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.

Terminology:

• 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 accretion event.
• 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 with.
• 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.

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 terranes?

• 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.
• ophiolites.
• 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 question.
• are they detached from underlying mantle?

• The case for North American Cordillera and Alaska.
• 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.
  • USGS site with map of some of the West coast terranes.
  • Detailed map of Klamath terranes from USGS.
  • 
• 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.