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Understanding thermogenic degassing in large igneous provinces; inferences from the geological and statistical characteristics of breccia pipes in the western parts of the Karoo Basin

Henrik H. Svensen, Oyvind Hammer, Luc Chevallier, Dougal A. Jerram, Petter Silkoset, Stephane Polteau and Sverre Planke
Understanding thermogenic degassing in large igneous provinces; inferences from the geological and statistical characteristics of breccia pipes in the western parts of the Karoo Basin (in Mass extinctions, volcanism, and impacts; new developments, Thierry Adatte (editor), David P. G. Bond (editor) and Gerta Keller (editor))
Special Paper - Geological Society of America (December 2019) 544

Abstract

Fluid release structures resulting from the interaction of igneous intrusions with sedimentary basins form an important part of the evolution of large igneous provinces. Hydrothermal breccia pipes formed in the Karoo Basin in South Africa during emplacement of igneous sills in the Karoo large igneous province represent one of the best-exposed expressions of such venting structures. Earlier work has shown that degassing of thermogenic CO (sub 2) and CH (sub 4) through the breccia pipes may have contributed to the Early Jurassic environmental changes. Here, we present the first detailed analysis of the distribution of breccia pipes in the western parts of the Karoo Basin. We mapped 431 pipes in a 650 km (super 2) area using outcrop data. The pipes are rooted in contact aureoles around four sills emplaced in organic-rich Ecca Group shale, and thermal modeling of sill cooling and contact metamorphism gives a maximum temperature of 675 degrees C near the sill contacts, sufficient to convert a significant fraction of the organic carbon to gas. Model estimates indicate that metamorphism in the 650 km (super 2) area generated 75-88 Gt of CO (sub 2) , depending on actual sill thicknesses and emplacement levels. When further up-scaled, an area of 7400-8700 km (super 2) (i.e., less than 2% of the area in the Karoo Basin intruded by sills) would be required to generate 1000 Gt of CO (sub 2) . In order to characterize the degassing pipes, their geographical positions and diameters were analyzed using several point-pattern methods. The results showed that the pipes (1) have diameters in the 11-177 m range (average 44 m), (2) are spaced with an average nearest-neighbor distance of 452 m, and (3) are overall randomly spaced but with weak overdispersion at very small scales (<50 m) and weak clusters at larger scales (400-3000 m). In contrast to studies of volcanic pipe spacing, this study on breccia pipes does not indicate that the pipe spacing is controlled by any large-scale geophysical parameters such as crustal or basin thicknesses. Conclusions point to the pipes being formed following sill emplacement and pressure increase in the low-permeability organic-rich shale, followed by rapid carbon degassing, emphasizing their important role in the Early Jurassic climate change and oceanic anoxic event.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 544
Title: Understanding thermogenic degassing in large igneous provinces; inferences from the geological and statistical characteristics of breccia pipes in the western parts of the Karoo Basin
Title: Mass extinctions, volcanism, and impacts; new developments
Author(s): Svensen, Henrik H.Hammer, OyvindChevallier, LucJerram, Dougal A.Silkoset, PetterPolteau, StephanePlanke, Sverre
Author(s): Adatte, Thierryeditor
Author(s): Bond, David P. G.editor
Author(s): Keller, Gertaeditor
Affiliation: University of Oslo, Centre for Earth Evolution and Dynamics, Oslo, Norway
Affiliation: University of Lausanne, Institute of Earth Sciences, Geopolis, Lausanne, Switzerland
Published: 20191231
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 62
Accession Number: 2020-009245
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, strat. cols., geol. sketch maps
S31°00'00" - S30°30'00", E19°15'00" - E19°45'00"
Secondary Affiliation: Council for Geosciences, ZAF, South AfricaOslo Innovation Center, NOR, Norway
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute.
Update Code: 202007
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