GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 116-6
Presentation Time: 9:00 AM-6:30 PM


HARMON, Lydia J., Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, GUALDA, Guilherme A.R., Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235 and GRAVLEY, Darren M., Geological Sciences, University of Canterbury, Christchurch, 8041, New Zealand

How volcanic eruptive styles change through time is particularly important to understand the development of large silicic components of LIPs. Paraná silicic volcanics, southern Brazil, includes some of the largest SLIP deposits on record, related to the South Atlantic rifting. They include both effusive and explosive units, exposed at the plateau and in ~400-800 m canyon walls.

Distinguishing between effusive and explosive deposits in the Paraná volcanics has been challenging, largely due to the high degree of welding and rheomorphic deformation. We have previously identified effusive and explosive units, and we infer the presence of lava flows and pyroclastic deposits formed by fire-fountaining.

We focus on the canyons, where the continuous exposures provide unparalleled access to vertically stacked packages. We document the units at multiple scales, from remote observations from viewpoints 10’s-100’s m away to directly at the canyon walls.

Lava-dominated canyons are identified by imbricated flow units and range from massive to sparsely flow banded. Columnar jointed layers (1-70 m) are also seen. In detail, lava flow units are not horizontal and follow paleotopography created by successive flows.

In contrast, canyons dominated by pyroclastic units exhibit long, tabular, horizontally continuous packages that can extend for kilometers. Horizontal fracturing dominates, and horizontal subunits can often be recognized within the larger packages. In one package we studied in detail, the rocks are characterized by a lenticular fabric throughout the unit that gradually transitions from ~3 cm to the mm scale. We interpret the lenses to be juvenile volcanic material. At the base of the unit the lenticular fabric transitions to a fine lamination and we see kinematic indicators, indicating rheomorphic flow.

The detailed vertical work in the thick, continuous canyon sequences allows us to investigate the temporal evolution of these volcanic systems. We locally see transitions from pyroclastic units to lava flows, indicating a change in eruptive style over time. This work begins to resolve the distribution of eruptive vents, and the waxing and waning of these volcanic centers through time. The connections between canyons may ultimately unlock the volcanism associated with SLIP and continental break-up processes.