Reading in text: p. 198-208

There is a tectonic history seen in most mountain belts that occurs before final ocean basin closure and the collision of two continents. Subduction closes oceanic basins, but 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 the behavior is much richer than this.

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.

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!!!!!
• 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.
• 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 tool.

1) Take the paleolatitude for the Wrangellia terrane and compute a relative velocity in cms/yr if the span of time is 70 Ma for the move from 2 to 15 degrees latitude. What is this motion relative to? Is this a minimum or maximum estimate of the actual velocity of terrane movement?

2) Plot the paleolatitude position versus time for the Wrangellia, Alexander and Peninsular terranes? Use the midpoints of the geologic periods given. You can use Excel to do this. What can you conclude from this plot? Give it some thought.

3) What additional information could be added to the above graph that would help understand the terrane history?

Types of lithotectonic elements and accreted terranes?

• oceanic plateaus:
  • Kerguelen, Ontong Java.
  • formed as LIPs, primarily basaltic.
• 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 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 delaminated from underlying mantle.

• The case for North American Cordillera and Alaska: case study of Wrangellia.
  • 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.
• Carolina terrane in the Appalachians.

Are these just microplates? Not quite - detached from lithosphere, not internally rigid.

Is this a process by which continental mass has grown with time?

• This is a matter of debate as to degree.
• Map of orogens/terranes and the growth of the North American continent in the PreCambrian.

• 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.