Summary traits of southern Appalachian orogen
major components (described from foreland to hinterland): Map from USGS shows some of these major divisions and the distinction between the Northern and Southern Appalachians, with New York City close to the switch over location. One questions that can be asked is what in the geologic history led to the difference between the northern and southern sections. We are focusing on the Southern Appalachians simply because I know it a bit better.
- Valley and Ridge foreland fold-thrust belt.
- thin-skinned, no basement involvement.
- Pennsylvanian (Alleghanian in age).
- North American shelf sediments involved in the deformation, Cambrian
and Ordovician passive margin wedge of strata.
- In above map from the USGS, while the details are not clear, the thickening of the Paleozoic strata to the east is clear. This is the passive margin associated with the Iapetus, which when closed produced the Appalachians.
- Rome shale as a detachment
- Blue Ridge:
- North American basement (Grenville) involvement
and Iapetan rift sediments in western part.
- rocks are metamorphosed, with polyphase,
- eclogites and ultramafites in eastern part.
- folded thrust nappes, but thin skinned, and underlain by Paleozoic carbonates.
- seems likely that any suture in here somewhere.
- Brevard zone - major fault separating Piedmont
from Blue Ridge. Early thrust component, large late major dextral motion.
- complex of terranes. Larger ones Carolina
terrane and Avalon terrane.
- Pine Mountain thrust window - see through
crystalline thrust sheet into underlying N. American cratonic
- large scale folds, and an extensivenetwork of dextral
strike-slip faults active in Alleghanian.
- Alleghanian magmatism also occurs, mainly granites, some gabbros. Alleghanian granites carry dextral shear fabric.
- Mauritanides in northwest Africa are the
other half of orogen.
- Triassic rift basins along its interior (in the Piedmont).
- Coastal Plain composed of Cretaceous and Tertiary passive margin sediments.
Valley and Ridge province of the Appalachians outlined by two white lines in the USGS Tapestry Map, Image source: http://tapestry.usgs.gov/features/features.html .
This is a shaded relief DEM image taked from the NASA Visible earth site . See if from the description you can pick out the following: anticlines and possiple thrusts in the Valley and Ridge, the approximate contact between the Valley and Ridge, the Brevard zone. The land surface as a geologic signature simply because erosion selectively etches into the underlying geologic structure.
Some major events:
- Taconian deformation
(Late Ordovician, circa 440 Ma) of North American marigin clear
in the northern Appalachians. First major accretion event - obduction
- A slew of other events inbetween Taconian
and Alleghanian - assigned to Acadian.
- Significant Alleghanian crustal thickening,
and hinterland dextral strike-slip suggests decoupled oblique
convergence. Associated clastics from Appalachian extent all
the way into Nebraska.
- Map above showing simplified Pennsylvanian paleogeography. Note the extensive shoreline 'swamp areas' where coals were deposited. Image from USGS site http://pubs.usgs.gov/gip/continents/ .
- Map above showing the details of the Appalachian coal basin, with the eastern edge involved in the Valley and Ridge folding. Image source from USGS site http://pubs.usgs.gov/fs/fs115-99/fs115-99.html .
- Subsequent Triassic rifting.
- Some things as basic as the polarity of subduction
are still in debate.
Map showing two of the better exposed, studied and known Triassic grabens in the Appalachians - the Newark and Connecticutt River basins. More basins lie buried underneath the Coastal Plain Province. This rifting started perhaps some 50-60 millions after the Alleghanian deformation stopped - not along time. Image source: http://3dparks.wr.usgs.gov/nyc/mesozoic/mesozoicbasins.htm .
Penetration of Appalachian strain and fluids into the continent.
- calcite twin analysis
- squee-gee model and Mississippi Pb-Zn deposits.
- why are they still high, still mountains?
- are the Alleghanian granitoids from subduction,
crustal anatexis, lithospheric delamination?
- what was the polarity of subduction before
final continental collision?
- when was the Carolina terrane accreted?
materials for Plate Tectonics, GEOL 3700, University of Nebraska
at Omaha. Instructor: H. D. Maher Jr., copyright. This material
may be used for non-profit educational purposes with appropriate
attribution of authorship. Otherwise please contact author.