User:Dr. Dunc/Lateral Transport

Introduction
Lateral transport of terranes, also called coastwise transport, is defined as the transport of fragments of lithospheric crust along the edge of a continent, possibly over long distances. Coastwise transport of slivers of continental plate driven by oblique subduction is likely a principal cause of rearrangement of crustal blocks in orogenic belts. It appears likely that oblique subduction can drive very large displacements (of greater than 1000 kilometers) only under somewhat unusual circumstances, so they are probably fairly uncommon.

Causes of Lateral Transport
The dominant geologic structures responsible for coastwise transport are strike-slip faults. It has been suggested that the principal factors which affect strike-slip faulting behind oblique subduction zones are the angle of obliquity, the strength of the overriding crust, and the degree of coupling between subducting and overriding plates. The strike-slip velocity is very sensitive to the obliquity as well as the convergence velocity. All overriding plates are continental, probably because continental lithosphere is weaker than oceanic, and thus easier to fracture, and oceanic lithosphere is too strong to permit strike-slip faults to develop. Thermal weakening of the arc may be needed to permit sliver motion to begin, although some early faulting may occur in the forearc region.

Modes of Translation

 * Nearly pure transform motion (San Andreas fault)


 * Oblique convergence along a major strike slip fault within the volcanic arc (Sumatra)


 * Oblique divergence along en echelon transform faults (Gulf of California)


 * Lateral motion of smaller fore-arc slivers (Western Cordillera)

Locations
Coastwise transport (margin parallel displacement of arc and fore arc slivers) is a common feature of zones of oblique subduction. Oblique convergence in modern subduction zones is partitioned between sliver motion (coastwise transport of a detached sliver of overriding crust) and oblique subduction, but the partitioning factor is highly variable.

North American Cordillera (Late Cretaceous to Early Tertiary)

 * The North American Cordillera, sometimes called the Western or Northern Cordillera, is a Phanerozoic mountain chain that runs along the edge of North America from the Beaufort Sea in the south to Alaska in the north.


 * Large scale movement here was made possible by long lasting, highly oblique convergence, the specifics of the plate margin geometry, and the fact that it lacks a buttress.


 * The Yakutat terrane is a small composite oceanic-continental terrane that has migrated northwestward with the pacific plate along the western edge of North America until it collided obliquely with the continent in the Gulf of Alaska.


 * There was a period of very rapid, very oblique convergence at the end of the Cretaceous for the western cordillera.

Baja British Columbia

 * Translated north in the range of a few hundred kilometers up to 3000 kilometers. Exactly how far it was moved is debated.

Salina Block

 * Originated either immediately south of the Sierra Nevada before translation on the San Andreas fault system or even farther south.

Baja California

 * Did it originate about 300 kilometers south of where it is today during the early Cretaceous, and then move due to rifting of the Gulf of California? Or did it originate in southern Mexico about 1000 kilometers south of its present location?

The Rates at which the Process Occurs
The rates at which the process occurs are rarely more than a few tens of kilometers per million years, but were likely several times higher in the Cretaceous and Tertiary. The maximum margin-parallel velocity of sliver motion recorded is 3.6 centimeters per year in Sumatra. In 45 million years, Sumatra will have been displaced slightly more than 1600 kilometers; forearc slivers in most cases would move a fraction of this amount. From available evidence of currently active subduction zones, it appears that forearc displacement by coastwise transport requires special conditions to attain displacements of thousands of kilometers.

These conditions apparently are:


 * 1) Continuous subduction at a highly oblique angle for a large time period
 * 2) The absence of a buttress

These conditions were likely present along western North America during Late Cretaceous to Early Tertiary times. In general it appears that oblique subduction can be expected to produce margin-parallel displacements (lateral transport) of hundreds of kilometers in many subduction zones. However displacements of several thousand kilometers are probably much less common. In western North America, northward displacements of up to several thousand kilometers have been suggested on the basis of paleomagnetic measurements; geology and other paleomagnetic measurements require that this be accomplished within about 45 million years.

How far do they move?
How far terranes translate along the margin of a continent after accretion is very controversial.

Evidence
Paleomagnetic studies strongly indicate that coastwise transport in the North American Cordillera went on during the Cretaceous and Tertiary. The strike slip faults where transport occurs (particularly oblique convergence and divergence,) have a low preservation potential, so the data for recognizing the translation of terranes often relies on secondary structures more than the main faults. The history of a terrane’s transport and accretion can be determined by provenance analysis. This can show the different source areas of the sedimentary cover, and the transport history can then be found out.

The Buttress Effect
A buttress is a physical impediment to sliver motion at its leading edge, and slows the rate of transport. Any geometrical factor that might prevent free motion of the forearc sliver could be defined as a buttress.

Key factors of buttress formation:


 * 1) The greater strength of oceanic vs. continental lithosphere
 * 2) The energy required to thicken continental crust

It can be difficult to quantify the effects of buttressing. Even a sliver with an absolute buttress, which is when the leading edge of the sliver cannot move at all, can experience some displacement because some sliver motion can be taken up by crustal thickening and deformation all along its length. Displacement would die out along the trend of such a fault system and decrease in the direction of sliver motion.

Yakutat Terrane
Multiple terranes have collided with Southern Alaska during the Cenezoic, including the Yakutat. Its size is about 600 kilometers long and 200 kilometers wide. It arrived where it is today more recently than the Pliocene, and according to GPS measurements is moving north at about 44 millimeters per year (slightly less than the Pacific plate which in that region moves at about 52 millimeters per year). It is currently being underthrust beneath the Chugach terrane at about 0.56 millimeters per year, forming the Cugach/St. Elias Range, and is attached to the Pacific plate. It was transported northwestward along the Alaskan continental margin.

There are two proposed hypotheses for how the Yakutat terrane was transported to where it is today (since about 50 million years ago):

The Northern Option (Short Transport Hypothesis)


 * The terrane fragment underwent a relatively short distance of southward Neogene transport (about 600 kilometers)
 * The cover strata was derived from local sources
 * Significant shortening of the terrane

The Southern Option (Long Transport Hypothesis)


 * Based on the studies of magnetic anomalies reconstructions
 * The basement rocks were originally around Northern California or Oregon in the Eocene
 * There was about 1500 to 2000 kilometers of transport north along the Cordilleran margin
 * Cover strata would show evidence of being transported

The northern option seems to be considerably supported, for several reasons, including the grain age distributions, rock composition, and lack of evidence of volcanic activity or zircons. The far travelled option is ruled out for the younger cover strata, but not for the basement rocks. In fact, there is evidence that the basement rocks were transported for quite a distance, such as the similarities of the metamorphic rocks on Baranof Island in southeast Alaska to the Leech River Schist in southern Vancouver Island. This would imply that they originated around the Washington/British Columbia border around 50 million years ago, and that at they time they were contiguous. Then a chunk was detached and moved about 1100 kilometers north to the northern margin of the Cordillera, and was likely in its current position by 40 million years ago.

How long has this process been occurring?
Lateral accretion involving horizontal contraction has been an important process of crustal addition in the Phanerozoic, but it is uncertain how important it was in the formation of Archean cratons, and it is possibly just a minor crustal growth process when compared to the emplacement of mantle driven magmas. The same plate tectonic processes of lateral transport and accretion could have been functioning in the Archean as well as they do today to produce composite provinces such as the Klamath Mountain Province and the Wyoming Province.