Mechanisms of noncollisional orogenesis

Orogenesis at ocean-continent convergent margins initiates where two conditions are met (Dewey & Bird, 1970): (i) the upper continental plate is thrown into compression and (ii) the converging plates are sufficiently coupled to allow compressional stresses to be transmitted into the interior of the upper plate.

Studies of subduction zones in general suggest that the stress regime in the overriding plate is influenced by the rate and age of subducting oceanic lithosphere (Uyeda & Kanamori, 1979; Jarrard, 1986). High convergence rates and the underthrusting of young, thick, and/or buoyant lithosphere tend to induce compression, decrease slab dips, and enhance the transfer of compressional stresses (Section 10.2.2). However, although these factors may explain general differences between Chilean-type and Mariana-type subduction zones (Section 9.6), they do not explain the along-strike differences in the structure and evolution of the Andean orogen (Sections 10.2.3, 10.2.4). The Andean example shows that neither flat subduction nor the underthrusting of young and/or buoyant oceanic lithosphere control areas of maximum shortening and crustal thickening (Yanez & Cembrano, 2004). From the Altiplano region northward and southward, there is a decrease in the total amount of crustal shortening and thickening with no direct correspondence to either the slab age (Jordan et al., 1983; McQuarrie, 2002) or the convergence rate (Jordan et al., 2001). These observations indicate that factors other than the geometry, rate, and age of subducting lithosphere control the response of the upper plate to compression. Among the most important of these other factors are: (i) the strength of inter-plate coupling at the trench and (ii) the internal structure and rheology of the continental plate.

1 Interplate coupling at the trench. Yanez &

Cembrano (2004) used a continuum mechanics approach to examine the effects of variable amounts of inter-plate coupling at the trench on upper plate deformation. These authors noted that patterns of seismicity in the Andes suggest that flat subduction controls some areas of strong inter-plate coupling (Section 10.2.2). However, the largest seismic energy release above flat segments occurs up to several hundred kilometers inland from the trench (Gutscher et al., 2000). By contrast, seismicity at the Peru-Chile Trench is approximately equivalent in both flat and steep slab segments. This observation, and the lack of correlation between the amount of intra-plate shortening and the flat slab segments, suggests that the degree of inter-plate coupling at the trench may be equally or more important in controlling deformation of the upper plate. To test this idea, Yanez & Cembrano (2004) divided the South American plate into two tectonic domains that are characterized by different force balances: the forearc and the backarc-foreland. In the forearc, the age of the ocean crust and the convergent velocity control the strength of coupling across the ocean-continent interface. The strength of the coupling regulates the amount of deformation. The authors derived an empirical relationship between trench topography and the degree of coupling across the slipping interface using along-strike variations in the shape of the inner trench slope (Fig. 10.8a). This approach is based on the work of Wdowinski (1992) who suggested that after an equilibration period of 5-10 Ma, trench topography reflects the balance between the tectonic and buoyancy forces associated with subduction. Buoyancy forces associated with continental crust dominate the force balance if the strength of the plate interface is low, resulting in an upward movement of the forearc (Fig. 10.8b). Tectonic forces associated with the sinking of oceanic lithosphere dominate if the strength of the plate interface is high, causing downward movement of the forearc. By assuming the trench topography is in equilibrium with these forces, Yanez &

Profile: 4 Profile: 3 Profile: 2 Profile: 1

Profile: 15 Profile: 14 Profile: 13 Profile: 12 Profile: 11

Profile: 4 Profile: 3 Profile: 2 Profile: 1

Profile: 15 Profile: 14 Profile: 13 Profile: 12 Profile: 11

Buoyancy Force

Buoyancy Force

468 Distance (km/100)

468 Distance (km/100)

Buoyancy force

Tectonic force

Tectonic force

Nazca. R. Carnegie R.

km g a. co -a

50

0

0

km O ci o -a

40 20-

o

0 50

age (Ma)

0

e

10

P 'ra > ยง

5

Co

0

isc. Pa s)

X(1022

10 11 12 13 14 15

nnniirinnnnnnnnnn

10 11 12 13 14 15

10 11 12 13 14 15

Was this article helpful?

0 0
Boating Secrets Uncovered

Boating Secrets Uncovered

If you're wanting to learn about boating. Then this may be the most important letter you'll ever read! You Are Going To Get An In-Depth Look At One Of The Most Remarkable Boating Guides There Is Available On The Market Today. It doesn't matter if you are just for the first time looking into going boating, this boating guide will get you on the right track to a fun filled experience.

Get My Free Ebook


Post a comment