AMS Constraints on Initial Magma Flow in Sills: An Icelandic Analog For Crustal Accretion At Mid-Ocean Ridges

Sill intrusion is an important process of crustal growth within extensional tectonic regimes such as mid-ocean ridges. However, details of accretion and magma flow kinematics within intrusive centers remain poorly constrained due to lack of exposure along much of the ridge system. Deeply eroded Tertiary exposures of intrusive centers in Iceland offer details into the structural geometry and kinematics of magma flow beneath central volcanoes analogous to those in other rift systems. Structural analysis and anisotropy of magnetic susceptibility (AMS) data of coarse- to fine-grained mafic sills related to an intrusive center in northwest-central Iceland reveal aspects of crustal accretion that are more complex than commonly assumed in sill/cone sheet assemblages. Sills are distributed in an arc-shaped fashion around, and dipping towards, a large gabbro body near the town of Skagiströnd. Consistent cross-cutting relationships demonstrate that massive gabbroic sills, many >10 m thick, intrude screens of altered, vesicular basaltic breccia thought to be remnants of previous lava flows or cone sheets. These more massive gabbroic sills are commonly cut by a series of much thinner sills that range from approximately 0.25 m to 4 m. Thicker intrusions coarsen to gabbro in their interior, often within 0.5 to 0.75 m of a chilled margin, while sills <0.75 m thick are typically diabase texture. In some areas late basaltic diabase dikes cut all sills. The sills consistently have shallow to moderate dips projecting westward generally below a large gabbro body that is partially exposed near Skagiströnd. Their orientations approximately define half of an inverted conical geometry documented in many other intrusive centers in both Iceland and other central volcanoes in rift zones. Preliminary paleomagnetic data suggest that these sills have not been significantly tilted. Surface indicators of magma flow present on some top glassy chilled margins typically have steep pitches suggesting steep initial flow. All flow directions inferred from AMS data determined by the imbrication of Kmax directions relative to the margin are thought to reflect initial intrusion direction. Although most of the flow directions are steep and either up- or down-dip, few Kmax directions have shallow to moderate inclinations suggesting the presence of initial subhorizontal flow within some shallow-dipping sills. These data provide insight into the complex magma flow during crustal growth by injection of discrete gabbroic and basaltic sills below a central volcano. Evidence from this study emphasizes that magmatic construction beneath central volcanoes is likely to be more complex than generally assumed. These processes have analogs in other spreading centers with high magma supply.