This paper describes three mid-Tertiary intrusions from the Henry Mountains (Utah, USA) that were assembled from amalgamation of multiple horizontal sheet-like magma pulses in the absence of regional deformation. The three-dimensional intrusion geometries are exceptionally well preserved and include: (1) a highly lobate sill; (2) a laccolith; and (3) a bysmalith (a cylindrical, fault-bounded, piston-like laccolith). Individual intrusive sheets are recognised on the margins of the bodies by stacked lobate contacts, and within the intrusions by both intercalated sedimentary wallrock and formation of solid-state fabrics. Finally, conduits feeding these intrusions were mostly sub-horizontal and pipe-like, as determined by both direct observation and modelling of geophysical data. The intrusion geometries, in aggregate, are interpreted to reflect the time evolution of an idealised upper crustal pluton. These intrusions initiate as sills, evolve into laccoliths, and eventually become piston-like bysmaliths. The emplacement of multiple magma sheets was rapid and pulsed; the largest intrusion was assembled in less than 100 years. The magmatic fabrics are interpreted as recording the internal flow of the sheets preserved by fast cooling rates in the upper crust. Because there are multiple magma sheets, fabrics may vary vertically as different sheets are traversed. These bodies provide unambiguous evidence that some intrusions are emplaced in multiple pulses, and that igneous assembly can be highly heterogeneous in both space and time. The features diagnostic of pulsed assembly observed in these small intrusions can be easily destroyed in larger plutons, particularly in tectonically active regions.

Abstract Small intrusions (< km 2 ) on the margins of the Henry Mountains intrusive complex of southern Utah are exceptionally well exposed in three dimensions and have a variety of shapes. Our examination of the geometry, structures, and fabric of the Maiden Creek sill, Trachyte Mesa laccolith, and the Black Mesa bysmalith (cylindrical intrusion bounded by vertical faults) suggests that this range of intrusion geometry may reflect a continuum of igneous emplacement as volume increases through magma sheeting. Intrusions begin as thin sills and through incremental injection of additional sheets, inflate into laccoliths. Marginal wall rocks are strained and rotated upward. Further sheet emplacement leads to the formation of a fault at the margin of the inflating intrusion. This fault accommodates piston-like uplift of the intrusion’s roof and results in the formation of a bysmalith. All three of these intrusions exhibit evidence for sheeting, although the evidence is weakest on the margins of the Black Mesa bysmalith. Solid-state shear zones exist between sheets in the Maiden Creek sill and on the margins of the Trachyte Mesa laccolith. Cataclastic zones also separate sheets within the Trachyte Mesa laccolith. Evidence for sheeting in the interior of the Trachyte Mesa laccolith is solely based on differences in weathering and jointing patterns. Evidence for sheeting on the margins of the Black Mesa bysmalith is based on the differences in lineation patterns and also on the distribution of cataclastic zones.

Abstract At shallow-crustal levels, the most efficient process for the accommodation of magma emplacement is roof lifting, which induces an upward vertical displacement of the Earth's surface. Estimates of the rate and duration of this process have rarely been published. One of the most spectacular places where plutons constructed by such mechanisms are exposed is the Henry Mountains in Utah. In this place, Pollard & Johnson (1973) derive a time of ‘less than several weeks’ for the construction of the Black Mesa bysmalith (BMb), by coupling a mechanical approach with a model for the flow rate of Bingham magma in a tabular conduit with a constant driving pressure at the feeder. The aim of this new study of the BMb is to evaluate the maximum duration of its emplacement and propose a feasible scenario for its construction. Our study of the pluton's internal structures suggest that the BMb is a multi-pulse pluton. We have constrained the duration of BMb emplacement by simulating the thermal evolution of the growing pluton and its wallrocks for different construction scenarios. We have adjusted the number, the thickness and the frequency of the pulses with our textural ‘time’ constraints, which are the absence of solid-state textures around internal contacts, which implies that a melted zone was maintained in the intrusion during its construction; and the absence of significant contact metamorphism or recrystallization which means that the increase of temperature in the host rock was relatively small, or short-lived, or both. In accordance with the previous estimates of Pollard & Johnson (1973), we propose that the emplacement of the BMb was a very short geological event, with a maximum duration in the order of 60 years, implying minimum vertical displacement rates of the wallrocks above the pluton of 4 metres per year. Moreover, our simulations indicate that pulses around 20 metres thick rapidly injected approximately every three months are the most consistent with the constraints from field observations.

Abstract The central White-Inyo Range in eastern California has been an excellent field area to study concordant (deformed aureoles) and discordant, or “cookie cutter” plutons (see Nelson and Sylvester, 1971 ; Nelson et al., 1978 ; Sylvester et al, 1978a ; Sylvester et al, 1978b ; Paterson et al., 1991 ; Law et al., 1992 , 1993 ; Bilodeau and Nelson, 1993 ; Morgan and Law, 1994 ; Morgan et al., 1998a and b , Saint Blanquat et al., in review), because: 1) the range is dominantly composed of a well-mapped sedimentary section where the dominant contractional regional deformation predates pluton emplacement, and 2) the plutons are well exposed at various structural levels. In this contribution, we present some of our work on three of the plutons in the range, the Eureka Valley-Joshua Flat-Beer Creek (EJB) composite pluton, the Sage Hen Flat pluton, and the Papoose Flat pluton. These three plutons exhibit drastic differences in the style and amount of deformation at their margins. Description and examination of the deformation (or lack of) at the margins of these plutons, which accommodated their emplacement, along with a characterization of the internal magmatic/solid state/magnetic foliations, are the foci of this paper and field trip. Specifically, field trip participants will have the opportunity to observe the following; 1) Total lack of strain at a pluton contact (Sage Hen Flat pluton), 2) Intense strain at two pluton contacts related to emplacement (Papoose