Kristian E. Meisling

Segmentation of an obliquely rifted margin, Campos and Santos basins, southeastern Brazil

We make the case for Early Cretaceous transfer zones that segment the obliquely rifted Atlantic margin of southeastern Brazil. Our interpretation is based on published literature, Bouguer-corrected gravity, regional reflection seismic profiles, and well data. In the Santos and Campos basins, Neocomian rift architecture was strongly influenced by preexisting fabric and structures of the Late Proterozoic (Brasiliano orogeny). The Atlantic margin inherited an east-northeast-west-southwest orientation so that rifting was oblique to the margin. On a regional map of Bouguer-corrected gravity, a nearshore belt of positive anomalies correlates with an interpreted broad Moho uplift in the footwall of Neocomian extensional faults. Farther offshore, a second belt of positive anomalies correlates with a presalt ridge of eroded volcanic or basement anticlines covered by thin Aptian evaporites, interpreted as a failed spreading center. Intervening negative anomalies coincide with the main rift basin. All three belts show apparent offsets along linear zones trending west-northwest-east-southeast, which we interpret as transfer zones. The vergence of half rifts tends to change across transfer zones, compartmentalizing the rifted margin into subbasins. Our results have implications for the risks associated with distribution, maturation, and migration of hydrocarbons within the prolific Early Cretaceous lacustrine petroleum system of the Campos and Santos basins.

Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California

The late Cenozoic structural and stratigraphic history of the northwestern San Bernardino Mountains supports two distinct episodes of uplift, in late Miocene to earliest Pliocene and Quaternary time, that we hypothesize are related to movements on low-angle structures beneath the range. In this paper, we document the nature, distribution, and timing of late Cenozoic deformation and deposition in the northwestern San Bernardino Mountains, and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams. In the first episode of deformation, late Miocene to earliest Pliocene motion on the south-southwest-directed Squaw Peak thrust system disrupted drainage in pre-existing Miocene nonmarine basins and uplifted the western third of the present range to form the ancestral San Bernardino Mountains. Crystalline rocks of the San Bernardino Mountains were thrust southward across the present site of the San Andreas fault between 9.5 and 4.1 Ma, at a time when the San Gabriel fault was the active strand of the San Andreas transform system. We speculate that the Liebre Mountain crystalline block at the northern margin of the Ridge Basin may be the missing upper plate of the Squaw Peak thrust, now offset along the San Andreas fault. The second episode of deformation began with uplift of the northern plateau of the modern San Bernardino Mountains on north-directed, range-front thrusts in early Pleistocene time, between 2.0 and 1.5 Ma. Synchronous uplift of the northern plateau, recorded in early Pleistocene fanglomerates on the northwestern margin of the range, is interpreted to be the result of movement of a relatively coherent crustal block northward up a south-dipping detachment ramp beneath the central range. In middle Pleistocene time, activity on the northern range front began to wane, and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the San Andreas fault, which subsequently migrated rapidly northwestward along the San Andreas fault from the western San Bernardino Mountains into the northeastern San Gabriel Mountains. We attribute this pattern of deformation to the passage of a bulge or strike-slip ramp attached to the southwest side of the San Andreas fault at depth.

Forced folding and basement-detached normal faulting in the Haltenbanken area, offshore Norway

Triassic evaporites strongly influenced the structural development of the Haltenbanken area of offshore Norway during Late Jurassic and Early Cretaceous time by mechanically decoupling Triassic and younger strata from older strata and basement. Many folds in the Haltenbanken area are forced folds above basement-involved normal faults. Seismic data show that they are asymmetric flexures affecting Triassic, Jurassic, and Lower Cretaceous strata above the evaporites. They commonly are cut by, or die out along strike into, basement-involved normal faults. Extensional forced folds formed, at least in part, because Triassic evaporites behaved in a ductile manner, decoupling overlying strata from underlying faulted strata and basement. Many normal faults in the Haltenbanken rea are basement-detached faults that flatten within the Triassic evaporites. Seismic data show that rollover anticlines and secondary normal faults affect Triassic, Jurassic, and Lower Cretaceous strata within the hanging walls of these basement-detached normal faults. Strata beneath the Triassic evaporites are unaffected by this deformation.

