Fault zone processes during caldera collapse: Jangsan Caldera, Korea. (July 2019)

• Record Type: Journal Article
• Authors: Kim, Chang-Min; Han, Raehee; Kim, Jong-Sun; Sohn, Young Kwan; Jeong, Jong Ok; Jeong, Gi Young; Yi, Keewook; Kim, Jeong Chan
• Is Part Of: Journal of structural geology. Volume 124(2019)
• Page Start: 197
• Page End: 210
• Publication Date: 2019-07
• Subjects: Caldera -- Caldera fault zone -- Pseudotachylyte -- Frictional melting -- Tuffisite -- Fault intrusions; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2019.05.002
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: Caldera fault zones, identified in many modern and ancient volcanoes, have been the subject of geological and geophysical observations and of analog and numerical modeling. However, the physicochemical processes in fault zones during a caldera collapse are still poorly understood. Here, we present field observations from a caldera fault zone in the Cretaceous Jangsan Caldera, SE Korea. The fault zone is ∼30 m wide and juxtaposes an intracaldera rhyolitic volcanic complex against older dacitic rocks; it consists of a minor fault, a main fault, and a series of fault-related intrusions (rhyolite and tuffisite). The main fault dips 90°–88°NW (vertically to steeply inward) and strikes N30°–40°E. A layer of pseudotachylyte (less than ∼10 cm thick) with some injection veins occurs along the main fault. The pseudotachylyte contains rounded to sub-rounded clasts, commonly displaying fuzzy or embayed boundaries, as well as euhedral feldspar microlites (<10 μm) and flow structures similar to those typically observed in tectonic pseudotachylytes. The tuffisite not only intrudes sharply into the pseudotachylyte and dacitic wall rocks but also displays a mingling structure with the pseudotachylyte. The contact between the rhyolitic intrusion and the tuffisite is nonplanar and complex. Based on the observed field relationships, we conclude that frictional melting and magmatic intrusion were near-coeval and that intrusion outlasted frictional melting by a short period. Although volcano–tectonic pseudotachylytes have rarely been identified in calderas (e.g., in the Glencoe Caldera, Scotland), our results suggest that frictional melting may be an important fault zone process during caldera collapse. Highlights: Pseudotachylyte, rhyolitic intrusions and tuffisite are identified in a caldera fault zone of the deeply eroded and ancient Jangsan Caldera, SE Korea. The intrusion of pyroclastic and magmatic materials occurred shortly after frictional melting and outlasted by a short period. Frictional melting may be an important fault zone process during caldera collapse.