Kattenhorn, S.A. (2005)


Ferrar Dolerite Sill Emplacement Styles, Dry Valleys, Antarctica

Eos, Transactions of the American Geophysical Union, 86.

The Ferrar Dolerite of the Dry Valleys region of Antarctica, 100 km WNW of McMurdo Station, provides an unparalleled opportunity to examine both the physical and chemical characteristics of a magmatic plumbing system across a crustal depth range of several km. The mid-Jurassic Ferrar system consists of 4 distinct sill levels (100 to >300 m thick) and an extrusive component: the Kirkpatrick Basalt. The upper three sills (Mt. Fleming, Asgard, and Peneplain sills) intruded the Devonian-Triassic sedimentary section (the Beacon Sandstone). The lowermost sill (Basement sill) intruded Ordovician granite at a depth of ~3-4 km and is >330 m in thickness in places. The Basement sill is unique in this system in that: (1) It intruded into igneous rocks, resulting in sill geometries that are distinct from overlying sills that intruded along bedding planes; (2) It contains a laterally restricted, internally layered core of predominantly orthopyroxenite (the Opx Tongue); (3) Sill boundaries show evidence of initial brittle emplacement with subsequent thermal reintegration of sill walls in places, particularly adjacent to the Opx Tongue.

The Basement sill intruded the granite from several to >100 m below the contact with the overlying sediments, defined by a Devonian erosional peneplain (the Kukri surface). Abutting relationships suggest that the Basement sill event postdated the overlying Peneplain sill. Emplacement features of the sills were examined along the walls of several valleys to characterize the styles of intrusion. At the W end of Wright Valley, the upper sills exhibit numerous vertical steps and jogs indicative of where adjacent sill segments intruded bedding planes at slightly different stratigraphic levels, then linked by fracturing through bridges of intervening host rock which became entrained within the sills. In contrast, a 7-km-long exposure of the Basement sill along the N wall of E Wright Valley maintains a fairly planar geometry. However, numerous vertical steps along the sill margins imply that intrusion here involved at least 10 initially separate sill segments that ultimately coalesced. Bent and rotated bridges at segment linkage points imply that nascent brittle fractures were intruded by magma, inducing an elastic host rock response during inflation. Adjacent segments linked together when the strength of intervening bridges was overcome. Theoretically, segmentation is most evident in planes orthogonal to the flow direction. The 3D geometry of segments and linkage features was somewhat discernable due to the slope of the valley walls and an ~65° angular range of cross-section views through the sill provided by topography. Analysis of such features imply magma transport through the sill segments along a N to NNE-trending axis. In Wright and Victoria valleys, the Basement sill transitions abruptly at the lateral tips into upward-feeding dikes. Such dike locations are consistent with theoretical predictions for a laccolith-like intrusion in an elastic body.

The Opx Tongue only occurs in the thicker parts of the Basement sill, representing about 25% of the sill thickness. At least one lateral discontinuity in the tongue indicates that the linked sill segments at that location had already cooled sufficiently by the time of tongue emplacement that the liquid centers were disconnected; however, the tongue is continuous across other segment linkage points. The tongue thus appears to be a late-stage feature that was restricted to active magma channels within the partially cooled sill system. The tongue localizes between the center and upper margin of the sill, suggesting more rapid cooling through the base, perhaps due to the presence of an overlying heat source from the already present Peneplain sill.