ANTILLEAN-CARIBBEAN REGION 





Colombian Basin 



The Colombian basin appears to have the same volcanic layer as the 

 Venezuelan but the gabbroic layer is much thicker. The gabbroic layer 

 is especially thick under the Nicaraguan rise but thins somewhat under 

 the basin. 



ORIGIN OF THE CARIBBEAN BASINS, TRENCHES, AND RISES 



Magmatic Activity 



It appears evident from the widespread occurrence of volcanic and 

 intrusive rocks in the islands of the Greater and Lesser Antilles, and in 

 the submerged areas as interpreted from the seismic layers, that the de- 

 velopment of the Caribbean region started with oceanic crust and pro- 

 ceeded to evolve by abundant and widespread magmatic activity (J. I. 

 Ewing et al., 1957 ) . Although previous theories have held the magmatic 

 activity to be secondary to tectonic forces of compression or shear, J. I. 

 Ewing et al. believe it may be the primary cause of the island arc struc- 

 ture. As suggested in Chapter 33 on the igneous provinces, the upper 

 mantle is believed to melt partially at times and in places, and to yield a 

 liquid of basaltic composition which intrudes the gabbroic layer and 

 adds to it. In continental areas the heat released from the basaltic intru- 

 sive masses may result in the melting of lower parts of the sillicic crystal- 

 line basement and produce large volumes of monzonitic magma, but in 

 the oceanic areas no silicic layer is present to be melted, and only frac- 

 tionation of the basaltic magma can occur to produce eruptives other than 

 basalt. Perhaps large volumes of the extruded rock is andesite. ( See Chap- 

 ter 33 on origin of andesite.) In the major basins perhaps the volcanics 

 are fissure flows and mostly basalt. The variable velocities will depend 

 on the relative amounts of basalt and andesite, on the porosity of the erup- 

 tives, and on the presence and volume of later intrusives into the vol- 

 canics. 



The thickening of the gabbroic layer and the accumulation of a number 

 of kilometers of volcanics on the thickened basalt layer, plus intrusives 

 in the volcanics will make up a crust which stands higher than the 

 adjacent oceanic crust. Therefore, in the manner postulated by Benioff 



in Chapter 32 for the orogenic belts along the Pacific margin, the higher 

 crust of the Venezuelan and Colombian basins will tend to flow outward 

 toward the lower Atlantic and Pacific crusts and override them. This ac- 

 counts for the compressional structures at the junction and the formation 

 of the complementary upfold and downfold (rise and trench) of the 

 Puerto Rico and Lesser Antilles arc, and also for the arcuate map pattern 

 of the belt of deformation (theory of J. I. Ewing et al. [ 1957] and Officer 

 et al. [1957] ). The overriding of thick crust on thin crust generates a shear 

 which dips under the thick crust to great depths (Chapter 32 and Fig. 

 38.3) and provides an avenue for volatiles and perhaps even magma to rise 

 further from the mantle. This engenders additional volcanic activity in 

 the uplift inside the trench. 



Since continental Venezuela and Colombia stand higher than the adja- 

 cent basins, the tendency will be for the continental margin to move north- 

 ward and override the basin. The deformed belt of the Dutch Leeward 

 Islands and the Leeward and Los Roques trenches, together with the belt 

 of negative gravity anomalies, support this postulate. The Colombian 

 basin crust may have tended to flow westward toward the Pacific, and 

 the Central American trench suggests this idea (Chapter 32), but the 

 trench continues northwestward to southern Mexico beyond the sphere of 

 influence of the Colombian basin. Conditions are complex in Central 

 America and will be commented on later. 



Mexican, Yucatan, and Cayman Depressions 



It has been pointed out that M. Ewing and associates believe the Cay- 

 man trench and the thinned crust under it to denote a structure which 

 is the result of tension. In cross section the structure is like a necked por- 

 tion of a steel rod which has been deformed under tension. The Yucatan, 

 Mexican and southern part of the Colombian basin are structures which 

 in line of cross section (Fig. 41.15) appear to be similar to the Cayman, 

 but their shape in plan view must also be regarded. 



The Cayman and Yucatan basins are relatively narrow and long, and 

 are marked by strong positive gravity fields. Tin- Mexican and Colombian 

 basins are broad, but also are marked by positive gravity fields. The active 

 seismic belt passes westward from Puerto Rico through Hispaniola to the 



