Letters to the Editor
Sir:
During the recent (4-11-05) HGS dinner presentation, J. Blickwede/UNOCAL in an excellent presentation showed some data that will profoundly influence our understanding of the Gulf of Mexico basin development: On a seismic section across the general Trident deepwater area, primary Jurassic salt was shown to pinch out from NW to SE into the present abyssal basin. A similar section has been published in AAPG from the deepwater Mississippi Delta area, and Industry may have more. If indeed Jurassic salt has pinched out against the area of the present abyssal depth portion of the Gulf Basin, all attempts to use present absence of salt here as proving a breakup and drift-apart of a once continuous salt basin would be wrong. Rather, a Jurassic topographic high may have prevented salt deposition in the present basin center. This present basin center then more likely was a structural uplift, caused by a thermal mantle dome. And, when we extend this thought further back, metamorphism and South-to-North nappe tectonics in the Texas-Oklahoma Marathon/Ouachita belts can be seen as result of mantle dome events that began in late Pennsylvanian time. Collapse of such a dome led to deposition of thick Permian marine and continental sediments in northern Louisiana, finally to the Triassic/Jurassic and younger Gulf Coast Basin. In this context a recent article in the AAPG bulletin of February 2005 by D.E.Bird et al is quite interesting: It requires acceptance of a Jurassic mantle dome for Gulf Coast Basin development, a major step forward from past theoretical plate tectonic models.
J. Chris Pratsch
Geologist
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The following, in blue italics, are excerpts from a letter from J. Chris Pratsch, published earlier, with responses by Alan Morgan. [Ed]
(Pratsch) Dear Mr. Berman:
I''d like to congratulate you to your article in the February 2005 HGS Bulletin ”The Sumatra Earthquake of 2004: Forty Years of Ignoring Plate Tectonics”. The article brings a welcome fresh wind to the Society Bulletin and is well placed because our society is not a Houston Petroleum-Geological Society that many assume, but a Geological Society; and so we can and should discuss general/alternate non-petroleum geology as well. I am a petroleum geologist but just love to see more from other fields.
The only thing is that it appears highly questionable to what degree plate tectonics or their acceptance have anything to do with this earthquake and its horrible effect on people; at least it appears that we do not yet have the right answers here. Why is this earthquake just on a dot along a multi-1,000 mile zone within a basically identical geological framework? It would seem that a point-like event occurred, like the uplift of a magmatically-driven body of rocks (not necessarily tectonically driven magmatics, as your model requires). This would be in line with volcanic-magmatic events elsewhere in onshore Sumatra through the Tertiary.
(Morgan)
January 30, 2005
Mr. Arthur E. Berman
Editor
Houston Geological Society
Here is your answer as to why the seismic data appears to be a point event. Seismic recording stations depend on wave data and time of recording to determine the distance from the station to the event. Three stations are required to triangulate an actual position of origin, as a single station cannot tell which direction the wave came from. The reason the event is recorded as a point lies within the method of determining the epicenter and the definition of epicenter. Seismic waves travel through the ground at velocities tied to the rocks they travel through. If an event takes place over a 1000km area, the method to determine the distance still will only use the p-wave and s-wave. The event had to start somewhere, which was the rupture event recorded as the event epicenter. A simple experiment will illustrate this:
Take a cardboard box containing moderately heavy contents inside and place it on a rough concrete surface. Using one hand, apply force to one side of the box to the point that it barely moves. You will notice that the box started moving at a single point, not along the entire box. Empty the box, turn it over and examine the scratches on its bottom side. You will notice that you will be able to identify the friction point where the box first started to move. This is analogous to the Sumatra event. You cannot claim that the box only moved at the single point.
(Pratsch) And, subduction-related melting of basic crust can hardly ever lead to magmatism of granite, as you explain for Krakatau acidics. Therefore, we may need another explanation for the magmatism in Sumatra and elsewhere.
(Morgan) Rejecting the plate tectonic model leaves you at a disadvantage for the answer to this. Oceanic (basaltic) plates subduct along continental (granitic) plates. The Indonesian island arc is composed of continental material. The crust subducting beneath this island arc is oceanic. Friction along this subduction path along with heat flow migration pathways allows for melting of BOTH materials along their interface. Mixing of basaltic and granitic magmas can and does often occur. The surface expulsion of these magmas does not always reflect the entire contents of the original melt, as the laws of physics like to keep more dense material (melted basalt) beneath lighter material (melted granite).
(Pratsch) As one result of these thoughts one can hardly see “articulation of the plate tectonic model ”would “prepare…. citizens for the inevitability of an event like this…..”. Plate tectonics or not, people know tsunamis for hundreds of years, and the guilt of Indian Ocean-surrounding countries (for not having tsunami warning system in place) lies more in their lack of acceptance of available technology that can be applied for sufficient tsunami warnings; it is not that they know nothing about plate tectonics (may be it would even be better for them if they don’t) and therefore presumably have not organized a tsunami warning system.
This brings me to another point - are plate tectonics really valid today? Is this a theory that is proven or useful? I doubt that it is. In no case known to me are there unique one-sided data or facts that prove the plate tectonic theory as real, thus as useful: We only can detect relative movements of plates, which leave always questions unanswered: It may not be subduction of the oceanic plate but obduction of the continental plate that is the basic mechanism (why else are there mantle domes and their final collapse as main tectonic events causing such regional features as Alboran Sea/Rif/Subbetics, Hungarian Plains/Carpathians, Black Sea, northern Gulf of Mexico/Ouachitas, Michigan Basin...?) It would be quite educating for all if one could get together a conference or other way of an objective discussion of pros and contras of plate tectonic theory as seen today.
(Morgan) You offer obduction as an alternate to subduction. One cannot exist without the other, much like the hanging wall and foot wall of a thrust. As to relative motion, is