The Fold Illusion: Structural and Thermo-Rheological Constraints on the Deformation of Silicic Lavas During Emplacement

Abstract:

Surface folds are reported on all silicic lavas implying ductile-compressional deformation of the upper carapace during emplacement. However, this interpretation is contradicted by analog models, and now new structural and thermo-rheological analyses that imply brittle-tensile deformation dominates. Extensive field examinations of Holocene silicic lavas in California and Oregon fail to find evidence to support the interpretation that parabolic ridges and troughs on the upper surface (ogives) are antiformal and synformal folds, respectively. Smaller-scale folds undoubtedly exist but they and their accompanying flow-banding are truncated by the ogives and not folded around them. Instead adjacent ridges have steeply-dipping curviplanar surfaces that meet along the axis of the trough similar to crease structures documented in many silicic domes and lavas. Moreover, the flow-banding and folds on opposing ridge faces are often identical and can be restored together by shortening the across the trough (see figure). The upper surface is pervasively fractured and the crease structures forming ogive troughs are the largest scale manifestation of brittle-tensile deformation. This should not be surprising given (1) the very low tensile strength of the pumiceous obsidian that is ubiquitous at the free surface, (2) that this pumiceous obsidian is largely degassed and dry, and (3) it cooled very rapidly. If the pumiceous obsidian carapace was in compression thrust faults would form rather than folds. To form folds the size of ogives would require strain rates similar to tectonic rates and timescales in excess of 10⁵ years.

Recognition of the brittle-tensile origin of ogives on silicic lavas makes them analogous to crevasses on ice and salt glaciers and undermines analyses where folding is assumed, for example, studies of fold wavelength and amplitude on remote lavas to estimate lava composition using Biot's fold theory. This has significant implications for remote sensing investigations of gravity flows on other planetary bodies and otherwise inaccessible lavas on Earth.