Abstract

The northern Gulf of Mexico is believed to have formed between north and south Americas during the Triassic-Jurassic time. Its rifted northern margin appears to have been right laterally segmented by a series of northwest-southeast trending transfer faults that date back to Triassic-Jurassic rifting Pilger, 1981, Salvador, 1987, Buffler, 1991; Huh et al., 1996) (Fig. 1).

In general, the post-rift tectonics of the Gulf of Mexico are described in terms of thin-skinned growth faulting, salt movements, and gravity tectonics, none of which are supposed to affect basement below the Louann Salt. However, a cursory look at the tectonics of the continental interior and historic seismicity of the Gulf of Mexico reveal that this entire region is active and, along with the surrounding areas, has been episodically active since the Mesozoic. Also, some of the more recent deep hypocenters on the Gulf Coast and the Gulf of Mexico apparently represent basement deformation below the Louann Salt (Figs. 2–3).

Aeromagnetic and gravity anomalies on the Gulf Coast and the Gulf of Mexico shelf show northwest-southeast lineaments that parallel the transfer faults. In southern Louisiana, these northwest-southeast lineaments laterally offset (wrench) east-northeast trending gravity highs and lows that follow the half graben and horsts of the buried rift margin (Fig. 4), which control the general east-west strike of Tertiary growth faults.

Based on such observations, this writer is of the opinion that basement involved wrenching of the Gulf Coast (as evidenced by deep hypocenters) is real and constitutes a hitherto ignored factor contributing to coastal subsidence and land loss along the Gulf Coast (Sarwar, 2002, 2003; Sarwar and Bohlinger, 2005; Dokka, 2006; Gagliano, 2008; Stephens, 2010).

Further, given the above neo-tectonic framework and seismicity patterns, we should also study the risk of a sudden and more catastrophic collapse somewhere along the Louisiana-Texas coast than has so far been considered (see, for example, Edwards, 2000; Gagliano, 2008). The eastern tectonic province, as defined by Peel et al. (1995), involves active growth faults like the Baton Rouge Fault, and the overloaded Mississippi River slope fan. Because this province also overlies active northwest-southeast transfer faults, (like the Pearl River; Sarwar, unpublished), it seems to be highly vulnerable to seismic jolts even if they travel from far away.

The buried Miocene clastic sequence, south of the Lake Pontchartrain near New Orleans, shows a 10–20 mi long train of piggyback folds and thrust faults apparently related to a gravity slide that travelled south from the then shelf margin (Sarwar, unpublished). One wonders whether such an event was relatively gradual or could have occurred suddenly with catastrophic results. And what does it take to trigger such a slide along an unstable shelf margin?

Any sudden or slow collapse of the coastal region resulting in a catastrophic shift of shoreline can be a mega-disaster both in human and environmental terms. So, in this writer’s opinion, it would be wise to study the seismicity of the Gulf Coast to develop a better understanding of how the basement and overlying strata are being deformed along the unstable growth-faulted Gulf Coast, for consideration of risk.