Ian O. Norton

Chapter 5 Palaeogeographic and tectonic evolution of the Arctic region during the Palaeozoic

Abstract The Palaeozoic motion of the future Arctic continents is presented in the animation found in the accompanying CD-ROM. The animation shows snapshots of the motion of the tectonic blocks from 550 to 250 Ma in 3 million year steps. The locations of the blocks are controlled mainly by palaeomagnetic pole values for the blocks tied to known geological events, particularly the three main Arctic orogenies: the Scandian Caledonian which began in the Silurian, the Ellesmerian in the Late Devonian and the Uralian that began in the Late Pennsylvanian. Perhaps the most significant observation to come out of the animation is that the future Arctic continents were never very far from one another during the Palaeozoic. The maximum distance from Baltica to Laurentia may have reached 6000 km during the Middle Cambrian but the Arctic continents all surrounded the same eastern Iapetus Ocean and, by Silurian, they were quite close. Reliance on the ‘Y-loop’ palaeomagnetic data causes extremely rapid motion of Gondwana during the Silurian. Consequently the ‘X-path’ for that period is used. The palaeomagnetic poles for 422 and 406 Ma have been eliminated so that Gondwana motion is within the bounds of present day plate motion. Supplementary material: A Quicktime™ movie of palaeogeographic and tectonic evolution of the Arctic region during the Palaeozoic is available at http://www.geolsoc.org.uk/SUP18472.

Variations in rift symmetry: Cautionary examples from the Southern Rift System (Australia–Antarctica)

Abstract We present a synthesis based on the interpretation of two pairs of deep seismic reflection crustal sections within the Southern Rift System (SRS) separating Australia and Antarctica. One pair of sections is from the conjugate margins between the Great Australian Bight (GAB) and Wilkes Land, in the central sector of the SRS, which broke up in the Campanian. The second pair of conjugate sections is located approximately 400 km further east, between the Otway Basin and Terre Adélie, which probably broke up in Maastrichtian time. Interpretations are based on an integrated synthesis of deep multi-channel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information, and the conjugate sections are presented with the oceanic crust removed beyond the continent–ocean boundary (COB). At first order, both conjugate pairs show a transition from thinned continental crust, through a wide and internally complex continent–ocean transition zone (COTZ), which shows features in common with magma-poor rifted margins worldwide, such as basement ridges interpreted as exhumed subcontinental mantle. In the central GAB sector, the COTZ is symmetric around the point of break-up and displays a pair of mantle ridges, one on each margin, outboard of which lies a deep-water rift basin. Break-up has occurred in the centre of this basin in this sector of the SRS. In contrast, the Terre Adélie margin is nearly 600 km wide and shows an abandoned crustal megaboudin, the Adélie Rift Block. This block is underlain by interpreted middle crust, and appears to have a mantle ridge structure inboard, as well as an outboard exhumed mantle complex from which mylonitized harzburgite has been dredged. The conjugate margin of the Beachport Sub-basin is relatively narrow (c. 100 km wide) and does not appear to contain an exhumed mantle ridge, as observed along strike in the GAB. These observations from a single rift spreading compartment show that radically different break-up symmetries and margin architectures can result from an essentially symmetric rifting process involving multiple, paired detachment systems. This indicates the need for caution in interpreting causative mechanisms of rifting from limited conjugate sections in other rifts. We speculate that the underlying crustal composition, rheology and structural preconditioning play a significant role in partitioning strain during the transition to break-up.

Deep crustal structure in the eastern Gulf of Mexico

We use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along Gulf of Mexico Basin Opening Line 4, a profile extending from the northwestern Florida peninsula across the Florida Escarpment to the central Gulf of Mexico. Moderately thinned continental crust with a Moho depth of 32–33 km, average sediment thickness of 6 km, and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore Florida. Thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6–7 km; mean seismic velocities of the crust in this region increase from 6.55 km/s to 6.95 km/s from northeast to southwest and are evidence for increased magmatic input as rifting developed. Oceanic crust with an average thickness of 5.6–5.7 km is observed over a distance of 175 km on the southwest end of the profile, with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile. Anomalously high upper oceanic crust velocities of 6.0–6.7 km/s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement. Integrating well, seismic reflection and our seismic refraction data allow us to estimate a full-spreading rate of 2.2 cm/yr for seafloor spreading along the profile; this indicates that oceanic crust was emplaced at a slow-spreading center.

Deep crustal structure of the northeastern Gulf of Mexico: Implications for rift evolution and seafloor spreading

We image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean-bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends ~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0–4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23–10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn-rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full-spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor-spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

Two-stage formation of Death Valley

Extension in Death Valley is usually interpreted as a combination of low-angle Basin and Range–style extension and strike slip associated with the developing Pacific-North America plate boundary in western North America, with these two tectonic regimes operating synchronously in Death Valley. Examination of structural, stratigraphic, and timing relationships in the region suggests that this interpretation needs revision. Evolution of Death Valley is best described as a two-stage process. In the first stage, lasting from ca. 18 to 5 Ma, low-angle Basin and Range extension transported allochthons consisting of Late Proterozoic through Early Paleozoic miogeoclinal section along detachment surfaces that, as extension continued, were exhumed from mid-lower crustal levels to the surface. Near the end of this extensional phase and lasting until ca. 3 Ma, deposition of a thick sequence of volcanics, clastics, and some lacustrine carbonates signaled a period of relative tectonic quiescence, with sediments in some areas covering the exhumed detachment surfaces. At ca. 3 Ma, initiation of the East California Shear Zone started development of the present-day topographic depression of Death Valley, formed as a pull-apart basin associated with this strike slip. Faulting broke the older, inactive, Basin and Range detachment surfaces, with high-angle transtensional faulting along the Black Mountains front. The Black Mountains were elevated as a result of footwall uplift, with the well-known turtleback structures being megamullions along these bounding faults. These megamullions are similar to those seen at oceanic spreading centers. The Panamint Range has previously been interpreted as an extensional allochthon, with the entire range transported from on top of or east of the Black Mountains. A new interpretation presented here is that the range is a large core complex similar to the core complex at Tucki Mountain, at the northern end of the range. The Basin and Range extensional detachment tracks over the top of the range, with extensional allochthons perched on the eastern flanks of the range. This modified model for evolution of Death Valley suggests a strong link between timing and style of deformation in the basin with the developing Pacific-North America plate boundary, particularly eastward propagation of this boundary.

