From the Editor - March 2016

From the Editor - March 2016

Hall observed that the thickness of the marine sedimentary section in the Appalachians (nearly 40,000’) is ten times the thickness of the equivalent section further west in the Mississippi River Valley. Hall’s assumption was that the thickness of the sedimentary pile was somehow responsible for its consequent deformation. “The key element for Hall was the accumulation of thick sedimentary layers, which he imagined must depress the crust and in the process become wrinkled to form the structures seen in mountain ranges, such as the familiar Appalachians. He envisioned compression of the upper layers and tension of the lower ones as subsidence occurred much as one can imagine by bending a ream of paper.” (Dott, 2005)

Dana built on the foundation that Hall had laid, but pointed out that Hall had not explained the critical part of the story, of how mountain belts were uplifted. But ultimately even Dana and his followers were never able to propose a satisfactory mechanism for the uplift of thick sediment prisms. Nevertheless, the geosynclinal theory not only survived as a unifying paradigm for more than 100 years, it produced a dizzying array of terminology that is today mostly obsolete. When was the last time you heard the term “zeugogeosyncline?”

As recently as the 1960’s, the geosynclinal theory was still considered by some to be the keystone of modern geology. Clark and Stern (1960) stated “The geosynclinal theory is one of the great unifying principles in geology. In many ways its role in geology is similar to that of the theory of evolution which serves to integrate the many branchesof the biological sciences. The geosynclinal theory is of fundamental importance to sedimentation, petrology, geomorphology, ore deposits, structural geology, geophysics, and in fact all branches of geological science… Just as the doctrine of evolution is universally accepted among biologists, so also the geosynclinal origin of the major mountain systems is an established principle in geology.” Just a few years after the publishing of Clark & Stern’s textbook, the plate tectonic revolution was in full swing, and widely accepted by the geological community as having totally supplanted geosynclinal theory.

Even though the geosynclinal theory is now generally considered to be dead, and entirely replaced by plate tectonic theory, I’ve wondered from time to time if there could be some element(s) of truth to Hall’s original assumption that there can be some sort of cause and effect relationship between sediment thickness and orogenesis—albeit an indirect relationship, not a direct one as he had imagined. The idea is this: that the sedimentary prisms developed on passive continental margins, if they become thick enough, will cause the underlying lithosphere to mechanically fail, with the lithosphere on the denser side (toward the ocean basin) bending downward into the asthenosphere and initiating subduction under the less dense side (toward the continent) by slab pull. I’ll probably be quickly shot down on this idea by those of you with much more knowledge of crust and mantle dynamics. But if it were to be plausible, it might explain the repeated opening and closing of ocean basins (the Wilson Cycle), because diverging passive margins (at least those with significant sediment accumulation) would ultimately be doomed to be converted to subduction zones, and reverse the opening of the oceanic basin by slab pull of the newly-downgoing oceanic lithosphere. I recently discovered this idea isn’t novel at all. Indeed, Stern (2004) even refers to it as a broadly accepted mechanism for initiating subduction zones that he calls “spontaneous nucleation of a subduction zone by passive margin collapse” (see cartoon). But the big problem is that there is apparently no documented evidence (e.g. from earthquake hypocenters or seismic tomography) of any present-day passive continental margin being transformed into a convergent margin. But perhaps this is another case (see my column in the February issue) of the present not necessarily being the key to the past.

So – could there be an incipient subduction zone lurking beneath us in the northern Gulf of Mexico Basin, beneath the 40,000+ feet-thick geosyncline offshore Texas & Louisiana, one of the thickest sedimentary accumulations on the planet (see map)… and destined to become a mountain belt in the distant future? Personally I’m not sure… but I think James Hall would have said “absolutely!”

References
Clark, T.H., and C.W. Stern, 1960, Geologic History of North America: Ronald Press Company, 434 pp.
Dana, J.D., 1866, Observations on the origin of some of Earth’s features: American Journal of Science, vol. 42, p. 195-211
Dott, R.H. Jr., 2005, James Hall, Jr., 1811-1898: Biographical Memoirs, vol. 87, The National Academies Press, Washington, D.C.
Hall, J., 1859, Geological Survey of New York, Paleontology, vol. 3, part 1, p. 66-96
Stern, R.J., 2004, Subduction initiation: spontaneous and induced: Earth and Planetary Science Letters, vol. 226, p. 275– 292