Sweeping the oceans - suspect and
Reading: Introduction of Howell, Jones & Schermer, 1985, Tectonostratigraphic terranes of the Circum-Pacific Region, p. 1-9 (available at course Blackboard site).
There is a significant tectonic history seen in most mountain
belts that occurs before final ocean basin closure with the collision
of two continents, and that can't be explained by simple subduction processes. The Appalachians are an excellent example (as captured in the two diagrams below). The final act of closing an ocean basin (as in the Wilson Cycle) if continental collision and suturing, but some important
pre-closure tectonic history can't be accounted for by simple subduction
tectonics as discussed in this class so far. Something else was
occurring at times during the oceanic basin closure - the accretion
of terranes. There is a uniformitarian component to this idea,
in that, when we look at present oceanic basins there is a striking
amount of small country or state sized real estate embedded in oceanic crust
that looks like it should have difficulty being subducted. What happens to this real estate when it encounters
a subduction zone? The basic idea is that it gets plastered on
to the edge of the overriding plate (it accretes). As you will see terrane behavior
is much richer than this simplified description.
The idea of terrane accretion is another distinct
paradigm revolution/expansion in the growth of plate tectonic
theory. Sometimes this has been referred to as collage tectonics.
This serial cross section diagram depicts the evolution in the New England Appalachians with the accretion of two terranes prior to the final collision with Africa. Before the recognition of the terrane accretion phenomena it was clear that the rocks were deformed by several deformation episodes. The Taconic was one of the earlier recognized such events, and it can be understood as the accretion of the Taconic terrane. A good questions to ask is - how do we 'know' this? Image source: http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm .
This serial cross section from the USGS site - http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html - shows some of the details of the earlier formation of the Taconic terrane and its accretion. Note how the accretion is basically a modification of the Wilson cycle. In this case, one can consider the idea of terrane accretion as a paradigm expansion, and not a replacement or revolution.
Terminology associated with terrane accretion
There is a rich terminology associated with the development of the terrane accretion concept, some of which is as follows:
- exotic terranes:
a terrane that had geologic history initially independent of
that of the continent it is now part of.
- suspect terranes:
status uncertain, could be exotic - its suspect!!!!!
Often the bounding faults have yet to be defined.
- composite terranes: a terrane formed from identifiable terranes, often amalgamated as part of the outboard history.
- outboard vs. inboard events: events that occur before accretion to the continental
margin vs. those that happen after accretion.
- docking: the
- strike-slip dispersal, rift dispersal: dismemberment of the terrane by inboard events. The map below shows the Swakane terrane being offset by a major fault with dextral offset. Image source USGS site: http://geomaps.wr.usgs.gov/parks/noca/t7swakane.html .
- stitching plutons:
plutons that intrude terrane boundaries and help to constrain
time of accretion.
- successor basin or overlap sequence: basins or sequences that overlap terrane boundaries
and help to constrain the time of accretion.
How are suspect terranes recognized?
Rarely is one line of evidence convincing on its own, but the convergence of the types of evidence listed below help in establishing the existence of a terrane:
- fault boundaries
(mostly strike-slip and thrust faults).
- paleomagnetic position: basically a terrane has a separate polar wander paths
from adjacent terranes and from the continent they have docked
- Images showing paleolatitude plotted by time for terranes in south-central Alaska as compared to that for the North American craton. Compare the two and then draw your conclusions. Image sources from Nockleberg et al. 1995, PHANEROZOIC TECTONIC EVOLUTION OF THE CIRCUM-NORTH PACIFIC: Open File Report 98-754 pubs.usgs.gov/of/1998/0754/report.pdf .
- geologic dissimilarity to surrounding terranes and to the continent it accretes to.
- different thermo-tectonic histories from neighbors.
- different geochemical signatures (basement signatures and isotopic ratios).
- What is the variance within vs. between terranes
is a crucial question! Which of the above evidence might you
find more convincing in defining an exotic terrane? It can get tricky, with a sliver of a continent rifting away, but then redocking, so that an earlier history is shared.
- faunal assemblages, e.g. trilobites in the Carolina terrane of the southern Appalachians having Baltic affinities.
- dissimilarities in ages of single-zircon
detrital age distributions. This is a relatively
new, but powerful tool, enabled by technologic advances that permit dating of a single zircon grain.
