Mid‐Tertiary ash flow tuff cauldrons, southwestern New Mexico

Abstract

This summary characterizes 28 known or suspected mid‐Tertiary ash flow tuff (ignimbrite) cauldrons, up to 40 km in diameter, in southwestern New Mexico. A combination of postcauldron block faulting and erosion has resulted in widely ranging levels of three‐dimensional exposures, down to plutonic roots. The evolution of the largest resurgent cauldrons followed the course determined by other workers for the Valles caldera (New Mexico) and Lake City cauldron (Colorado), with local variations. Five principal stages can be recognized: (1) precursor, (2) caldera collapse, (3) early post‐collapse volcanism, (4) major ringfracture volcanism, and (5) hydrothermal activity. Development can be terminated at any stage; stages can also be repeated. Resurgent doming can occur during stage 3 or later; doming can also be enhanced by reactivation of cauldron structures by postvolcanic basin and range faulting. Even where the surface of caldera‐fill tuff is domed, the base may have quite a different configuration. Recognition of resurgent doming may be difficult in deeply eroded cauldrons. A proposed modification in the definition of “resurgence” places emphasis on one or more magma pulses after a caldera‐forming eruption; doming is one of several effects that may or may not occur when magma invades a previously subsided cauldron. In some cauldrons, postcollapse lavas can be divided into (1) “cauldron lavas,” i.e., relatively mafic crystal‐rich disequilibrium assemblages derived from shallow defluidized residues of caldera‐forming ash flow tuff eruptions, and (2) “framework lavas,” i.e., relatively siliceous crystal‐poor near‐equilibrium assemblages derived from an evolving siliceous pluton at deeper levels, below the cauldron complex. Special features of New Mexico cauldrons include zoned outflow sheets; zoning may be normal (most siliceous at the bottom) or reversed. Posteruption potassium metasomatism may mask some chemical trends. Several styles of eruption can be recognized among mid‐Tertiary cauldrons of southwestern New Mexico. The largest ones tend to occur in clusters, interpreted as surface expressions of buried composite plutons (e.g., Mogollon Plateau). In an early stage of development, or on the fringe of a cluster, cauldrons tend to be asymmetrical trapdoor structures, formed by repeated small eruptions separated by quiescent periods. During the height of activity, cauldrons formed by eviscerating eruptions, commonly followed by resurgence. The youngest known cauldron is shallow and resulted from asymmetrical subsidence and collapse of caldera walls.

Eruptive fountains of silicic magma and their possible effects on the tin content of fountain-fed lavas, Taylor Creek Rhyolite, New Mexico

“…1). The field consists of about 10,000 km 3 of mafic to silicic lavas and pyroclastic rocks, and it includes several calderas that formed by collapse in response to eruptions of large volumes of silicic magma that fed pyroclastic flows (Ratte and others, 1984;Elston, 1984). The first-order gravity signature forms a roughly 200-km-wide circular Bouguer low of about 40 milligals (Cordell and others, 1982) that is interpreted to reflect an underlying composite granitic batholith, the intrusive roots of the volcanic rocks.…”

Estimation of magma depth for resurgent domes: An experimental approach

“…Post-collapse resurgence, defined as the long-term structural uplift of the caldera floor, is a classical key step of the caldera cycle and has been described within many calderas (Smith and Bailey, 1968;Lipman, 1984;Elston, 1984). Several causes have been proposed to explain this process (Smith and Bailey, 1968), of which pressure changes in a magma chamber is the most probable (Marsh, 1984).…”

Experimental study of caldera formation

“…Examples include Silverton [Steven and Lipman, 1976], Bonanza [Varga and Smith, 1984], Mule Creek [Elston, 1984] [Vincent, 1963], Fernandina [Simkin and Howard, 1970], and Snowdon [Howells et al, 1986]. At Snowdon the point of maximum subsidence is located near the main eruptive vent, as recognized by a concentration of co-ignimbrite lag breccias.…”