Geology and Relief

There are four major mountain ranges in the Dominican Republic which run west-northwest to east-southeast and parallel to each other. From north to south these are the Cordillera Septentrional, the Cordillera Central, the Sierra de Neiba and the Sierra de Bahoruco. They are separated by basins, the Cibao Valley, the San Juan Valley, and the Enriquillo Basin. The less mountainous eastern part of the country consists of the Eastern Plains (Llanura Costera) and the Cordillera Oriental, a minor mountain range, to their north (figure 4).

The Cordillera Central is the highest of the Dominican Republic's mountain ranges. Its highest peak, the Pico Duarte (3175 m a.s.l.), is at the same time the highest mountain of the Caribbean.

The Sierra de Bahoruco and the Sierra de Neiba, with the sub-sea-level depression of the Enriquillo Basin between them, both reach elevations of over 2000 m, while the Cordillera Septentrional has just one peak above 1000 m.

The upper catchment area of the Río Yaque del Norte has elevations between 320 m (Tavera reservoir) and 3038 m (Loma La Rucilla). The area is characterized by large and small valleys with steep slopes. Slopes of more than 40 % make up the majority of the UCRYN. The only extended area with low gradients is the valley around Jarabacoa.

The mountainous nature of the Dominican Republic and the whole island of Hispaniola is due to its position at a plate boundary zone and the resulting tectonic activities.

Geologic and tectonic development

Hispaniola is situated in the northern Caribbean plate boundary zone where the Caribbean plate meets the North America plate (Erikson et al. 1998). Since Jurassic time, convergent and transcurrent tectonics in this zone have led to the development of the Greater Antilles islands (Lewis & Draper 1990).

The core of the island of Hispaniola consists of Early Cretaceous to Eocene island arc terranes formed at a convergent plate boundary. During these times, phases of subduction and the associated island-arc plutonism, volcanism, and metamorphism (mainly in a submarine environment) alter-nated with phases of uplift, deformation and erosion (Mann et al. 1991a). The resulting meta-morphic rocks can be found today in central and northern Hispaniola. In addition, there are several terranes of volcanic, volcaniclastic and associated sedimentary rocks of mainly Upper Cretaceous to Eocene age, and a fragment of a back-arc basin with marine sandstone, siltstone and limestone.

Intrusive rocks were emplaced mainly in Central Hispaniola during episodes of magmatism until the Early Eocene (Kesler et al. 1991). To the south of the island, an oceanic plateau grew during Late Cretaceous time, with basalts, pelagic limestone and chert. Paleocene to early Miocene subsidence was accompanied by continued deep-sea carbonate sedimentation in this area (Lewis &

Draper 1990).

In the Eocene, the island arc collided with the southern edge of the North America plate (Bahama carbonate platform), leading to uplift of the submerged arc and putting an end to island-arc volcanism and plutonism. The relative movement of the plates in the boundary region changed from convergent to mainly east-west strike-slip faulting, with the Caribbean plate moving east relative to the North America plate. In northern Hispaniola, Oligocene sedimentary basins developed within a strike-slip fault system at the edges of today's Cibao valley, resulting in the Tavera belt in the

northern foothills of the Cordillera Central and the El Mamey Belt of the Cordillera Septentrional (Dolan et al. 1991).

The oblique collision of the southern oceanic plateau with island-arc Hispaniola during Miocene time resulted in the suturing of the involved terranes and initiated the Miocene to recent transpressional phase of Hispaniola's tectonic history (Mann et al. 1991a). Uplift and erosion occured across Hispaniola. The southern oceanic plateau terrane was folded and emerged in the form of the southern limestone mountain ranges (Sierra de Bahoruco in the Dominican Republic).

Under the conditions of oblique convergence in the northern Caribbean plate boundary zone, many terrane boundaries were reactivated as oblique-slip faults. Today's fault-bounded west-northwest striking mountain ranges with interjacent basins were formed. Three deep sedimentary basins developed and were filled with neogene clastic sediments: the Cibao, the San Juan-Azua, and the Enriquillo-Cul-de-Sac Basin. Left-lateral strike-slip faults, in accordance with the relative plate motion, bound the northern edge of the Cibao basin and transect the Enriquillo-Cul-de-Sac basin, while the margins of the San Juan and Enriquillo basins are bounded by thrust and reverse faults due to transpression (Mann et al. 1991b).

Earthquakes occurring in the Dominican Republic make it clear that the region is still tectonically active (Cross 1992), and rapid uplift and erosion of the mountain ranges in central and western Hispaniola are continuing (Mann et al. 1991b).

Regional geology of the upper catchment area of the Río Yaque del Norte

The UCRYN is situated in the north-eastern Cordillera Central, which is dominated by Cretaceous volcanic rocks with upper Cretaceous to Eocene granitoid intrusions (figure 6). To the south of the Guacara fault, the El Rio batholith has intruded into volcanic and volcaniclastic rocks of the Tireo formation. This large intrusion consists of unfoliated tonalite, a silicic coarse-grained intrusive rock (Kesler et al. 1991). The rocks of the Tireo Group which surround the batholith are of upper Cretaceous age. They comprise mainly vitric tuffs with intercalated sediments (mudstones, limestones), besides basaltic flows (for instance around the Pico Duarte) and more acidic rocks like quartz keratophyres (Lewis et al. 1991).

To the north of the Guacara fault, the Duarte Complex forms an extensive metamorphic belt, which is intruded by several tonalite plutons (Draper & Lewis 1991). In addition, there are smaller areas of hornblendite and diorite intrusive rocks. The Duarte Complex, which is probably of lower Cretaceous age, comprises greenschist, weakly metamorphosed subgreenschist and amphibolite metamorphic facies. Mafic metabasalts of the Duarte Complex dominate the area along the Río Yaque del Norte to the north of Jarabacoa, with some subgreenschist facies rocks further north.

Amphibolites are developed in the proximity of the plutons. Only small parts of the UCRYN are taken up by the serpentinized peridotite which forms a belt at the north-eastern border of the Duarte

Complex. These rocks are very conspicuous for example in Landsat images, especially in the main part of the belt just outside the study area, because their ultramafic nature results in infertile soils and a different vegetation (pine forest) compared to the surrounding areas.

The northern boundary of the Duarte Complex is formed by the Hispaniola Fault Zone. This broad fault zone contains, in the area of the Tavera reservoir, Oligocene sedimentary rocks of the Tavera belt, namely conglomerate, mudstone, sandstone, and limestone (Dolan et al 1991).

Quarternary fluvial deposits cover the valley floors, especially where the valleys widen around Jarabacoa and in the area of Manabao.

Figure 6: Geological map of the study area. D: Duarte Complex, T: Tireo Group, to: tonalites, um: ultramafic, pi:

hornblendite, ar: sandstone, conglomerate et al. of Tavera belt, q'f: alluvium. Source: Mapa Geológico de la República Dominicana 1:250,000, BGR 1991, modified.