The possible contribution of circumferential fault intrusion to caldera resurgence

Abstract

In the geological record, the intrusion of substantial amounts of magma into circumferential faults and ring fractures is commonly observed. Finite element modelling is used here to investigate the strain field that may be expected from such intrusive events. Two simple vertical scenarios are explored, one for a caldera with a central block of thickness to diameter ratio of~1:1 (similar to Rabaul) and one with a ratio much less than 1:1 (similar to the 'Valles type'). Surface deformation in both cases is similar with central uplift, the development of a moat (or trough) like feature just outside of the intersection of the azimuth of the intruded ring fault and the free surface, and broader scale tumescence at a scale several times larger than the caldera's radius. The response of the block and sub-caldera magma chamber for the two scenarios, however, is different. The blocks are in effect squeezed; the high aspect ratio one deforms upwards at the surface and downwards at its base, whereas the low aspect ration one experiences up arching (or bending) of the central part of the caldera block. Central uplift still occurs when only a short arc of a ring fracture system or a circumferential fault is intruded. In both models, tumescence in the centre of the caldera from single ring dyke intrusion can only account for decimetres to metres of surface uplift. Repeated intrusions over tens to hundreds of thousands of years, however, may cause incremental up doming of the caldera block leading to larger scale resurgent features. The amount of uplift possible due to squeezing of a high aspect ratio block is limited. It is proposed, however, that where bending of plate-like blocks occur above a decompressible and/or malleable magma body, ring fault intrusion may be a significant contributor to resurgence. In the simple conceptual models shown here, the amount of ring dykeinduced central uplift will be >40-50% of the width of the ring complex. In the geological record the accumulation of intrusions into some ring fractures has led to annular or arcuate plutons of hundreds of meters to several kilometres in thickness. At certain calderas such intrusions may be a control on the marked concentration of uplift within the restricted area defined by the caldera faults. The complex nature of the horizontal displacements associated with the intrusion of ring and arcuate dykes is also explored. Intrusion into ring fracture zones will tend to take place into those sectors of the annular zone which are perpendicular to the least compressive stress vector. This may be a factor in the observed difference for caldera evolution in extensional and compressional areas. The unrest at several modern calderas is tentatively related to circumferential fault intrusion.

Structure of volcano plumbing systems: A review of multi-parametric effects

“…This model differs from previous models of ring dykes, which were directly linked with the underlying magma chamber (e.g. Saunders, 2005) (Fig. 44B).…”

“…44B). Saunders (2005) also proposes that arching of the caldera floor induces extension in the extrados, whereas in the intrados compression is caused by both arching and the force exerted laterally by the intruding ring dykes, preventing intrusions in the caldera block. Several eroded calderas show that substantial amounts of magma are intruded along ring faults that channel the magma flow upward (Clough et al, 1909;Bailey and Maufe, 1960;McCall and Bristow, 1965;Smith and Bailey, 1968;Almond, 1977;Sparks, 1988;Miura, 1999;Johnson et al, 2002;Sewell et al, 2012).…”

“…A number of inclined sheets intersected the ring faults that have been previously developed during the caldera collapse, and became deflected up along the fault to form multiple ring dykes(after Browning and Gudmundsson, 2015). (B) Model of ring dykes directly linked with the underlying magma chamber (afterSaunders, 2005).…”

Resurgent Toba—field, chronologic, and model constraints on time scales and mechanisms of resurgence at large calderas

“…As such, a numerical finite element model (FEM) approach is necessary to investigate resurgent timing in greater detail. Previous numerical investigations using elastic finite element models indicate magmatic initiation via intrusion along a ring dike may promote resurgent uplift observed at caldera systems (Saunders, 2001(Saunders, , 2004. However, while these models illustrate the elastic response due to magmatic intrusion, they do not provide means to calculate uplift rates.…”

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

“…Portanto, não representam diques ou intrusões anelares, dado que a definição original de tais termos inclui critérios descritivos e genéticos, específicos do momento da subsidência (Roberts 1970;Stevenson et al 2008). Sua geometria é consistente com episódios finais na câmara magmática, quando intrusões anulares ressurgentes reativam falhas periféricas ou antigos diques anelares (Johnson et al 2002;Saunders 2005;Kennedy & Stix 2007). O crescimento dessas intrusões, em incrementos de magma, envolve mecanismos do tipo crack-seal que dependem das condições de extensão, da pressão do magma e do restabelecimento do comportamento reológico favorável à fratura a cada incremento (Bartley et al 2008).…”