Basement rocks of the Transantarctic Mountains are believed to record a change in the paleo‐Pacific margin of Gondwana from a passive to a tectonically active margin. Widespread emplacement of calc‐alkaline batholiths (Granite Harbor intrusives) occurred during the active margin phase. We present new concordant zircon and titanite U‐Pb ages for these magmatic rocks in southern Victoria Land and the Scott Glacier area. Most magmatic rocks previously associated with a pre‐late Early Cambrian (>530 Ma) deformational event(s) (Beardmore orogeny) have yielded younger crystallization ages. The lack of definite arc magmatism prior to ∼530 Ma suggests that deformation may have been associated with a strike or oblique‐slip regime, although shallow subduction without significant arc magmatism cannot be ruled out. Local transpressional and transtensional domains may account for compressional deformation and rare alkaline and carbonatite magmatism during this early period. The oldest and most voluminous magmatic rocks were emplaced after ∼530 Ma. This magmatism has been associated with active subduction, and suggests a fundamental change in the plate boundary at ∼530 Ma. Ductile shearing of plutons and contractional deformation of supracrustal rocks after ∼530 Ma (Ross orogeny) may have been due to transpressional tectonics in an oblique subduction setting and/or a collision. Compressional deformation associated with the Ross orogeny may have ceased by ∼500 Ma along the southern Victoria Land‐Scott Glacier segment of the Antarctic margin, as indicated by undeformed magmatic rocks of this age, although magmatic activity continued to at least ∼485 Ma.

New Rb‐Sr and K‐Ar geochronological data are presented for the majority of known pre‐Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc‐alkaline igneous or metaigneous rocks, and indicate long‐standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle‐derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust‐contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite‐bearing granite with an Early Jurassic age of 198±2 Ma; its initial 87Sr/86Sr ratio of 0.710 and ϵNdt values of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I‐type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial 87Sr/86Sr ratios of 0.705–0.706 and ϵNdt values of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean‐type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial 87Sr/86Sr ratios of 0.706–0.709 and ϵNdt values of −3 to −6), as subduction‐related magmatism ceased in this part of the margin.

New paleomagnetic data presented here further constrain the relative motion of the Antarctic Peninsula relative to East Antarctica during the Mesozoic development of the southern ocean basins. The Antarctic Peninsula (AP) is one of four crustal blocks that define West Antarctica, the others being the Ellsworth‐Whitmore Mountains (EWM), Thurston Island‐Eights Coast (TI), and Marie Byrd Land (MBL). A Jurassic pole (∼155Ma) (124°E, 64°S, A95=7.1°, N=10 VGPs) was obtained from the AP block which suggests that the AP block rotated clockwise between ∼175 and ∼155 Ma due to significant early opening in the Weddell Sea basin. A new Early Cretaceous paleomagnetic pole (182°E, 74S, A95=5.9°, N=6 site mean VGPs) indicates that the AP block was in or near to its present‐day position with respect to East Antarctica by ∼130 Ma. Between ∼155 and 130 Ma, counterclockwise rotation of the AP‐TI blocks, together with the southward motion of East Gondwanaland, probably resulted in subduction of Weddell Sea ocean floor beneath the southern AP block and initiated the Palmer Land deformational event. A ∼130 Ma pole from the TI block requires clockwise rotation of the TI and possibly the EWM blocks between 130 and ∼110 Ma producing sinsistral strike‐slip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI‐EWM blocks and the AP block. New AP block ∼110 Ma and ∼85 Ma poles from this study (199°E, 74°S, A95=6.9°, N=13 VGPs; 152°E, 86°S, A95 = 7.5°, N=6 VGPs, respectively) are similar to equivalent age poles from East Antarctica and suggest little or no relative motion between the Antarctic Peninsula and East Antarctica. Northern and southern Antarctic Peninsula mid‐Cretaceous poles are very much alike suggesting that the “S” shape of the Antarctic Peninsula is not due to oroclinal bending since ∼110 Ma.

Paleomagnetic data from three West Antarctic crustal blocks (Antarctic Peninsula (AP), Thurston Island‐Eights Coast (TI), and the Ellsworth‐Whitmore Mountains (EWM) indicate that there has been motion between the individual blocks and motion relative to East Antarctica during the Mesozoic. A Triassic paleomagnetic pole from the TI block (116°E, 61°S, A95 = 19.4°, N = 3 VGPs) appears to indicate that the block has rotated ∼90° relative to East Antarctica between 230 Ma and 110 Ma. Our previously reported Middle Jurassic paleomagnetic pole from the EWM block indicates that a 90° rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90° counterclockwise EWM rotation occurred between ∼220 Ma and 175 Ma related to the development of post‐Gondwanide Orogeny shear zones. The motion of the AP, TI, and EWM blocks appears to be linked during the mid‐to late Mesozoic to three major events in the evolution of the southern ocean basins. Opening in the Mozambique‐Somali‐Weddell Sea basins may have produced major counterclockwise rotation of the TI block with respect to East Antarctica between the Jurassic and Early Cretaceous based on new Late Jurassic (145°E, 64.5°S, A95 = 7°,N = 5 VGPs) poles. We believe that the TI rotation, as well as deformation in the southern AP block, was caused by collision and shearing of the EWM block against the other two as the EWM block moved southward with East Antarctica. An Early Cretaceous paleomagnetic pole (232°E, 49°S, A95 = 7.9°, N = 5 VGPs) from the TI block requires that between the Early and mid‐Cretaceous there was clockwise rotation, with respect to East Antarctica, of the AP‐TI‐EWM blocks (an entity we call Weddellia). A change in the opening history of the Weddell Sea basin caused by initiation of spreading in the South Atlantic ocean basin at ∼130 Ma probably started Weddellia's clockwise rotation. Two new ∼110 and ∼90 Ma poles from the TI block (210°E, 73°S, A95 = 7.6°,N = 7 VGPs and 161°E, 81°S, A95 = 3.9°,N = 18 VGPs, respectively) are similar to equivalent age poles from the AP block and East Antarctica and indicate that Weddellia was at or near its present‐day position with respect to East Antarctica by ∼110 Ma. This corresponds to a time of major plate reorganization in the South Atlantic and southeast Indian Oceans. Based on both the new TI paleomagnetic data and previously reported data from Marie Byrd Land (MBL), dextral shearing would be expected to have occurred between MBL and Weddellia since the mid‐Cretaceous. Pine Island Bay, the area between the TI and MBL blocks, marks a fundamental and complex tectonic boundary in West Antarctica that we propose has largely been a zone of transcurrent shearing.

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