Frank,

Thanks for catching the typo. You are obviously correct that it should have read 0.5 cm/yr. Teach me to try to fire off a quick response between meetings. ;-)

I'm not aware of a published version of these stimates unless perhaps Mike Hudec included something similar in his work on the restorations he did a few years back in the Mad Dog area. Do you know of a published source, Frank?

Scot Krueger

PS - My error reminds me of a T-shirt I saw yesterday which said "5 out of 4 people have trouble with statistics".

-----Original Message-----
From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Peel, Frank FJ
Sent: Thursday, January 12, 2012 10:20 AM
To: [log in to unmask]
Subject: Re: help - velocity of salt tectonics

Dear Colleagues,
My own work confirms the rates that Scot Krueger and Herman Lebit provided for the Gulf of Mexico, based on palinspastic restoration (using Locace software) of many balanced cross sections. For the Miocene-Pliocene Atwater Fold Belt the gross rate of basinward translation averaged over time is of the order of 1mm/yr, and this is the case for the three major fold belt lobes (Western, Central and Eastern Atwater Fold Belt, or WAFB, CAFB and EAFB for short in industry parlance). The stratal patterns of deepwater sediments onlapping the growing anticlines indicate that the movement was continuous, but that it was not uniform (starting slow in the Early Miocene, building up to a peak rate then dying off in the Early Pliocene, rather than having multiple surges of movement, as far as I could tell). The average rate is slower in the WAFB and higher in the CAFB and EAFB. The Atwater Fold Belt is a gravity-spreading-dominant system detaching on a deep salt allochthon.

The rate of emplacement of the shallower (Sigsbee) allochthon is much faster, reaching the order of magnitude of centimeters per year, as Scot K states. But Scot made a typo error, because of course 25km in 5Ma is 0.5cm/yr not 2.5cm/yr! Sorry Scot..... The timing and direction of the initial salt allochthon advance is extremely well defined by successive footwall cutoffs of stratigraphy which are well-constrained in location and age (good seismic imaging and many well penetrations). In contrast to the Atwater Fold Belt, the rate of Sigsbee advance varied significantly with position along strike and also with time (there are distinct local surges) Consequently there are places and times where the figure or 2.5cm/yr may have been achieved. In my study area (the region which encompasses the Atwater Fold Belt), the gross flow direction for initial salt emplacement is overall towards S to SSE but there are local variations in local emplacement direction.

So, you probably ask, why are the rates of the deep system and the overlying shallow system different in the greater Atwater Fold Belt region? They are mechanically separate from one another, but they are both salt-detached, both are dominantly driven by gravity spreading, and both are going in the same general direction. They even partially overlap in time (emplacement of the Sigsbee allochthon started while the Atwater Fold Belt was still contracting).  The two big differences are, I think, that
(i) the shallow (Sigsbee) system did not have a continuous thick sediment cover when it was moving, where the deep system did which slowed it down
(ii) the shallow Sigsbee system is smaller in updip-downdip scale and it is responding only to the Neogene sediment dump (which is very spatially localized), whereas the deeper AFB system is bigger in scale in the dip direction and it encompasses the whole Jurassic to Neogene sediment pile, which is less spatially localized.



Hope this helps
PS I've got lots of palinspastically restored cross sections from Angola, Gabon, and other salt-floored systems, and could do the same sums there.
Frank Peel

Points of terminology for the record: the Eastern Atwater Fold Belt (EAFB) lobe is also sometimes known as the Mississippi Fan Fold Belt (MFFB). In addition to the Mio-Pliocene phase of folding, the Atwater Fold Belt  also had an older history of allochthonous salt emplacement and folding, which I didn't discuss here. Rates of movement for the older phases are more poorly constrained (we don't have precise dates for the footwall cutoffs beneath the deep salt allochthon, and the stratigraphy onlapping the late Cretaceous to early Paleogene fold belt is poorly defined).

-----Original Message-----
From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Krueger, Scot
Sent: Thursday, 12 January 2012 8:23 AM
To: [log in to unmask]
Subject: Re: help - velocity of salt tectonics

A good example of the rapid end of the spectrum would be the salt "surge" of the Sigsbee salt glacier in the central deepwater Gulf of Mexico, which has advanced by up to 25 km in the last 5 Ma, or an average geologic rate of 2.5 cm/yr. The rates in the GOM for an average location and an average time would almost always be below that rate. And since the local behavior at the front appears to be stick-slip over periods of a few million years, the shorter term slip rate during the slipping phase may be somewhat higher. Hope this helps.

Scot Krueger

-----Original Message-----
From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Andrea Billi
Sent: Wednesday, January 11, 2012 5:09 AM
To: [log in to unmask]
Subject: help - velocity of salt tectonics

Dear All,

I was looking for typical ranges of velocities for sedimentary overburdens sliding over a salt layer along passive margins.

In models by Gemmer et alii (Basin Research 2004, 16, 199-218) and Albertz et alii (Tectonics 2010, 29, TC4017, doi:10.1029/2009TC002539) I see velocities between a few millimetres to a few centimeters per year, depending on various boundary conditions.

Do you know other natural or experimental data concerning these velocities?

Thanks a lot

Andrea

_________________________________________________
Andrea Billi (PhD)
Consiglio Nazionale delle Ricerche, IGAG, c.o. Dipartimento Scienze della Terra, Sapienza Università di Roma, P.le A. Moro 5, 00185, Rome, Italy.

Phone: +39 06-49914955
Skype: a.billi
Email: [log in to unmask]
Web site: http://www.andreabilli.com
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