The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Monday, July 21, 2014

July 21. Marathon Orogeny



Allegheny-Ouachita-Marathon orogenic belt. From Thomas (1983) via this link.
 
The Marathon Orogeny in West Texas wasn’t really a discrete, separate mountain-building event, but just another aspect of the complex collision between Gondwana and North America.  In the east, from New York to Alabama, it was more or less the head-on interaction between what is now West Africa and eastern North America. Further south and west, the geometry of the North American margin together with complexities in northwestern Gondwana made for a less straightforward collision.

Folds in Caballos Novaculite, Marathon region, West Texas
The heart of the eastern and southern margins of North America was built up in the Precambrian, during the Grenville Orogeny, when a long relatively narrow strip of continental material was accreted, added, to the ancient Archean core of the continent. That included somewhat discrete blocks such as the Nashville Dome in and around central Tennessee, the Ozark Dome in Missouri and Arkansas, and the Llano Uplift in what is now central Texas. These blocks served as buttresses to the impinging parts of Gondwana.

As a result, there are two big salients, or places where the big push worked its way further into the continent. One formed the Ouachita Mountains in Arkansas and Oklahoma, where the tectonic push worked its way between the Nashville and Ozark Domes on one side and the Llano Uplift on the other. On the far southwestern side of the Llano Uplift, another salient developed where rocks were pushed further into the continent. This became the Marathon Orogeny.

Think of the Llano Uplift as a big round chunk of concrete sitting in the sand on a wave-washed beach. As the waves come in, pushing sand with them, they break around the concrete block and push sand around the flanks of it. The zones on either side, where sand and shells and other debris push furthest up the beach are the analogs to the Ouachita and Marathon Orogenies.

As the South American corner of Gondwana approached North America, it approached somewhat obliquely. I think it’s fair to think of this part of the collision as something like a zipper closing the ocean between the two continents – two continents with edges that were definitely NOT straight lines. The zipper closed from east to west, so the Marathon region was one of the later aspects of this complex collision.

The push caught up the older rocks – everything older than Pennsylvanian, and even including some Pennsylvanian rocks that had been laid down in the seaway, a fairly deep trough between the continents, a depression probably caused by subduction associated with the onset of the collision. The subduction zone probably plunged to the south, beneath Gondwana, where a volcanic belt developed. The volcanic zone erupted into the deep trough and even it was eventually caught up in the collision. Everything was pushed to the northwest in modern coordinates, up and over the margin of North America in what is now West Texas. Pennsylvanian rocks on the shallower slope were pushed over the deep-water Pennsylvanian rocks in the trough. This happened more or less in late Pennsylvanian time and into the following Permian Period.

Complex folds and faults in Marathon region, west Texas. From King (1937). Purple (Dc) is Devonian Caballos Novaculite.

The push and collision were enough to break the North American continental crust far beyond the margin. We talked about the Ancestral Rockies of Colorado and adjacent areas a few days ago, but they must be a consequence of this collision.

The Appalachian-Alleghenian-Ouachita-Marathon Orogeny continues into Mexico where it is called the Sonoran Orogeny. It may or may not have been shifted into its present location Mexico along a long complex shear zone similar to the San Andreas Fault, some time after the Marathon-Sonora fold belt was formed. The hypothesis of the Mojave-Sonora Megashear, as it’s called, is still somewhat controversial, and there is evidence on both sides. I have a link on the blog to a Geological Society of America Special Paper on the topic. However the tectonic energy was transferred, the impacts of the collision of Gondwana were definitely far-reaching, and even they have been changed over later time.

All of these orogenies, mountain-building events, from the Allegheny to the Sonora, were part of the assembly of the huge supercontinent called Pangaea, which means “all earth.” We’ll talk more about it next month, in the Permian, when its final assembly was complete.

Thanks to my friend Pat Dickerson for one of the images above. Pat has worked extensively on the geology of West Texas and its former connections to South America.

—Richard I. Gibson

References and links:

Ouachita-Marathon Orogeny 

Structural Style of the Marathon Fold Belt (Hickman et al.) 

Geology of the Marathon Region, P.B. King, 1937, USGS Prof. Paper 187 (1937)

Ouachita-Marathon and Ancestral Rockies

The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives: edited by Thomas Howard Anderson and others; Geol. Soc. Amer. Special Paper 393, 2005.

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