Sheathfolds in rheomorphic ignimbrites

Abstract

Structural reappraisal of several classic rheomorphic ignimbrites in Colorado, Idaho, the Canary Islands and Italy has, for the first time, revealed abundant oblique folds, curvilinear folds and sheathfolds which formed during emplacement. Like their equivalents in tectonic shear-zones, the sheathfold axes lie sub-parallel to a pervasive elongation lineation, and appear as eye structures on rock surfaces normal to the transport direction. With the recognition of sheathfolds, ignimbrites previously inferred to have undergone complex rheomorphic deformation histories are re-interpreted as recording a single, progressive deformation event. In some examples, the trends of sheathfolds and related lineations change with height through a single ignimbrite suggesting that rheomorphism did not affect the entire thickness of ignimbrite synchronously. Instead, we infer that in these ignimbrites a thin ductile shear-zone rose gradually through the aggrading agglutinating mass whilst the flow direction varied with time. This suggests that, in some cases, both welding and rheomorphism can be extremely rapid, with ductile strain rates significantly exceeding rates of ignimbrite aggradation.

Rheomorphic ignimbrites of the Rogerson Formation, central Snake River plain, USA: record of mid-Miocene rhyolitic explosive eruptions and associated crustal subsidence along the Yellowstone hotspot track

“…d Site mean TRM directions (corrected for post-emplacement tectonic dips) for the Brown's View Member are consistent between correlatives and distinct from the underlying Wooden Shoe Member. e Inferred distributions of the Brown's View and Wooden Shoe members in the central Snake River Basin, with known and inferred thickness contours yielding conservative volume estimates of ∼100 and ∼130 km ignimbrite has been well described and interpreted because it has been influential in the understanding of how rhyolitic rheomorphic ignimbrites are emplaced, rapidly welded, deformed and cool (Branney et al 2004;Branney et al 2008;Andrews et al 2008;Andrews and Branney 2011;Robert et al 2013;Finn et al 2015;Ellis et al 2015). However, previous accounts included nearby exposures (W on Fig.…”

“…The overlying, red-brown, lithoidal zone is c. 57 m thick and has a lower 'flat-zone' with sub-horizontal intrafolial isoclines and sheath folds and an upper 'steep-zone' with upright to overturned ≥10 m scale antiforms and synforms that refold the earlier isoclines (Branney et al 2004;Andrews and Branney 2011). The upper vitrophyre, ≥1 m thick, is perlitic, locally highly inflated by post-welding vesiculation, and folded into tight upright folds with rheomorphic autobrecciation.…”

“…'Fused' ash is welded as the result of contact with a formerly hot adjacent rock (Fisher and Schmincke 1984). 'Intensely welded' tuff shows high strain and lacks remnant interstitial porosity (Smith 1960) and is 'rheomorphic' where there is evidence for ductile extension or folding prior to cooling (Branney et al 2004). 'Vitrophyre' is chilled, mostly glassy welded tuff, whereas 'lithoidal' welded tuff is microcrystalline (including devitrified tuff).…”

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

“…6 and 7). Progressive shear modified early-formed folds, producing sheath folds (Farrell 1984;Smith 2002;Branney et al 2004;Andrews and Branney 2011). The time period in which the Rocche Rosse progressively deformed was likely to be weeks following effusion (Gottsmann and Dingwell 2001), and it has been suggested that deformation in rhyolitic lava flows can span a year (Lu et al 2004;Tuffen et al 2013).…”

Syn-eruptive, soft-sediment deformation of deposits from dilute pyroclastic density current: triggers from granular shear, dynamic pore pressure, ballistic impacts and shock waves

“…This may be associated with subaqueous eruptions (Fiske, 1963) or subaqueous deposition (Brand and White, 2007;Brand and Clarke, 2009;Jordan et al, 2013), but also importantly, subaerial emplacement (Vazquez and Ort, 2006). Hot-state, plastic deformation including partial deformation of the clasts themselves is referred to as rheomorphism (Branney et al, 2004;Andrews and Branney, 2011). Lava flows may also deform underlying soft-sediment beds (Rawcliffe and Brown, 2014).…”