Structure, emplacement mechanism and magma-flow significance of igneous fingers ? Implications for sill emplacement in sedimentary basins

RABBEL, OLE; EIKEN, TROND; GALLAND, OLIVIER; MAIR, KAREN; SCHIUMA, MARIO; SPACAPAN, JUAN B.; GONZALEZ SOTO, FEDERICO; LEANZA HÉCTOR A.

JOURNAL OF STRUCTURAL GEOLOGY

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2019 vol. 124 p. 120 - 135

0191-8141

Field and seismic observations show that numerous sills exhibit lobate morphologies. Each lobe corresponds toa distinct igneous segment exhibiting a finger-like shape, the long axis of which is commonly interpreted as amagma-flow indicator. Robust understanding of the emplacement mechanisms of finger-shaped sills, and directobservations supporting finger orientation as magma-flow indicator are lacking. In this paper, we present theresults of detailed structural mapping on an exceptional, easily accessible 1-km long outcrop in the NeuquénBasin, Argentina, that exhibits a sill, its contacts and the structures in the finely layered sedimentary host rock.We show that the sill is made of distinct segments that grew, inflated or coalesced. We also demonstrate that thefingers were emplaced according to the viscoelastic fingering or viscous indenter models, with no field evidenceof tensile elastic fracture mechanism as commonly assumed in mechanical models of sill emplacement. We identified new structural criteria at the intrusion´s contacts for inferring magma flow direction during the magmaemplacement. Our small-scale structural observations carried out on a seismic-scale outcrop have the potentialto considerably aid the structural interpretation of seismic data imaging igneous sills, i.e. to fill the standard gapbetween outcrop-scale field observations and seismic-scale geophysical data.