This is an example of a single-zircon detrital age plot from a sample of schist collected in Alaska taken from Bradley et al. 2009 (reference and link below). Since the schist had a sedimentary protolith the zircons are almost certainly detrital, and the ages reflect the time of igneous activity (and zircon crystallization) in the source material that was eroded to produce the sediment. Within a drainage-deposition basin system one would expect the age spectra (where the peaks are) to be similar, and differing terranes will likely have different spectra. One of broader results to recognize here is the total lack of PreCambrian ages, indicating that North American Precambrian basement did not serve as a source, consistent with this being unconnected to North America. The youngest ages constrain the depositional age of the schist. In a terrane with active volcanism in source areas the youngest zircon crystallization ages may overlap with the depositional age.
Exercise: Assessing the paleolatitude
history of some Alaskan terranes. Take
the data table and simple map from Panuska & Stone, 1985 and
use it to answer the following questions.
1) Take the paleolatitude for the Wrangellia
terrane and compute a relative velocity in cms/yr if the span
of time is roughly 100 Ma (Triassic to Cretaceous) for the move from 2 to 12 degrees latitude (remember that the earth's radius is 6373 km). What
is this motion relative to? Is this a minimum or maximum estimate
of the actual velocity of terrane movement?
2) Look at Figure 3 or 5 from this paper. What can you conclude from this plot? Give it some thought.
Paleomagnetic pole database: http://www.ngu.no/geodynamics/gpmdb/.
Types of accreted
- oceanic plateaus:
- formed as LIPs, primarily basaltic, can have abundant pillow basalts.
- examples of existing plateaus: Kerguelen, Ontong Java.
- image to right is topographic image of Kerguelen plateau, a Cretaceous LIP near Antarctica. The source of the image is orginally from NOAA, but directly from http://en.wikipedia.org/wiki/Image:Kerguelen-Plateau-Topography.jpg. Portion of Antarctica lies below.
- oceanic islands, seamounts, guyots:
- hot spot related, primarily basaltic, difference from LIP primarily is smaller size.
- would Hawaii ever be subducted?
- island arcs:
- probably one of the most common types of terranes.
- continental fragments/slivers:
- examples: Lomonosov ridge and Madagascar.
- continental rifting can be messy, leaving
embedded continental fragments within oceanic crust.
- Image from NOAA ( http://www.ngdc.noaa.gov/mgg/bathymetry/arctic/currentmap.html) of the bathymetry of the Arctic Ocean basin. The Lomonsov Ridge is the long skinny area with to heights similar to the surrounding continental shelf and which separates the Eurasian from the Amerasian basins. It is thought to be continental crust, and actually has some geopolitical significance when it comes to natural resources in the Arctic. Not as high but also evident is the more irregular Alpha Ridge to the west or left of the Lomonosov Ridge, which is of unknown character.
- back arc basins: because of their position this material may not be subducted.
- slices of oceanic crust preserved in mountain belts (have discussed before.
- schematic diagram to right is of the major components of an ophiolite. Source: USGS site: http://pubs.usgs.gov/bul/b1693/html/bull3lr9.htm
- thickness of the various components highly variable from ophiolite to ophiolite.
- well known example is the Bay of Island ophiolite in Newfoundland.
- typical or atypical oceanic crust?
- oceanic crust obducted instead of subducted
- why not subducted?
- composite terranes:
- e.g. composite arc terranes particularly common.
- alternate and less interpretative and more descriptive terminology/approach:
- stratified terranes.
- disrupted terranes.
- metamorphic terranes.
- composite terranes.
Pillow basalts from Cape Disappointment State Park coastal Washington. These extensive pillow basalt deposits are thought to be part of the Crescent terrane and are of oceanic affinity. In my limited experience these are the best pillow basalts I have ever seen, and the locality is well worth the visit.
Glassy rinds on some of the smaller pillows at Cape Disappointment.
Mechanics of terrane accretion/dispersal?
These are complex and include:
- thrusting, nappes: often associated with docking and emplacement.
- rigid indentor tectonics (will discuss more in next lecture).
- localized character of associated orogenesis.
- older to younger inboard to outboard is general expectation.
- partitioning of oblique subduction, Sunda
style tectonics, may play a major role.
- strike slip dispersal (e.g. Denali fault).
- extension can also play a role.
- migration mechanics of subduction zone a
question. Likely restablished from the ends of where terrane docking initially occurs.
Plate tectonic diagram for Alaska. Note the Denali fault well within the interior. This is an active strike-slip fault (dextral), which is slicing through and dispersing accreted exotic terranes . Image is from the NOAA site: http://earthobservatory.nasa.gov/Features/denali/ , which has a lot more information on the Denali fault and the associated tectonics.
Examples of the terrane accretion
North American Cordillera and
This map from the USGS web site http://pubs.usgs.gov/gip/dynamic/Pangaea.html , shows general types of terranes, some of the larger terranes, and the amount of material added to the North American continent through this terrane accretion.
