The roots of ash flow calderas in western North America: Windows into the tops of granitic batholiths

Abstract

Large‐volume ash flow eruptions and associated caldera collapses provide a direct link with subvolcanic granitic plutons of batholithic dimensions. The eruptive history, structural features, and petrologic evolution of ash flow calderas provide data on early stages of the evolution of an associated subvolcanic magmatic system. Broadly cogenetic, erosionally unroofed granitic plutons provide a record mainly of the late stages of emplacement and crystallization of silicic magmas. This review summarizes features of well‐studied calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a Volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the worjd. Primary examples include San Juan, Mogollon‐Datil, Marysvale, Latir‐Questa, Chiricahua‐Turkey Creek, Challis, and Boulder Batholith‐Elkhorn Mountains. Most ash flows have erupted from sites of preceding volcanism that records shallow accumulation of caldera‐related magma. Structural boundaries of calderas are single ring faults or composite ring fault zones that dip vertically to steeply inward; outward dipping boundary faults favored by some models have not been identified in North American calderas. The area and volume of caldera collapse are roughly proportional to the amount of erupted material. Pyroclastic eruptions of relatively small volume (less than 50–100 km3) may cause incomplete hinged caldera subsidences or structural sags; larger systems are bounded by complete ring faults. Few ash flow vent structures have been related to major calderas; vent geometry, as determined by size analyses of pyroclastic materials, may shift complexly during caldera collapse. Scalloped topographic walls beyond the structural boundaries of most calderas are due to secondary gravitational slumping during subsidence. Most exposed floors are a structurally coherent plate or cylinder bounded by a ring fault or dike, indicating pistonlike caldera collapse; chaotically brecciated floors predicted by models of piecemeal collapse have not been identified. Deviations from circular shape commonly reflect influence of regional structures; some calderas in extensional terranes are elongate in the direction of extension. Large calderas (greater than 100 km3 of erupted material) collapse concurrently with eruption, as indicated by thick intracaldera ash flow fill and interleaved collapse slide breccias. Volumes of intracaldera and outflow tuff tend to be subequal; correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithic fragments, degree of welding, devitrification, alteration, and even chemical composition of magmatie material. Postcollapse volcanism may occur from varied vent geometries within ash flow calderas; ring vent eruptions are most common in resurgent calderas, reflecting renewed magmatic pressure. Large intrusions related to resurgence are exposed centrally within some calderas; ring dikes and other intr...

The Volcanic-Plutonic Connection

“…This hypothesis specifically links rhyolite to coeval granodiorite plutons, consistent with the field-based studies summarized by Lipman (1984), and makes specific predictions about the geochemical and geochronological connections between related volcanic and plutonic rocks.…”

“…He noted that although compositional gradients are evident in both volcanic and plutonic systems, they may be produced by different mechanisms. Lipman (1984Lipman ( , 2007 reiterated the strong field connection between roughly coeval plutonic and volcanic rocks in caldera complexes.…”

Maars to calderas: end-members on a spectrum of explosive volcanic depressions

“…Calderas range from several kilometers to several tens of kilometers in diameter, with subsidence depths from a few 100 m to a few kilometers (Acocella, 2007). Beneath the surface expressions of calderas are thick accumulations of intracaldera ignimbrite, often exceeding 1 km thickness, with interstratified breccias and megabreccias associated with caldera collapse (Lipman, 1976(Lipman, , 1984. Intrusions include dikes, sills, and stocks.…”

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

“…While resurgence is often seen as the waning stage of a caldera cycle, it is also one of the most dynamic phases as evinced by the "restless" nature of currently active calderas. Structural uplift promotes faulting and permeability that may result in further eruptions or the development of hydrothermal and geothermal systems through percolation of meteoric fluids downwards, enhancing the conditions for ore formation (Smith and Bailey, 1968;Lipman, 1984;Kennedy et al, 2012). Many calderas host large lakes, and these may pose considerable hazard if rapidly drained (e.g., Goff et al, 1992) and may involve flooding and tsunami hazard if rapid uplift triggers landslides and collapse (Chen et al, 1995;Tibaldi and Vezzoli, 2004).…”