PRELIMINARY ASSESSMENT OF THE STRUCTURAL HISTORY OF THE NORTHERN
RANGES OF
TRINIDAD
ST. ALGAR and J.L. PINDELL
Department of Earth Sciences
Dartmouth College
Hanover, NH 03755, U.S.A.
The Northern Ranges of Trinidad are bounded by the El Pilar and Northern Boundary faults, which are both major
tectonic boundaries within the southern Caribbean plate boundary zone. Preliminary results from a study of the
origin and evolution of the Northern Range agree with the existence of an overturned antiform (Potter 1973), but
suggest that the area has since experienced a period of transpression (compressive strike-slip), followed by a
period of transtension (extensional strike-slip).
The transtension is seen both onshore as roughly E-W striking normal faults crosscutting the pre-existing structures,
and in offshore seismic profiles as faults with a normal component of slip, downthrowing from the Northern Ranges.
The period of transpression is suggested because of three main lines of evidence: Firstly, the area has been uplifted
from a burial depth of at least 5 km (indicated by the greenschist facies of metamorphism), suggesting that a compressional
force acted on the block to push it up. Secondly is the parallelism to the major boundary faults of most cornpressional
structures observed in the Northern Ranges. This is not consistent with pure strike-slip, but is thought to be
consistent with transpression. Thirdly, transpression seems to be the only possible way of explaining the complex
relationship between minor folds and faults observed throughout the Northern Ranges.
Transpression is particularly prevalent in the Toco region in the northeastern corner of the Northen Ranges. There,
compressional structures are observed between and within zones of previously unidentified NE-SW trending strike-slip
faults which juxtapose a variety of lithologies as diverse as cherts and quartzose grits. This juxtaposition occurs
at and around the "unconformity" between the lower Cretaceous Toco Formation, and the upper Cretaceous
Galera Formation. We suggest that this boundary is in fact faulted, and that a significant amount of horizontal
translation has occurred along this and surrounding faults. Observation of this type of faulting throughout the
Toco region has prompted a complete re-examination of the stratigraphy of the area, which is currently in progress.
Thus, it is concluded that strike-slip, compression and extension have played important roles in the formation
of the block that now makes up the Northern Ranges, and that these processes have caused a great deal of previously
unrecognized or unexplained structural discontinuity within the Northern Ranges.
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