Active and recent faulting along the main north—south road in Tibet is dominated by normal faulting occurring on northerly-trending planes and by strike-slip faulting, both of which reflect an east-west extension of the plateau. Normal faulting is prevalent in the southern half of the plateau, but we saw no evidence for any major graben in the northern half. Strike-slip faulting on roughly easterly-trending structures is m ore prevalent in the northern half, but conjugate faulting, with right-lateral slip on northwesterly-trending planes and left-lateral slip on northeasterly-trending planes, is common in the southern half. In two areas, we also observed components of thrust faulting, apparently in association with young strikeslip faulting. Our most important results are bounds on the rates of slip on the two main strands of the Kunlun strike-slip fault system, which trends east-w est through the Kunlun range. Ground moraine containing boulders of pyroxenite is separated by 30 km from the nearest outcrop of such rock, implying that amount of displacement in the last 1.5 to 3 M a. Therefore the average rate of slip during the Quaternary period has been between 10 and 20 mm/a , with a likely value of 13 mm/a . Abundant fresh tension cracks and mole tracks imply continued slip on the main strand, the Xidatan -Tuosuohu-Maqu fault, and the likely occurrence of a major earthquake in the last few hundred years. Consistent offsets of gullies and dry stream channels of about 10 m may reflect slip of that amount during such an earthquake, and possible multiple offsets at one site suggest that slip may occur by large displacements of 10 m during infrequent great earthquakes. Along the other strand, the Kunlun Pass fault, offsets of roughly 50 to 150 m of, apparently, post-glacial valleys and of one glacier and its terminal moraine suggest a Holocene rate of slip between 5 and 20 mm/a , and most likely about 10 mm/a , on this fault. These rapid rates of displacement imply that Tibet is being extruded rapidly eastward, at a rate com parable to the rate at which India is penetrating into Eurasia, and therefore that, at present, a substantial fraction of this penetration is being absorbed by the eastward extrusion of Tibet.
Southern Baie Verte Peninsula (Newfoundland and Labrador, NTS 12-H/9) is underlain by the Mesoproterozoic East Pond Metamorphic Suite, Neoproterozoic Birchy Complex, and Neoproterozoic to Ordovician Fleur de Lys Supergroup, which make up the Humber continental margin. East of the Baie Verte Line, the continental margin units are tectonically overlain by Cambrian rocks of the Advocate Complex (Baie Verte Oceanic Tract), and ophiolite cover of the Ordovician Snooks Arm Group and Black Brook group. These are stitched by a continental overlap assemblage including Ordovician-Silurian Burlington plutonic suite, and Silurian Micmac Lake Group, King's Point volcanic complex, and west of the Baie Verte Line, Wild Cove Pond Igneous Suite and late tectonic, Trap Pond granite. Four phases of regional deformation have affected this area including D1 best documented in the Birchy Complex and related to ophiolite obduction; D2 regional, penetrative deformation associated with isoclinal folds and shear zones and accompanied by greenschist- to amphibolite-facies metamorphism; D3 related to asymmetric and chevron folds near the Baie Verte Line; and D4 related to extensional and dextral faults and reactivation of faults.
Localized intensive ductile and ductile-brittle extension of along-the-arc orientation is prominent in Phyllite-Quartzite unit (PQU) rocks in Kythera and the adjacent southeastern Peloponnese but declines in prominence to the north and to the south along the Cretan-Peloponnese ridge. Using zircon and apatite fission track dating we find that this structural characteristic is correlated with the youngest zircon fission track cooling ages of 14 to 9 Ma, and with the highest pressure-temperature condition recorded in the metamorphic rocks below the structural detachment. The cooling ages of the high pressure Phyllite-Quartzite unit (PQU) rocks along the Hellenic forearc ridge show that exhumation migrated from both Crete and from the Peloponnese to the area with the youngest ages, in Kythera and the southeastern Peloponnese. Starting in the Early Miocene, and continuing to the present, trench-rollback and slab retreat expanded the Hellenic arc, and this bending of the arc from an initial more rectilinear geometry increased areas of oblique convergence and we suggest this localized the subsequent arc-parallel extension and local enhanced exhumation of HP-rocks. The Zircon FT exhumation ages from the arc-parallel stretching episode restrict the ductile part of this episode in exposed rocks to between about 14 and 9 Ma. Apatite FT ages suggest brittle along-arc extension continued to about 7 Ma. This episode is proposed to be the result of a temporary higher rate of rollback and slab retreat. Younger normal faults, including those which define the margins of the present Hellenic Arc, show return to arc-normal extension.
This paper reports detailed stratigraphic analysis, whole rock geochemistry and Nd isotopic ratios of basalts from the northern Maine inliers. These data place constraints on first-order controversies about the tectonic conditions leading up to and during the early stages of the Acadian Orogeny in northern Maine. Late Silurian and early Devonian stratigraphic sequences indicate a progressive change from shallow water and subaerial exposure (Ripogenus and The Forks formations, East Branch Group) to a rapidly subsiding basin prior to the onset of Acadian deformation. Subsidence was accompanied by mafic to intermediate volcanism of the West Branch, Spider Lake, and Fish Pond volcanics, and that continued with intrusion of the syn-deformational Greenville Plutonic Belt, including the Flagstaff Lake Igneous Complex, whose properties we report in this paper. Trace element geochemistry indicates that magmatism is transdiscriminant, showing aspects of within-plate, volcanic arc, and back-arc affinities and Nd isotopic ratios are moderately positive (+2.3 to +3.8) indicating either an uncontaminated and moderately enriched mantle source, or a depleted mantle source contaminated by continental crust. Four possible tectonic models of the Acadian Orogeny and the Siluro-Devonian sedimentary-volcanic sequences of northern Maine are evaluated in the context of a subsiding basin associated with this magmatism developed on the amalgamated Laurentian plate. These include 1) slab detachment during southeast-directed subduction of the Laurentian continental margin; 2) "Laramide-style" thrust basins above a shallow, northwest-dipping subduction zone; 3) back-arc extension followed by thin-skinned shortening above a northwest-dipping subduction zone; and 4) "Moluccan-style" dual-dipping subduction zones.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)