Abstract

Precambrian, Paleozoic, and Mesozoic(?) metamorphic rocks form the basement of the central Colombian Andes, which was intruded by Mesozoic quartz diorite batholiths and was the site of Tertiary volcanism. Mesozoic miogeosynclinal rocks are preserved to the east in the Magdalena Valley, and eugeosynclinal rocks are preserved on the west flank of the Cordillera Central. The Cordillera Occidental includes highly deformed rocks of the Mesozoic eugeosyncline, intruded by Tertiary(?) dioritic to granitic plutons and overlain locally by Tertiary volcanic rocks. The fault-bordered Cauca Valley separates these two Andean ranges and was the site of deposition of fluviatile, lacustrine, and volcanic Tertiary rocks. Thick sequences of marine Tertiary sedimentary rocks occupy the Atrato-San Juan basin to the west and the Sinú-Urabá basin at the northern end of the Andes. Pillow basalts, gabbros, and andesites, are the chief rocks forming the basement complex of the Serrania de Baudó along the Pacific coast of Colombia.

Bouguer anomalies attain values as high as + 135 mgals over pillow basalts and gabbros of the Serrania de Baudó. A negative anomaly with values as low as −90 mgals is found over the Atrato-San Juan basin. Values increase to +95 mgals over the Sautatá arch and then decrease to −45 mgals over the Sinú-Urabá basin. Bouguer anomalies are negative along the Cordillera Occidental southward from Mutatá to near lat 5° N., but there the values become positive. These positive values are a southeastern continuation of the gravity high over the Sautatá arch, and they persist southward along the Cordillera, despite topographic elevations greater than 3,000 m, which indicates an isostatic imbalance in the crust or upper mantle. This anomaly, the West Colombian gravity high, extends southward into Ecuador. Bouguer anomalies are negative (−20 to −150 mgals) over the Cauca Valley, Cordillera Central, and Magdalena Valley.

These regional relations indicate that granitic crust is present under the Cordillera Central but may be thin or absent beneath the Mesozoic eugeosyncline. Mafic, perhaps oceanic, crust serves as floor of the Mesozoic eugeosyncline, the Pacific Lowlands, and the Serrania de Baudó. If an oceanic crust of 16 km is assumed in the Pacific area, west of the Colombian coast, the gravity data indicate an eastward increase in crustal thickness to 30 or 35 km beneath the Cordillera Central and Magdalena Valley.

Major tectonic boundaries may be inferred from gravity data as follows: (1) Steep marginal gravity gradients suggest that the Atrato-San Juan basin is like a gigantic graben. (2) Steep gradients on the flanks of the Suatatá arch suggest that it is a horst-like feature. (3) Rocks of the Mesozoic-early Tertiary eugeosyncline may locally override the mafic rocks that cause the West Colombian gravity high. (4) Steep gradients on the east side of this high mark the boundary between granitic crust to the east and oceanic crust to the west. South of lat 5° N., this zone coincides with the great Romeral-Cauca fault system.

The Mesozoic eugeosyncline is interpreted to be the site of Mesozoic underflow of the Pacific plate beneath nuclear South America; the Romeral fault, with associated serpentinites, is the trace of a Mesozoic Benioff zone. The Atrato-San Juan basin may represent a Tertiary zone of underflow that was an early branch of the Peru-Chile trench.

From analysis of regional distribution of earthquakes and focal depths in northern South America and vicinity it is concluded that relative motion between the South American plate and the Pacific plate is partitioned into two types: (1) some underthrusting occurs along the northern extension of the Peru-Chile trench, and (2) right-lateral movement occurs along the Dolores-Boconó-El Pilar megashears. The western Cordillera of northern Ecuador and Colombia and eastern Panama are semi-attached to the Pacific plate and have an east to northeast motion related to eastern spreading from the East Pacific rise and northward spreading along the Galápagos rift zone. Thus, gross relative motion of northern nuclear South America is counterclockwise with respect to the Pacific plate, Panama, and the Caribbean plate.

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