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).
Introductory statement: 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). Subduction closes oceanic basins, but some of this
pre-closure 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 country sized-real estate embedded in oceanic crust
that looks like it should have difficulty going back down below
during subduction. 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. 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,
which seems to fit pretty well to me.
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. 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 detail 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.
- 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!!!!!
A mystery terrane.
- composite terranes: a terrane formed from identifiable terranes.
- 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?
- fault boundaries
(mostly strike-slip and thrust faults).
- paleomagnetic position: basically they have separate polar wander paths
from adjacent terranes and from the continent they have docked
- geologic dissimilarity to surrounding terranes.
- 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?
- faunal assemblages.
- dissimilarities in ages of single-zircon
age distributions. This is a relatively
new, but powerful tool.
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 lithotectonic elements and accreted
- oceanic plateaus:
- examples of existing plateaus: Kerguelen, Ontong Java.
- formed as LIPs, primarily basaltic.
- 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.
- island arcs.
- remnant arcs (back arc rift slivers).
- continental fragments/slivers:
- examples: Lomonosov ridge, Madagascar, Taiwan.
- continental rifting can be messy, leaving
embedded continental fragments within oceanic crust.
- composite terranes.
- alternate and less interpretative terminology/approach:
- stratified terranes.
- disrupted terranes.
- metamorphic terranes.
- composite terranes.
Mechanics of accretion/dispersal?
- thrusting, nappes.
- indentation tectonics.
- localized character.
- older to younger inboard to outboard.
- partitioning of oblique subduction, Sunda
style tectonics may play a major role.
- extension can also play a role.
- migration mechanics of subduction zone a
- are they detached from underlying mantle?
Examples of the terrane approach.
- The case for 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.
- Link to description of Yukon area in terms of terranes.
- Terranes in the Appalachians.
- Carolina terrane, Cambrian volcanic arc (possibly composite arc terrane) with trilobites of non-North American affinity.
- 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/
Are these just microplates? Not quite - detached
from lithosphere(?), 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.
- 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.
- 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.
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.