch Leeward Islands and is about coincident with the Leeward 

 trench (Fig. 42.1). 



The anomalies are strongly positive over the Mexican, Colombian, 

 Venezuelan, and Yucatan basins, and also over the Cayman trench ( Fig. 

 42.9), which suggests that the Cayman trench is a different kind of tec- 

 tonic feature from the Puerto Rico trench. 



Concept of the Tectogene 



In order to account for the belt of strong negative anomalies, generally 

 parallel with the rises and troughs of the volcanic arcs but falling in- 

 discriminately on one and the other, Vening Meinesz (1930) concluded 

 that the cause was much more deep-seated than these topographic 

 features and due to masses of lighter density material of great volume 

 downfolded into the heavier subcrustal material. The great downbuckle 

 is illustrated in Fig. 42.10. It was named the tectogene by Kuenen 

 (1936). The gravity anomaly curve is also shown in Fig. 42.10; and it 



may be seen that the relation of the great downfold to the surficial 

 features is direct, but that they are puny in relative size, and that the 

 position of the negative anomaly axis to them is fortuitous. The downfold 

 is thought by some to be driven by convection currents in the mantle, 

 and by others the process of downfolding is thought to stimulate convec- 

 tion circulation. The downfold has been illustrated in model form by 

 Kuenen (1936), and the driving mechanism and nature of surficial de- 

 formation also in model form by Griggs ( 1939 ) . 



In the event that the driving mechanism slows or stops, the tectogene 

 will start to rise through isostatic adjustment, and two broad linear uplifts 

 will appear on either side of the axis of the downfold. Pursuing this 

 thought and mindful of the geology of the Greater and Lesser Antilles, 

 Hess (1933) has written as follows: 



A second great deformation has occurred a considerable time after the first 

 one, during which the tectogene originally was developed. In the interval be- 

 tween the first and second great deformations, one or both of the geanticlines 

 on either side of the tectogene may have emerged above sea level. Erosion of 

 these emergent portions, plus a great contribution of volcanics from the concave 

 side of the arc, may deposit great thicknesses of material in "geosynclines" 

 within the inner geanticline, and perhaps also outside of an outer geanticline, 

 as well as in the central basin over the tectogene itself. This basin over the 

 tectogene will henceforth be called the "geotectocline" because of its different 

 structural behaviour and in many cases its different type of sedimentary se- 

 quence than that which occurs in a geosyncline as the term is generally used 

 today. The second deformation will deform very intensely the material of the 

 geotectocline. Strong folding and perhaps thrusting of the interdeformational 

 sediments, if deposited, will occur, and probably further upthrusting of ma- 

 terial originally squeezed out of the tectogene, if present, will take place. This 

 happens because the material in the geotectocline is pinched between a sort of 

 jaw-crusher as the main crust moves toward the tectogene and down over its 

 rolling hinges. Furthermore, the material which may be on the geanticlines or 

 in the adjacent geosynclines on the sides (or side) away from the geotectocline 

 will be carried forward toward the geotectocline. This material may then im- 

 pinge against the upsqueezed mass in the geotectocline. Upon coming against 

 this bulwark, the weak upper part may be literally scraped off the main crust as 

 it rides forward and down into the tectogene. This is particularly true if very in- 

 competent horizons, such as salt beds or argillaceous sediments are contained in 

 it. The result will be that the cover will be thrown into folds and perhaps de- 

 velop a schuppen structure as its forward progress is stopped by the bulwark 

 and the main crust under-rides or in reality underthrusts it. 