Closing the Canada Basin: Detrital zircon geochronology relationships between the North Slope of Arctic Alaska and the Franklinian mobile belt of Arctic Canada

Constraining the pre-opening paleogeography of the Canadian and Alaskan margins of the Canada Basin is a first-order objective in resolving the plate tectonic evolution of the Amerasia Basin of the Arctic Ocean. The most widely accepted model for opening of the Canada Basin involves counterclockwise rotation of Arctic Alaska away from Arctic Canada about a pole of rotation in the Mackenzie Delta region, although numerous other kinematic models have been proposed. The rotation model is tested using detrital zircon U-Pb geochronology of 12 samples from Middle Mississippian to Early-Middle Jurassic strata (Ellesmerian and lower Beaufortian megasequences) obtained from wells and outcrop along Alaska’s North Slope. These northerly-derived strata were deposited in fluvial to nearshore marine environments along the south-facing (present-day) shelf margin of the Arctic Alaska Basin and contain 360–390 Ma, 415–470 Ma, 500–750 Ma, 0.9–2.1 Ga, and 2.4–3.2 Ga zircon populations. Detrital zircon age populations in Ellesmerian and lower Beaufortian strata are remarkably similar to detrital zircon populations from Devonian foreland clastic wedge strata in the Canadian Arctic Islands and northern Yukon Territory. A paleogeographic setting in which Arctic Alaska received sediments recycled from the Devonian foreland clastic wedge and underlying Franklinian Basin strata is most consistent with the model of [Embry (1990)][1] in which northern Alaska lay within the foreland fold and thrust belt of the Franklinian mobile belt prior to the opening of the Canada Basin. The sequences that are inferred to have been the long-lived source region for Ellesmerian and lower Beaufortian strata were uplifted by Paleozoic (predominantly Late Devonian) deformation that has been documented along the Canadian and Alaskan margins. Triassic and Jurassic strata deposited along the Arctic Canada, Arctic Alaska, and northern Yukon shelves have detrital zircon ages that are significantly older than the youngest detrital zircon ages (Mesozoic) in coeval strata that were deposited west of Hanna Trough and north of the Sverdrup Basin axis, supporting continuity of these bathymetric features prior to opening of the Canada Basin. [1]: #ref-32

First bedrock samples dredged from submarine outcrops in the Chukchi Borderland, Arctic Ocean

The Chukchi Borderland, a prominent bathymetric feature within the Arctic Ocean, has been interpreted as a fragment of an undeformed continental platform sequence rifted from the passive margin of Arctic Canada. Dredges collected for the U.S. Extended Continental Shelf project aboard the icebreaker U.S. Coast Guard Cutter Healy (cruise number HLY0905) recovered hundreds of kilograms of broken crystalline basement lithologies consisting of mylonitically deformed biotite-bearing amphibolite, garnet-bearing feldspathic gneiss, and augen-bearing orthogneiss from the Chukchi Border land. Metamorphic zircon within the amphibolite and associated leucogranitic seams within these rocks yielded U-Pb zircon ages between ca. 480 and 530 Ma. Garnet-bearing feldspathic gneisses contain variably discordant Mesoproterozoic zircon, ca. 600 Ma igneous zircon, and ca. 485–505 Ma metamorphic overgrowths. While we interpret these gneisses as deformed and metamorphosed granitoids, they could, instead, have a very immature sedimentary protolith. The youngest rocks sampled were K-feldspar augen orthogneisses that yield ca. 430 Ma zircon crystallization ages. Whole-rock geochemistry and Sr-Nd isotopic data indicate that the orthogneisses are I-type calc-alkaline granitoids. All of the basement rocks including the orthogneisses are variably metamorphosed and mylonitized. Collectively, the U-Pb age, geochemistry, and fabric of the dredged Chukchi Borderland basement samples indicate that they represent Neoproterozoic–Ordovician orogenic crust and Silurian arc batholithic rocks. This geologic origin is inconsistent with the Neoproterozoic to early Paleozoic passive margin history of western Arctic Canada to which the Chukchi Borderland has been previously correlated. We alternatively propose that the basement of the Chukchi Borderland is related to the peri-Laurentian composite terranes of Pearya and western Svalbard that have similar geologic histories.

