Summary traits of southern Appalachian orogen
major components (described from foreland to the west to hinterland to the east): 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 (because of the scale), 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 an important detachment level.
- Blue Ridge:
- North American basement (Grenville age) involvement
along with Iapetan rift sediments (Ocoee series) in western part.
- rocks are metamorphosed, with polyphase,
- eclogites and ultramafites in eastern part,
seems likely that any suture in here somewhere.
- folded thrust nappes, but thin skinned, and underlain by subhorizontal Paleozoic carbonates.
Image of Ocoee series metaconglomerate from Careter Lake, Georgia Spillway. The interpretation is that these were coarse grained rift-related sedimentary facies from the early development of the Iapetus passive margin of North America, that were exhumed during Alleganian thrust sheet development. Note the significant deformation. A natural question to ask is as to the age of the strain - pre- or post- emplacement? This was taken on the 2015 UNO field trip to Georgia and Florida.
- Brevard zone - major fault separating Piedmont
from Blue Ridge. Early thrust component, large late major dextral motion and part of Eastern Piedmont Fault Zone (EPFZ).
- complex of terranes. Larger ones Carolina
terrane and Avalon terrane.
- Pine Mountain thrust window - see through
crystalline thrust sheet into underlying deformed N. American cratonic
COCORP data also suggests exposed Piedmont is thin-skinned crystalline thrust sheet.
- large scale folds, and an extensive network of dextral
strike-slip faults active in Alleghanian, indicating that at this time convergence was oblique.
- 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).
- post-orogenic collapse and part of the Wilson cycle??
- 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
of ophiolites (covered in earlier lecture).
- 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 as part of a 'filled' foreland basin.
- Some hints of extensional deformation and gravitational collapse, but not worked out yet.
- 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 suggests low levels of strain penetrated into mid-contient region.
- 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?
- Dennis, A., Is the central Piedmont suture a low-angle normal fault? Geology, 1991, 1081-1084.
- Bobyarchick, A., 1981, The Eastern Piedmont Fault System and Its Relationship to Alleghanian Tectonics in the Southern Appalachians; The Journal of Geology, 89, 335-347 -
- Craddock, J. P., Jackson, M., van der Pluijm, B., Versical, R. T., 1993, Regional Shortening Farbics in Eastern North America: Far-field stress transmission from the Appalachian-QUachita Orogenic Belt; Tectonics, 12, 257-264.
- Hibbard, J., 1999, Docking Carolina: Mid-Paleozoic accretion in the southern Appalachians; Geology, 28, 127-130.
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
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