Putting It All Together Again

In the second and third parts of this four-article Geophysical Corner series we defined a kinematic framework for the evolution of the Gulf of Mexico region by restoring Andean deformations and progressively closing the Atlantic Ocean.

This month, we further evolve this framework to build a palinspastically quantitative reassembly of continents and continental blocks that were separated during the Mesozoic rifting and subsequent drift in the Gulf of Mexico region -- key features of which are shown in figure 1.

Figures 2-4 show primary developmental stages in the Gulf's evolution:

• Post-Gulf formation (figure 2).
• Post-salt/pre-seafloor spreading (figure 3).
• Early syn-rift (figure 4).

The kinematic elements applicable to the reconstructions are as follows.

♦   First, our Oligocene reconstruction of northern South America (article two) is further modified for Late Jurassic and Cretaceous time by removing island arc and other terranes that were accreted to in the Late Cretaceous and Early Tertiary (shape portrayed in figures 2 and 3).

We can then estimate and restore Jurassic extension in the rift basins of the Andes (using principles outlined in the August EXPLORER, which gives us an Early Jurassic shape for the northern Andes that can be closed against North America (figure 4).

♦   Second, figures 2-4 show that the entire region of Florida, the Blake Plateau and the Bahamas (and the "Cuban autochthon" beneath the Cuban arc) were strongly controlled by fracture zone trends of the early Atlantic.

In this region, plate separation was achieved by NW-SE stretching of crustal elements separated by transcurrent faults. Middle Jurassic basalt extrusion was commonplace in zones of high stretching.

Each crustal "corridor" between transcurrent faults underwent different amounts of stretching and displacements relative to the others. The conjugate margin to the Southern Bahamas flank is the transcurrent margin of Guyana.

♦   Third, unlike the Florida region, the Yucatan Block moved independently -- in two distinct stages -- of the larger continents as the Gulf opened.

At the time of figure 4, there is only a small range of paleo-positions in which Yucatan could have fit geometrically without overlap of palinspastically restored (i.e., rift-related stretching removed) areas of continental crusts. This position can be achieved by rotating present-day Yucatan clockwise about "pole A" (figure 4), which closes most of the Gulf by placing Yucatan snugly against the northeast Mexico-Texas-northwest Florida paleo-margin.

It definitely does not, however, close the southeastern Gulf. There, the crust of South Florida -- including that of the "Tampa Arch" -- must be retracted northwestward against Yucatan and out of an overlap position with Demerara Rise, off the Guyana margin.