MIOCENE METAMORPHISM AND NEOGENE TECTONICS OF NORTHERN RANGE ROCKS

R. SPEED
Dept. Geological Sciences,
Northwestern University,
Evanston, IL., U.S.A.
K.A. FOLAND
Dept. Geology and Mineralogy,
Ohio State University,
Columbus, OH., U.S.A.


We have obtained new data on the metamorphic rocks of the Northern Range of Trinidad by 4OAr/39Ar, vitrinite reflectance analyses and by structural studies on outcrop and thin section. The main findings are as follows.

1. The age of argon locking in metamorphic white micas in schist is 25-30 ma (mid-Cenozoic), implying unroofing through depths of about 10km at this time.

2. Some schist includes coarse detrital mica which retains Paleozoic or older argon ages.

3. The age of maximum metamorphism is mid-or early Cenozoic.

4. Northern Range rocks represent varied depth levels with respect to degree of re crystallization and quartz plasticity.

Bodies from different levels are juxtaposed by brittle faults, Implying structural reorganization at shallow levels. This probably occurred during Neogene unroofing and emplacement. Former assertions of systematic metamorphic gradients in the Northern Range are unsupported.
Structural fabrics and their sequence of development vary with position in the Northern Range, although there is a common theme of 1) one or two early phases of tight folding and development of foliation with or without stretching lineation, boudinage of stiff layers, and ductile strain of quartz, and 2) macro-to microscopic folding of foliation, commonly associated with semi-brittle to brittle faulting. The fabrics are indicative of persistent NW-SE contraction throughout the Range.
The Northern range can be interpreted to consist of the deeper rocks of a thick structural complex of sediments. Such sediments were deposited in deep marine settings in Mesozoic and probably, Paleogene times as continental turbidites and hemipelagites. The sites were probably on in-situ oceanic crust that fronted Trinidad and NE Venezuela in the early Cenozoic. The sediments were accreted at the leading edge of the southern Lesser Antilles arc system and overrun by it (Tobago terrain) and then thrust and greatly unroofed above the outer South American shelf in Trinidad during Neogene time. Although the Northern range may now be locked with the Northern Basin, it is probably moving S or SE with respect to the Guyana Shield, together with the stratigraphic cover of southern Trinidad, on a deep sole thrust.

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