Note that Alaska is basically a terrane assemblage. In this view North America appears to have grown substantially in size by terrane accretion, but there is the question of the underlying lithosphere and the extent of thin-skinned tectonics.
USGS site with
map of some of the West coast terranes.
Detailed map of Klamath terranes from USGS, with a good example of a tectono-stratigraphic diagram.
Terranes in the Appalachians.
- Times of significant orogenic events in the Appalachians include the Taconic, the Acadian and the Alleghanian. The Alleghanian is known to be the final closure of the Iapetus and the collision between Africa and North America. Are the earlier events accretion events? If so, which pieces were accreted and when? Should we expect the Taconic to be present along the length of the orogen??
- One difficulty is that because of metamorphism the NRM of the rocks that might reflect their earlier history has been obliterated, so the relevant paleomag data is very limited.
- Carolina terrane: Cambrian volcanic arc (possibly composite arc terrane) with trilobites of non-North American affinity, history of intra-arc rifting, geophysical evidence it is all thin-skinned.
- Example of some of the terranes identified in the Appalachians from Spear, Owens & Bailey, The Goochland-Chopawamsic Terrane Boundary, Central Virginia Piedmont, USGS site: http://pubs.usgs.gov/circ/2004/1264/html/trip7/ . Note the uncertain relationship between the Carolina slate belt (equivalent to the Carolina terrane) and the Goochland terrane here.
Outcrop of Persimmon Fork metavolcanics along Stevens Creek in western South Carolina, which are part of the Carolina terrane.
Close-up view of the Persimmon Fork volcanic, where the feldspar phenocrysts in a still relatively aphanitic matrix are evident. Cameral lense for scale.
Images from Prince William Forest just south of Washington DC in Virginia and of Piedmont rocks in the core of the Appalachian mountains. These are highly deformed metavolcanics of the Chopawamsic terrane. These arc volcanics are of roughly Cambrian age, and are of a distinctly different affinity than those clearly part of the eastern North American continental edge at that time. In the lower image one can clearly see the pyroclastic character of these metavolcanics, with fattened volcanic clasts, possible of what was once a lahar
Are exotic terranes just microplates? Not quite - most apper to be detached
from lithosphere(?), and they are not internally rigid.
Is this a process by which continental mass
has grown with time, or are we just shuffling around earlier formed pieces?
- This is a matter of debate as to degree.
- Below: Map of orogens/terranes and the growth of
the North American continent in the PreCambrian. Can you, from the position and ages given assemble the history of assembly of the North American continent. Image source: http://rst.gsfc.nasa.gov/Sect2/14strprv.jpg
- Smaller continental fragments leads to the suggestion that there was a permobile regime, perhaps fundamentally different. We will touch upon this material again and associated debates when we discuss Precambrian tectonics.
- Bradley, D., Haeussler, P., O’Sullivan, P., Friedman, R., Till, A., Bradley, D., and Trop, J., 2009, Detrital zircon geochronology of Cretaceous and Paleogene strata across the south-central Alaskan convergent margin, in Haeussler, P.J., and Galloway, J.P., Studies by the U.S. Geological Survey in Alaska, 2007: U.S. Geological Survey Professional Paper 1760-F, 36 p. - http://pubs.usgs.gov/pp/1760/f/ .
- Dennis, A. & Wright, J, 1997, The Carolina Terrane in northwestern South Carolina, U.S.A, Late Precambrian-Cambrian deformation and metamorphism in a peri-Gondwanan oceanic arc; Tectonics, 16, 460-473.
- Jones et al., 1977, Wrangellia - A displaced terrane in northwestern
North America; CJES, 14, 2565-2577. This is a good example of
an article defining a major terrane.
- Jones et al., 1982, The growth of Western North America;
Sci. Am., v. 247, 70-84.
- Howell, D. G., 1985, Terranes; Sci. Am. (Nov. p. 116-125.
- Howell. D. G. (ed.) , 1985, Tectonostratigraphic terranes
of the circum-Pacific region; Circum-Pacific Council for Energy
and Mineral Resources, Earth Science Series, vol. Houston, TX,
581 p. This is a huge compilation with some maps. Lots of data!
- Howell, D. G., 1989, Tectonics of Suspect Terranes; Chapman
& Hall, New York, 232 p.
- Searles, M. P. & Stevens, R.K., 1984, Obduction processes in ancient, modern and future ophiolites; Geological Society, London, Special Publications 1984, v. 13, p. 303-319
Course 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.