MESOSCOPIC STRUCTURAL OBSERVATIONS OF LARGE SILL EMPLACEMENT MECHANISMS: THE FERRAR LARGE IGNEOUS PROVINCE, SOUTH VICTORIA LAND, ANTARCTICA

The Jurassic Ferrar Large Igneous Province extends for over 3000 km along the Transantarctic Mountains. Its intrusive component is the Ferrar dolerites, a sheet swarm dominated by large sills (up to 1000 m thick) with subordinate dikes. The propagation and flow directions of these intrusions are of great interest because the number and location of source regions for the FLIP is not known, and long-distance lateral transport of magma in sills is mechanically unlikely.

As part of an integrated structural, petrofabric and AMS study, we classified and measured structures on dike and sill contact surfaces at eight different locations in the Dry Valleys region of South Victoria Land. Most data were collected on a 50 km east-west transect along the Taylor Glacier and Windy Gully, from the Basement Sill (intruding granite), through higher sills intruding subhorizontal sandstones and shales of the Beacon Supergroup. Data were also collected on a 100-km transect extending north to Elkhorn Ridge.

Six different types of structure were measured: (i) meter-scale steps and bridges; (ii) horns or bayonets; (iii) blocks (cusps) and scoops (buds) corresponding to the presence or absence of blocks of country rock; (iv) cm- to dm-scale fingers and grooves, which may be used to determine flow direction directly; (v) mm-scale lineations (“wrinkles”) which can be found on individual fingers and grooves, and (vi) cm-scale quartzite stringers, present in a few cases at the margins of large sills.

Interpreting these features in terms of flow direction can be challenging, for example fingers and grooves frequently occur in multiple orientations that can only be meaningfully interpreted where a large contact surface is exposed, and mm-scale lineations may be orthogonal to adjacent cm-scale fingers. Observations of horns, rotated sandstone blocks and detached xenoliths indicate that growth of these large sills was partially achieved by piecemeal block removal, effectively a lateral stoping mechanism.

Dikes can rotate to near-horizontal over a short distance, and show evidence for oblique flow, usually with an upward vertical component. At least one large sill was apparently fed by small dikes, indicating that dikes must have been relatively long-lived conduits that transported much greater volumes of magma than their present size would suggest.