Dredge samples from the Chukchi Borderland: Implications for paleogeographic reconstruction and tectonic evolution of the Amerasia Basin of the Arctic

The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. We present new lithologic descriptions, along with U-Pb, 40Ar/39Ar, and apatite fission track analyses, for rocks dredged from the Chukchi Borderland. These new results provide constraints on the Chukchi Borderlands crustal architecture, Paleozoic evolution, and pre-Cretaceous paleogeographic position prior to the formation of the Amerasia Basin. Based on the location and nature of dredged rocks, the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Early Paleozoic units recovered from Northwind Ridge in the eastern Chukchi Borderland consist of low-grade metasedimentary rocks with a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, high-grade gneisses and amphibolites dredged from the Chukchi Plateau in the central Chukchi Borderland yield U-Pb and 40Ar/39Ar ages that suggest an affinity with rock units of the Pearya terrane of Arctic Canada. U-Pb and 40Ar/39Ar ages reveal a Paleozoic tectonic history beginning in Early Cambrian to Early Ordovician time with subduction/arc-related high-T metamorphism. Intrusive relationships indicate that Late Ordovician to Silurian orthogneisses intruded the high-T units within an arc setting. Devonian exhumation of these high-grade units is recorded by U-Pb dating of sphene and 40Ar/39Ar thermochronology of micas and potassium feldspar. The exhumation of the high-grade rocks was coeval with the deposition of coarse-grained sediments bearing detrital zircon compositions similar to Silurian foreland basin sediments deposited in Pearya, as well as with greenschist facies metamorphism of metasedimentary rocks dredged from Northwind Ridge. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. This pre-rift position supports and refines the widely cited rotational models for opening of the Canada Basin.

Columbus basin, offshore Trinidad: A detached pull-apart basin in a transpressional foreland setting

The Columbus basin formed as a Miocene foreland basin to a transpressional fold-thrust belt along the Caribbean–South American plate boundary. The foreland basin was built atop a Cretaceous–Early Tertiary passive margin. During the Plio-Pleistocene, the basin evolved into a thin-skinned pull-apart basin that captured shallow-water sedimentary deposits of the prograding Orinoco River delta. Plio-Pleistocene structures within the basin include large-magnitude extensional faults, contractional folds, and strike-slip fault systems, all of which are regionally detached near the top of the underlying north-dipping passive-margin succession. This kinematically linked system transfers dextral displacement from the Caribbean–South American transcurrent plate boundary southeastward across the Columbus basin into the Caribbean–Atlantic subduction complex. The presence of a prolific Cretaceous source rock, a sand-rich Plio-Pleistocene shelf section, and a structural evolution that has created abundant traps and an efficient petroleum migration system has made this an economically important petroleum-producing basin.

Deformational history and thermochronology of Wrangel Island, East Siberian Shelf and coastal Chukotka, Arctic Russia

Abstract In Arctic Russia, south of Wrangel Island, Jura–Cretaceous fold belt structures are cut by c. 108–100 Ma plutonic rocks and a c. 103 Ma migmatitic complex (U–Pb, zircon) that cooled by c. 96 Ma (40Ar/39Ar biotite); the structures are unconformably overlain by c. 88 Ma and younger (U–Pb, zircon) volcanic rocks. Wrangel Island, with a similar stratigraphy and added exposure of Neoproterozoic basement rocks, was thought to represent the westwards continuation of the Jura–Cretaceous Brookian thrust belt of Alaska. A penetrative, high-strain, S-dipping foliation formed during north–south stretching in Triassic and older rocks, with stretched pebble aspect ratios of c. 2:1:0.5 to 10:1:0.1. Deformation was at greenschist facies (chlorite+white mica; biotite at depth; temperature c. 300–450°C). Microstructures suggest deformation mostly by pure shear and north–south stretching; the quartz textures and lattice preferred orientations suggest temperatures of c. 300–450°C. 40Ar/39Ar K-feldspar spectra (n=1) and muscovite (n=3) (total gas ages c. 611–514 Ma) in Neoproterozoic basement rocks are consistent with a short thermal pulse during deformation at 105–100 Ma. Apatite fission track ages (n=7) indicate cooling to near-surface conditions at c. 95 Ma. The shared thermal histories of Wrangel Island and Chukotka suggest that Wrangel deformation is related to post-shortening, north–south extension, not to fold–thrust belt deformation. Seismic data (line AR-5) indicate a sharp Moho and strong sub-horizontal reflectivity in the lower and middle crust beneath the region. Wrangel Island probably represents a crustal-scale extensional boudin between the North Chukchi and Longa basins.