TOREVA BLOCKS IN THE TRANSANTARCTIC MOUNTAINS: IMPLICATIONS FOR CENOZOIC PALEOCLIMATE

Toreva blocks are landslide deposits consisting of stratigraphically coherent blocks of sedimentary rock up to several hundred meters in strike length and scores of meters in stratigraphic thickness. Toreva blocks commonly are rotated backward on slip surfaces and dip toward the outcrops from which they originated at angles as much as 70°. Movement is attributed to slip on water-lubricated surfaces. In the central Transantarctic Mountains (CTM) there are more than a dozen examples of such features.

Some occurrences in CTM are close to their sources and could be relatively recent. Most are separated by some distance. These are in geomorphological positions incompatible with formation under present-day topography. Toreva blocks occur on shoulders of ridges (Mt. Falla), detached from their bedrock sources by as much as 12 km (Mt. Mackellar), or in one case form the summit of a mountain (Barnes Peak). For example, at Buttress Peak a coherent block of Jurassic Kirkpatrick Basalt, ca. 245 m thick and dipping about 40° SE, overlies horizontal Triassic Falla Fm. basal sandstones and on its north flank abuts a 300 m high cliff of in situ Falla strata; it lies 6 km north of the nearest in situ Kirkpatrick Basalt. Continuity of Jurassic sills in underlying strata precludes juxtaposing Falla strata and Kirkpatrick basalt by crustal tectonism. By the time of emplacement, erosion must have removed 1,000+ m of strata, and have formed the mesa topography implied by the contact relations. Present polar climate precludes recent movement along slip surfaces lubricated by water.

Assuming this mode of transport, emplacement must have occurred either during a warm interval such as that advocated for the Pliocene Sirius Group, or prior to onset of polar ice sheet glaciation in the Miocene. If the former, then unusually high rates of cliff retreat in a polar climate (ca. 2 km/m.y. for the last 3 Ma) are required to produce the geographic separation of the Buttress Peak block from the nearest basalts. If the latter, which seems more probable, then emplacement was pre-mid Miocene and possibly as old as late Eocene, following the onset of the principal uplift of the Transantarctic Mountains at 50-55 Ma. The toreva blocks record an early phase in the geomorphological evolution of the mountains and were emplaced under temperate climate conditions.