GEOLOGIC HISTORY OF TOBAGO, WEST INDIES
A.W. SNOKE, J.D. YULE, D.W. ROWE,
Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 -3006, USA;
G. WADGE
Department of Geography, University of Reading, Reading RG6 2 AB, UK; and SHARP, W.D.,
Department of Earth and Space Science, SUNY, Stony Brook, NY 11794-2100, USA.
Tobago, West Indies, is a basement high that forms the northeastern most corner of the present-day South American
continental shelf. However, its pro-Cenozoic history indicates that it has affinities with the Caribbean plate
and therefore is part of an allochthonous terrain that forms the easternmost fragment of the Caribbean Mountain
system.
Mesozoic oceanic island-arc crust is extensively exposed on Tobago and can be conveniently divided into three EW-trending
lithologic belts that transect the island: the North Coast Schist (NCS), the ultramafic to mafic plutonic suite,
and the Tobago Volcanic Group (TVG). A mafic dike swarm widely intruded the plutonic suite and TVG, whereas scattered
pre-metamorphic and post-metamorphic dikes occur in the NOS belt. 4OAr/39Ar hornblende plateau ages from the plutonic-volcanic-dike
complex indicate that this rock assemblage evolved in the Albian (105-1 03 Ma).
The NCS rocks are lower greenschist fades multiply deformed volcanogenic rocks. 4OAr/39Ar radiometric dating of
relict igneous hornblende from these greenschist fades rocks indicates a protolith age of at least Aptian and perhaps
as old as Late Jurassic. Synmetamorphic mesoscopic folds In the NCS are tight to isoclinal and display a penetrative
foliation subparallel to the axial surface of the folds. The fold hinges of these folds are subparallel to a widespread
stretching lineation (Lstr) defined chiefly by elongated mineral aggregates or relict lapilli. Late metamorphic
folds are essentially coaxial with the synmetamorphic folds but folds the SI penetrative foliation. Locally a penetrative
cleavage (82) is associated with these folds, especially in pelitic protoliths. Post-metamorphic folds include
and array of kink-like folds that probably developed under a complex and perhaps variable stress regime of uncertain
age and duration.
The plutonic suite is composite and consists of four mappable units: 1) deformed and metamorphosed mafic rocks;
2) ultramafic rocks; 3) dioritegabbro; and 4) biotite tonalite. Unit I was penetratively deformed under amphibolite
to pyroxene hornfels faces metamorphic conditions during the emplacement of adjacent intrusive ultramafic masses.
The protolith for this deformed oldest unit ranged from porphyritic gabbroic bodies to mafic fragmental volcanic
rocks (i.e., TVG). The ultramafic rocks include: dunite, wehrlite, olivine clinopyroxenite, horn-blonde, and hornblende-plagioclase
pegmatoid. The diorite-gabbro is exceptionally heterogeneous both in modal composition and texture, ranging from
quartz diorite to melagabbro and medium-grained, hypidiomorphic-granular to pegmatitic. The diorite gabbro is a
composite unit that evolved through piecemeal intrusion. The biotite+/-hornblende tonalite unit forms an EW-trending,
dike-like body that Is the youngest part of the plutonic suite.
As implied previously, parts of the plutonic suite intruded and contact metamorphosed the 1VG, although both igneous
suites are essentially consanguineous. The TVG rocks are chiefly volcaniclastic breccias, ranging from block-and-ash
flow deposits to sub aqueous debris flow deposits; however, locally lava (sometimes pillowed) forms prominent accumulations
in the volcanic pile. Volcanogenic argillite, sandstone, and grit form mappable subunits within the volcanic sequence.
Radiolaria in argillite layers suggest an Albian to Early Cenomanian age (D.L. Jones and P. Noble, pers.comm.,
1987), and ammonite molds from the same stratigraphic interval indicate an Albian age (W.A. Cobban, pers.comm.,1988).
Detailed geologic mapping coupled with the above radiometric and palaeontological dating constraints indicate that
the NCS was wallrock for the plutonic suite. A selvage of amphibolitic rocks (c250 m structural thickness) forms
a mappable belt between the NCS and plutonic suite. Metamorphic grade in the aureole decreases with increasing
structural depth, thereby exhibiting an inverted metamorphic gradient. Along several well-exposed cross-strike
transects, lower greenschist facies NCS rocks gradually grade into epidote amphibolite and eventually amphilbolite
facies rocks. The greenschist-grade penetrative foliation of NCS rocks, commonly containing a plunging, low-angle,
WSW-ENE stretching lineation, is overprinted by a dynamothermal foliation with a down-dip hornblende mineral lineation
apparently related to the emplacement of the plutonic suite. The primary igneous contact between the plutonic suite
and the metamorphosed NCS rocks is only locally preserved and more commonly this boundary has been overprinted
by brittle deformation and retrogression. Where preserved the primary igneous contact is interpreted as a zone
high shear strain where a hot, nearly crystallized mass of ultramafic-mafic rocks dynamothermally metamorphosed
older greenschist facies rocks during emplacement into the upper crust. During the progressive upward emplacement
of the plutonic suite, this zone evolved from a plastic shear zone into a brittle fault zone referred to here as
the "back-aureole (reverse) fault". Although the back-aureole fault virtually transects the island, it
is related to the emplacement of the plutonic suite and not a regional compressive stress regime. Furthermore,
in that this zone appears to have developed in close association with the mid-Cretaceous plutonic suites, it is
inferred to be a Cretaceous fault system.
Both normal and oblique-sup faults have fragmented and offset the back-aureole fault system. A normal fault system
subparallel to the back-aureole system has greatly complicated relations by attenuating the amphibolite aureole
and juxtaposing various elements of the three main belts into anastomosing, fault-bounded slices. A still younger
system of high-angle, crosscutting faults trend approxition. This separation is probably a result of a complex
oblique-slip displacement history. The age of the normal and oblique-slip faulting is uncertain but may be related
to the broad strike-slip regime that apparently existed in the southeastern Caribbean during the Neogene.
Finally, the Cenozoic rock record on Tobago, exclusive of Holocene surficial deposits, consists of the following
deposits (oldest to youngest):
Rockly Bay Formation (Pliocene): sandstone, conglomerate, and limestone of Montgomery [Pleistocene - Pliocene(?)];
and the coralline limestone of Booby Point (Quaternary). These deposits represent various shallow marine accumulations
and their preservation therefore records tectonic uplift and/or sea level change. A normal fault culling Rockly
Bay Formation in a sea cliff near Ju -C Beverages Bottlers Limited, Lower Scarborough, is oriented N85W54oS and
is the only known exposed fault cutting Cenozoic deposits. However, the Southern Tobago fault system, inferred
to transect the southwestern corner o the island, appears to tilt the coralline limestone of Booby Point to the
south. In summary, the Cenozoic history of Tobago is very poorly known but clearly only involves brittle deformation
rather than the plastic deformational history that was so important in mid-Mesozoic.
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