About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 614

Last Page: 614

Title: Displacements Across, and Strain in, Orogenic Belts: ABSTRACT

Author(s): J. F. Dewey

Article Type: Meeting abstract


The production of accurate palinspastic maps demands knowledge of displacements across orogenic belts. Traditionally this has involved attempts to measure linear crustal strains (^egr) across orogenic belts by fabric studies, unraveling folds, unscrambling thrusted superposed facies, etc. The inherent problems of this approach (e.g., distinguishing between stratal and crustal shortening) and the consequent difficulties of making meaningful strain measurements are minor compared with the complexities imposed by relating orogenic strain to displacements across consuming plate boundaries. It is possible in a general way to convert relative plate displacements (D) and displacement rates (D) directly into gross shortening and shortening rate values across a particular Mesozoic Tertiary orogen (e.g., Mediterranean fold belt). This has little meaning, however, for ^egr and ^egr· values in orogens for the following reasons. 1. Relative plate displacement vectors change with time. 2. Most of D is not converted into orogenic crustal strain, but is lost by subduction. Only where continental collision has occurred is there a chance that ^egr and ^egr· are direct functions of D and D·. 3. ^egr and ^egr· may be related to second-order consequences of plate motion; for example, high-level spreading and gliding of marginal nappes. 4. Mechanically significant rates depend upon determining instantaneous ^egr· and this in turn depends on the width of a zone deforming homogeneously at an instant f time. Using Le Pichon's D· values across the Alpine Himalayan fold belt, ^egr· values vary from 1.27 × 10-13 (^egr concentrated in Indus suture) to 1.59 × 10-15 (^egr across 900 km wide seismic belt from the Zagros crush zone to the Caspian Sea). These rates are far slower than rates (5 × 10-2 -10-8) at which ductile strains have been achieved in laboratory experiments. Brittle and semi-brittle structural behavior is common in orogenic belts and suggests that natural instantaneous ^egr· values are much higher than those calculated from D·. This may be a function of ^egr being concentrated instantaneously in narrow fault zones or, by incremental strain propagation, cross a particular zone. Even these factors, though, do not seem to modify ^egr· enough (e.g., in a 1-cm wide thrust zone in the Himalayas where D· = 5.6 cm/yr., ^egr· = 1.8 × 10-7). Yet, brittle deformation is evidenced by shallow earthquakes suggesting either that ^egr is concentrated along hairline fractures or that large strain accumulations precede rupture. Orogenic strains, however, are small compared with displacements across orogenic belts. The displacements can only be calculated from oceanic magnetic anomaly fitting and, less accurately, from paleomagnetic data from stable forelands and cratons.

End_of_Article - Last_Page 614------------

Copyright 1997 American Association of Petroleum Geologists