Crustal extension from exposed sheet intrusions: review and method proposal

Sheet intrusions are probably the most widespread magmatic features on Earth. They create a space for themselves by dilating the host rock. The exposed sheet intrusions allow an estimate of a minimum value of the extension to which the crustal layers and the volcanic edifices are subject. The correct evaluation of the contribution of sheet intrusions to the crustal extension is of great importance in various geologic settings. The current methods used to evaluate the extension due to exposed sheet intrusions are reviewed in this paper. The effect of the dip of sheet intrusions on the evaluation of horizontal crustal extension is analysed. If the dip effect is neglected and the intrusions are (sub)vertical, the error due to the dip effect in the extension evaluation is small; if the intrusions are shallow dipping, the error may be great and varies as a function of the dyke dip. For dykes with dip range 45-65 degrees, the error due to neglecting the dip effect is similar to 30 to similar to 15%. Increasing discrepancies of more than 25% are produced on the computed dilation when the intrusions progressively dip less than 50 degrees. A simple method, based on structural field measurements of exposed sheet intrusions and on trigonometric considerations, is presented as an improvement of pre-existing methods to estimate the horizontal component of the crustal extension. Using this method, field data of 1154 dykes from different geodynamic contexts are analysed. The horizontal component of extension is computed using dyke data from the Tertiary massifs of Lanzarote and Tenerife, and from the active stratovolcanoes of Monte Somma-Vesuvius and Etna. Since the analysed dykes are steeply dipping, the cumulative difference between the extension computed neglecting and taking into account the dip effect remains less than 10% for ail the data sets. The percentage difference is nearly constant and cumulatively higher for Monte Somma, where the mean dip of the intrusions is smaller. In Lanzarote, 237 dykes emplaced between similar to 15.5 and similar to3.7 Ma give a peak extension of 276 m in direction N135 degrees, and a time-averaged minimum extension rate of 1.9 x 10(-2) to 2.3 X 10(-2) mm a(-1). In Tenerife, an extension of 562 m is computed in the direction N65 degrees from 563 dykes emplaced from similar to7.4 to similar to3.3 Ma. The time-averaged minimum rate of extension for the Tertiary massifs of Tenerife ranges from 0.14 to 0.25 mm a(-1). The peak extension computed for Monte Somma is 81.7 m in the direction N90 degrees, based on 96 exposed sheets. Very likely, most of Monte Somma sheets intruded within similar to 12 ka, giving a time-averaged minimum extension rate of similar to7 mm a(-1). From 184 exposed dykes of Etna, a minimum extension rate of 6.2 mm a(-1) is computed (peak extension is 212 m in direction N35 degrees). The poor resolution of age constraints may affect deeply the reliability of the extension rates. However. from the examples presented it appears that the extension rates of pre-erosional successions from intraplate volcanic islands can be smaller than those of active stratovolcanoes. This may suggest a more active dynamics in the stratovolcanoes. (C) 2001 Elsevier Science B.V. All rights reserved.