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STRUCTURAL GEOLOGY OF NORTH AMERICA 



of the plutonic intrusive bodies which form a belt across the island. Moreover, 

 the northern major fault roughly coincides with the belt of pillow lavas and 

 volcanic breccias that were extruded during Robles (Late Cretaceous) time. 

 That belt of volcanism may have been located along a regional line of weakness 

 and the younger plutonic intrusives may have moved upward in part along this 

 same general zone. 



Although the general anticlinal structure of eastern Puerto Rico is probably 

 a result of doming by a batholith, several localized anticlines and synclines 

 have been formed by the movement of fault blocks (Fig. 42.6). The largest of 

 these secondary structures is the northeastward-plunging, faulted anticline near 

 the northeastern corner of the island. The Luquillo Range is the northwest limb 

 of this breached and faulted fold. This structure probably was formed by com- 

 pression from the northwest as the crustal block north of the transcurrent fault 

 moved eastward. Tight folding is localized near some of the faults but is not 

 extensively developed in eastern Puerto Rico. 



Ry comparing Kaye's and Rerryhill's analysis with that of Rutterlin 

 ( chart, Fig. 42.3 ) it will be seen that Rutterlin suggests older rocks than 

 they found on the island, and that late Eocene and early Oligocene forma- 

 tions are present whereas they indicate a hiatus for this interval. 



Rutterlin also points out that broad arching with an east-west axis was 

 the dominant part of the mid-Miocene disturbance. 



Rerryhill (1959) has elaborated on the transcurrent faulting to the 

 effect that the two principal faults or sets of faults divide the island into 

 three blocks, with the northeastern and southwestern blocks having 

 moved toward the southeast and the central block toward the northwest. 

 This is presumed to reflect eastward movement of the Caribbean block. 

 He assigns the major faults to an Eocene age, whereas Kaye recognized 

 the many "block-faults" as late Pliocene and early Pleistocene. 



Isla Mona and the Mona Passage 



Isla Mona, 21 square miles in area, and Isla Monito, less than one 

 quarter square mile, are situated in the Mona Passage between Puerto 

 Rico and Hispaniola. Isla Mona is a limestone tableland bounded by steep 

 to vertical cliffs except for a narrow coastal terrace about its southern 

 perimeter ( Kaye, 1959 ) . The Isla Mona limestone forms most of the mass 

 of both islands and is probably early or middle Miocene. Dips up to 3/2 

 degrees are visible in the cliffs. In places a thin cavernous lime- 

 stone, the Lirio, overlies the Isla Mona, and in one place a small angu- 



lar unconformity is visible. The Lirio is Pliocene or Pleistocene in age. 

 The great purity of the Isla Mona limestone indicates that it was de- 

 posited in an oceanic reef environment far from land, and from this it is 

 deduced that the Mona Passage was in existence in Miocene time ( Kaye, 

 1959). 



Jamaica 



Physiography. Jamaica measures 144 miles from east to west and has 

 a greatest width of 49 miles. It is very mountainous, with about one-half 

 of its area 1000 feet above sea level and much of it over 2000 feet. The 

 principal range, called the Rlue Mountains, occupies an axial position at 

 the east end of the island, and has a sharp crest and numerous, generally 

 cloud-wrapped peaks, the highest of which is 7520 feet above sea level. 

 From the sea on the north, the land rises in gentle hills to the higher 

 country, but on the south high cliffs and abrupt precipices mark the shore- 

 line. See Fig. 42.7. 



The relief of Jamaica is of four major types: (1) the interior mountain 

 ranges, constituting the nucleus of the island; (2) an elevated and dis- 

 sected, arched and karsted, white limestone plateau which surrounds the 

 interior mountains and ends abruptly toward the sea, occupying in all 

 fully four-fifths of the total area; (3) the coastal bluffs or back coast 

 border of the seaward margin of the plateau; and (4) a series of low flat 

 coastal plains between the sea and the back coast border (Schuchert, 

 1935). 



Jamaica is separated from Cuba by 90 miles of water, and the marine 

 basin between is the Cayman trench, here everywhere more than 15,000 

 feet deep and directly off Cuba, 21,000 feet deep. The long and narrow 

 peninsula of Haiti is about 90 miles northeast of Jamaica, and the two 

 islands are separated by water which has a general depth of over 4000 

 feet. On the south side of Jamaica lies the Caribbean Sea, whose bottom 

 sinks to 13,800 feet. From the island to Honduras it is 900 miles, and the 

 intervening area is mainly shallow water. It is a broad platform on which 

 the Mosquito, Rosalind, and Pedro banks occur, and which drops off 

 steeply into the Cayman trench on the north and slopes gently into the 

 Caribbean on the south. 