Continental rifting and sediment infill in the northwestern Gulf of Mexico

The opening of the Gulf of Mexico was an important Mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting. A new seismic velocity profile based on seismic refraction data in the northwestern Gulf of Mexico offshore Texas, where the basin started opening in the Early Jurassic, shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope. The structure of the thinned crust is consistent with large-scale extensional faulting and moderate amounts of synrift magmatism before continental breakup. These new seismic constraints do not indicate the presence of a volcanic margin along the Texas coast, as has sometimes been proposed based on magnetic data. The Laurentian continental lithosphere of central Texas may have been too thick at the onset of rifting (>100 km) to let magmatic diking control the extension. In contrast, the continental lithosphere of the northeastern Gulf of Mexico may have been thinner, such that magma-assisted rifting formed a volcanic margin there later in the Jurassic.

Southern Louisiana salt dome xenoliths: First glimpse of Jurassic (ca. 160 Ma) Gulf of Mexico crust

No direct information about the age and composition of rift-related igneous activity associated with the Late Jurassic opening of the Gulf of Mexico exists because the igneous rocks are deeply buried beneath sediments. Three salt diapirs from southern Louisiana exhume samples of alkalic igneous rocks; these salt domes rise from the base of the sedimentary pile and overlie an isolated magnetic high, which may mark the position of an ancient volcano. Three samples from two domes were studied; they are altered but preserve relict igneous minerals including strongly zoned clinopyroxene (diopside to Ti-augite) and Cr-rich spinel rimmed with titanite. 40 Ar/ 39 Ar ages of 158.6 ± 0.2 Ma and 160.1 ± 0.7 Ma for Ti-rich biotite and kaersutite from two different salt domes are interpreted to represent the time the igneous rock solidifi ed. Trace element compositions are strongly enriched in incompatible trace elements, indicating that the igneous rocks are low-degree melts of metasomatized upper mantle. Isotopic compositions of Nd and Hf indicate derivation from depleted mantle. This information supports the idea that crust beneath southern Louisiana formed as a magma-starved rifted margin on the northern fl ank of the Gulf of Mexico ca. 160 Ma. These results also confi rm that some magnetic highs mark accumulations of mafi c igneous rocks buried beneath thick sediments around the Gulf of Mexico margins.

Rift to drift transition in the South Atlantic salt basins: A new flavor of oceanic crust

The crustal structure of the continent-ocean transition zone in the South Atlantic salt basins is poorly understood. Current interpretations place the limits of oceanic crust at the distal salt limits, with sub-salt crust consisting of rifted continental crust and, in some versions, varying amounts of exhumed mantle. Plate reconstructions that map these limits of oceanic crust onto appropriate-age restorations show poor geometric fits, with unexplained gaps and overlaps. One possible reason for the poor fits is that the distal salt limits are not the real limits of oceanic crust. In this paper we investigate this option by mapping rift basins and seaward-dipping reflectors whose seaward edges mark significant structural boundaries as much as 300 km inboard of the distal salt limits. We interpret these boundaries, which match geometrically in a salt-age (Aptian) plate reconstruction, to be the limits of oceanic crust. We suggest that salt was deposited as seafloor spreading commenced and that, as the South Atlantic opened, salt flowed over the ridge axis, sealing off the extrusive component of oceanic crust, resulting in formation of intrusive oceanic crust. Seafloor spreading eventually broke through the thinning salt, forming breakthrough volcanoes preserved today as basement ramps at the distal salt limits. These ramps formed diachronously, so the distal salt limits are not isochrons, explaining the poor fit of these features in plate reconstructions.

Upper Jurassic Tithonian-centered source mapping in the deepwater northern Gulf of Mexico

AbstractLong the subject of speculation, the origin, distribution, and quality of Mesozoic source beds in the deepwater Gulf of Mexico (GOM) are now open to analytical study and hypothesis. We have developed new maps and concepts for organic richness and lithofacies patterns of the primary Upper Jurassic oil-prone source rock interval spanning the Kimmeridgian to Lower Berriasian in the northern GOM. This interval, previously referred to as the Tithonian-centered source, includes the Haynesville and Bossier shales, which lie within supersequences representing second-order transgressive and high-stand systems tracts, respectively. A newly developed gulf-wide Cotton Valley-Bossier paleogeographic map based on a novel paleotectonic model for the Mesozoic provides the framework for this source mapping study. Organic richness averages up to 4.7% and 6.5% total organic carbon for the Kimmeridgian and Tithonian-Lower Berriasian supersequences, respectively, based on the log overlay Δ log R technique and increase...

Seawater chemistry driven by supercontinent assembly, breakup, and dispersal: COMMENT

[Muller et al. (2013)][1] present an interesting hypothesis that quantifies a relationship between temporal changes in the ratio of Mg and Ca in seawater and global hydrothermal flux through oceanic crust. The relationship is quantified through estimates of the area of oceanic crust <65 m.y. old