Geology of Northeastern Venezuela

The transpressional plate boundary zone looks like a classical fold and thrust belt (Fig. 14) with a metamorphic hinterland in the north, the Caribbean Mountain system, and a non-metamorphic foreland fold and thrust belt in the south, the Serran’a del Interior. Recent studies have shown that the time of metamorphism and D₁ deformation in the hinterland is mid-Cretaceous (e.g., Loubet et al., 1985; Stockhert et al., 1995; Ave Lallemant, 1997; Smith, 1996; Smith et al., 1999), whereas the deformation in the foreland is Tertiary (Beck, 1978, 1985; Audemard, 1991). Like the arc, it appears that the metamorphic belts formed far to the west, migrated eastward, and were emplaced onto the South American plate in Tertiary time.

(1)South Caribbean Deformed Belt. This belt is the Tertiary accretionary wedge related to subduction of the Caribbean southward beneath the Leeward Antilles arc and South America, as discussed above. In the west it is very pronounced likely because it represents the earliest point of subduction polarity reversal.

(2) Leeward volcanic arc. The Leeward Antilles volcanic arc is the southern extension of the Lesser Antilles arc and the remnant Aves Ridge arc. Magmatic activity in the Leeward Antilles started in mid-Cretaceous time (e.g., Pindell, 1993). The basement rocks of some of the Dutch Leeward Antilles (Aruba, Cura?ao, Bonaire) are related to plateau basalt extrusion (e.g., White et al., 1999), and indeed the oceanic crust of much of the southern Caribbean is unusually thick (~12km). The younger volcanic and plutonic rocks however have intermediate to felsic compositions and are related to primitive island arc magmatism (e.g., Beets et al., 1984). It appears that magmatism in the arc gradually ceased from west to east: at about 85 Ma on the Dutch Leeward Antilles (Priem et al., 1979) and at about 45 Ma on Los Testigos Islands just east of Margarita Island (Figs. 2-3) (Santamar’a and Schubert, 1974), but most of the ages were determined using the K/Ar method which has been shown to be unreliable.

The boundary zone between the Leeward Antilles arc and the Caribbean Mountain system is mostly covered by the Caribbean Sea; seismic reflection studies (e.g., Silver et al., 1975; Erlich and Barrett, 1990; Ysaccis, 1998) suggest that this boundary is heavily disrupted by Neogene extensional basins (Bonaire, Cariaco, Carœpano, and Paria basins) resulting from displacement partitioning in the arc (Fig. 11; Ave Lallemant, 1997), by en-echelon EW strike-slip faults (Pull-aparts), or by transtension between 10 and 3 Ma (Fig. 5; Pindell et al., 1998; Babb and Mann, 1999; Flinch et al., 1999).

(3) Caribbean Mountain system. The Caribbean Mountain system is subdivided into a number of belts based on metamorphic grade and lithology. The northernmost belt consists mainly of high to intermediate grade metamorphic rocks (epidote amphibolite and greenschist facies) which include eclogites (1400-2200 MPa/ 450-700^oC) and blueschist (500-800 MPa/ 300-500^oC) metamorphic rocks that presumably formed in the Cretaceous subduction zone (Maresch, 1975; Ave Lallemant and Sisson, 1993; Stockhert et al., 1995; Sisson et al., 1997). The age of the epidote amphibolite metamorphism is mid-Cretaceous based on ⁴⁰Ar/³⁹Ar ages (Ave Lallemant, unpublished data), implying that they were subducted under the Great Arc of the Caribbean at that time. Moving southward the next belt contains Precambrian to Paleozoic metamorphic and igneous rocks and presumably is a fragment from the edge of the South American craton. The next belt consists of Lower Tertiary unmetamorphosed sedimentary rocks and ophiolites (Gonz‡lez de Juana et al., 1980; Bellizzia and Dengo, 1990). The southernmost belt, thought to be a "klippe" overlying the Caribbean Mountain system and the Serran’a del Interior. consists of volcanic and volcaniclastic rocks and chert metamorphosed at HP/LT conditions (blueschist facies) (Shagam, 1960, Navarro, 1983; Smith et al., 1996). These rocks were formed in the Cretaceous subduction zone as well (Smith, 1996; Smith et al., 1996, 1999).

(4) Serran’a del Interior foreland fold and thrust belt. This belt consists of Cretaceous passive margin and Tertiary orogenic sedimentary rocks. The basins now deformed in the folded belt formed progressively from the west in the Eocene to Miocene in the east. They were deformed and thrust south-southeastward over the extended South American continent in the west since the Eocene (e.g., Beck, 1978) and since Early Miocene time in the east (Rossi et al., 1987; Bally et al., 1995). South of the Serran’a del Interior foreland fold and thrust belt is the undeformed foreland basin that extends onto the Guyana Shield. The foreland basin also formed diachronously from the west in the Eocene to the east in the Miocene.

Structure. Deformation structures in the Caribbean Mountain system formed during two tectonic events. The first (D₁) is synmetamorphic, and is related to Cretaceous subduction, whereas the second (D₂) postmetamorphic fabric is related to Eocene to Present "obduction" of the Caribbean Mountain system onto the South American craton.

The D₁ structures in the HP/LT metamorphic belts originated during the upward (retrograde) migration of the unit from great (eclogite facies) to intermediate (blueschist facies) depth, and subsequently to relatively shallow (epidote amphibolite to greenschist facies) depth (Ave Lallemant and Sisson, 1993; Sisson et al., 1997). Penetrative structures indicate that in the present coordinates the rocks underwent north-south shortening, right lateral shear and margin parallel stretching along the east-west margin of South America in a mid-Cretaceous subduction zone (Ave Lallemant and Guth, 1990; Ave Lallemant and Sisson, 1993; Ave Lallemant, 1997).

D₂ structures in the HP/LT metamorphic belts are correlated with the oldest structures in the Serran’a del Interior fold and thrust belt. They are characterized by south- to southeast-directed thrust faults, east-trending right lateral strike-slip faults, and southeast-trending normal faults. They formed during the Eocene in the west and Miocene to present in the east (Pindell, 1993).

Cenozoic Basins: Sedimentary basins within and near the plate boundary zone record vertical and lateral movements associated with still active Cenozoic processes. The history of uplift, subsidence, and fault dislocation recoirded by basins is related to the larger and deeper processes of arc accretion, but such relationships are tentative at present, with strong discrepancies between models. The eastward migrating foredeep basin ahead of the Caribbean arc seems most clear, but the history of the basins in the strike-slip fault systems remains poorly understood.