Seismicity and fluid geochemistry at Lassen Volcanic National Park, California: Evidence for two circulation cells in the hydrothermal system

• Published in: Journal of volcanology and geothermal research Vol. 189; no. 3; pp. 257 - 277
• Main Authors: Janik, Cathy J., McLaren, Marcia K.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 15-01-2010; Elsevier
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Freshwater; geothermal fluids; hydrothermal system model; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Lassen Peak; Lassen Volcanic Center; seismic clusters; seismicity; stable and noble gas isotopes; water and gas geochemistry; Shasta County California; Lassen Peak; Lassen Volcanic National Park; California; United States; USA, California, Lassen Volcanic Natl. Park; USA, California; Lassen Peak; geothermal fluids; water and gas geochemistry; seismic clusters; stable and noble gas isotopes; Lassen Volcanic Center; seismicity; hydrothermal system model; extension; plumes; volcanic centers; reservoirs; shallow depth; earthquakes; North America; faults; depressions; meteoric water; hot springs; lakes; flow; focal mechanism; stress; inverse problem; shear; magmas; high temperature; boiling; geysers; interpretation; circulation; calderas; valleys
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2009.11.014

Seismic analysis and geochemical interpretations provide evidence that two separate hydrothermal cells circulate within the greater Lassen hydrothermal system. One cell originates south to SW of Lassen Peak and within the Brokeoff Volcano depression where it forms a reservoir of hot fluid (235–270
°C) that boils to feed steam to the high-temperature fumarolic areas, and has a plume of degassed reservoir liquid that flows southward to emerge at Growler and Morgan Hot Springs. The second cell originates SSE to SE of Lassen Peak and flows southeastward along inferred faults of the Walker Lane belt (WLB) where it forms a reservoir of hot fluid (220–240
°C) that boils beneath Devils Kitchen and Boiling Springs Lake, and has an outflow plume of degassed liquid that boils again beneath Terminal Geyser. Three distinct seismogenic zones (identified as the
West,
Middle, and
East seismic clusters) occur at shallow depths (<
6
km) in Lassen Volcanic National Park, SW to SSE of Lassen Peak and adjacent to areas of high-temperature (≤
161
°C) fumarolic activity (Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell) and an area of cold, weak gas emissions (Cold Boiling Lake). The three zones are located within the inferred Rockland caldera in response to interactions between deeply circulating meteoric water and hot brittle rock that overlies residual magma associated with the Lassen Volcanic Center. Earthquake focal mechanisms and stress inversions indicate primarily N–S oriented normal faulting and E–W extension, with some oblique faulting and right lateral shear in the
East cluster. The different focal mechanisms as well as spatial and temporal earthquake patterns for the
East cluster indicate a greater influence by regional tectonics and inferred faults within the WLB. A fourth, deeper (5–10
km) seismogenic zone (the
Devils Kitchen seismic cluster) occurs SE of the
East cluster and trends NNW from Sifford Mountain toward the Devils Kitchen thermal area where fumarolic temperatures are ≤
123
°C. Lassen fumaroles discharge geothermal gases that indicate mixing between a N
2-rich, arc-type component and gases derived from air-saturated meteoric recharge water. Most gases have relatively weak isotopic indicators of upper mantle or volcanic components, except for gas from Sulphur Works where
δ
13C–CO
2,
δ
34S–H
2S, and
δ
15N–N
2 values indicate a contribution from the mantle and a subducted sediment source in an arc volcanic setting.