Ron M. Clowes

CRUSTAL STRUCTURE OF ENDEAVOUR RIDGE SEGMENT, JUAN DE FUCA RIDGE, FROM A DETAILED SEISMIC REFRACTION SURVEY

A detailed seismic refraction survey was carried out over the Endeavour Segment of the Juan de Fuca Ridge, a medium-rate spreading center which lies off western North America, to investigate the creation and evolution of oceanic crust. A bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading. Travel time and amplitude data from 15 in-line air gun/ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two-dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust. Significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure, but they appear to be random rather than distributed symmetrically about the ridge. We attribute them to fracturing which causes porosity changes, hydrothermal circulation which fills voids and fractures with alteration products, and variations in magmatic and/or deformational processes at the spreading center. Layer 2A is found to have low (2.6–2.8 km/s) velocities, to average 0.4 km in thickness with variations up to 0.2 km, and to be bounded at its base by a sharp velocity increase to 4.8 km/s. Along the axial ridge, velocities 0.4–0.6 km/s higher than average are interpreted for layers 2B and 2C, but these values are confined to a 2-km-wide zone centered below the ridge. Velocities along ridge-parallel lines offset 10 km are normal, indicating that maturation to off-ridge structure has occurred within at most 0.3 Ma. Layer 3 velocities decrease by 0.1–0.2 km/s for arrivals traveling along and under the axial ridge, perhaps caused by higher temperatures. However, we find no anomalously low velocities beneath the ridge, indicating that no large crustal magma chamber exists. On the basis of this study, we conclude that magmatic accretion is a fully three-dimensional process within ridge segments such as Endeavour Ridge.

The big picture: A lithospheric cross section of the North American continent

A lithospheric cross section constructed within a 6000-kmlong corridor across southern Canada and its margins at 45– 55°N illuminates the assembly of the North American continent at an unprecedented scale. Based on coordinated, multidisciplinary research, the profile emphasizes lithospheric-scale relationships between orogens—plate collisions and accretions have sequentially stacked orogen upon orogen such that the older crust forms basement to the next younger. This largescale perspective highlights the similarities among crustal structures produced by orogenic processes despite the broad range of age from the Mesoarchean to the present. Heterogeneities in the lithospheric mantle suggest that, in certain situations, relict subducted or delaminated lithosphere can remain intact beneath, and eventually within, cratonic lithospheric mantle. In contrast, the dominantly subhorizontal Moho appears to be reequilibrated through mechanical and/or thermal processes; few crustal roots beneath orogens are preserved. INTRODUCTION A unique cross section of the North American continent represents a synthesis of more than two decades of coordinated research conducted by Lithoprobe, Canada’s national geoscience project. Based on existing interpretations within eight study regions, or transects, that are linked directly or by projection along strike, we have constructed a transcontinental lithospheric profile (Fig. 1 and poster insert). From west to east, this 6000-km profile crosses the Juan de Fuca oceanic plate, the active Cascadia subduction zone, the southern Cordillera (0.19 Ga–present), the Alberta and Trans-Hudson orogens (1.92–1.8 Ga), the Superior Province (3.82–2.60 Ga), the Mid-Continent Rift System (1.1–1.0 Ga), the Grenville orogen (1.19–0.99 Ga), the Newfoundland Appalachian orogen (0.47–0.28 Ga), the Grand Banks continental shelf, and the Atlantic passive margin (0.2 Ga). The diversity of tectonic history and ages included in the section facilitates direct comparison of the secular and spatial variation of orogenic processes. Data and interpretations are based on coordinated multidisciplinary research combined with a strong, steadily improving base of regional geotectonic knowledge. The structures displayed are primarily based on active-source seismic (reflection and refraction) data. However, the regional geometry and interpretations of the structure and tectonic processes utilize the full array of geological, geochemical, and geophysical data available for that region. Appendix 1 (see GSA’s supplemental data repository) summarizes how the cross section was constructed. A complete listing of references used to construct the cross section is provided in Appendix 2 (see footnote 2). In addition, Hammer et al. (2010) provide an in-depth description and two complementary lithospheric cross sections. The cross section is portrayed in terms of the “tectonic age” within the crust. We define this as the time since the most recent episode of significant tectonic deformation (Fig. 1 and insert [see footnote 1]). Tectonic age was chosen over more typical designations (e.g., geology or terranes/domains) because it simplifies the interpreted cross section to highlight comparative structures and to convey the sequence of orogenic development based on the current structural interpretations. In some areas, we chose to modify the tectonic age designations in order to convey key aspects of structure as well as the sequence of orogenic development based on current structural interpretations. For example, the Archean Sask, Hearne, and Superior continents were welded together in the Paleoproterozoic Trans-Hudson Orogen (1.92–1.80 Ga), yielding the core of the Laurentian craton. The largely unexposed Sask craton, discovered by Lithoprobe seismic studies (e.g., Lucas et al., 1993; Lewry et al., 1994; Hajnal et al., 2005), lies almost entirely beneath juvenile crustal imbricate structures. Although the Sask craton dates to 2.45–3.3 Ga, the lithospheric fragment was likely deformed by the Paleoproterozoic orogeny. However, to clarify its role in the assembly of Laurentia, we have chosen to label it with an Archean tectonic age but stippled to indicate Paleoproterozoic modification. Similar display procedures have been applied in other parts of the lithospheric cross section.

Deep, high-amplitude reflections from a major shear zone above the subducting Juan de Fuca plate

Seismic reflection data show that a regionally extensive band of landward-dipping reflections exists above the subducting Juan de Fuca plate at the western Canadian continental margin. The reflections truncate at depth a major terrane boundary mapped near the surface, and must have a structural origin. Many reflections are very strong, having reflection coefficients locally as high as 10%-20%.These amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts, if the pores are very thin; however, seismic data from the accreted sedimentary melange farther seaward, from which much of the deep reflective zone is derived, suggest that significant porosity contrasts do not occur alone. Because higher reflection amplitudes are found in regions of inferred high strain, both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks, possibly originating close to the subduction decollement.

Deep crustal seismic reflections at near-vertical incidence

Seismic reflections from discontinuities deep within the crust (reflection times of 8 to 16 sec) have been recorded along four different lines over a widespread area in southern Alberta, resulting in a total of 90 km of near‐vertical‐incidence profiling. Systems of geophone and hole patterns were designed to form an effective filter against long period surface waves. The data were recorded on FM analog magnetic tape and the results were digitized in order to apply digital processing techniques. Power‐spectra calculations indicate that the energy of the reflected wavelets is concentrated in the range 5 to 15 Hz. Synthetic seismograms were made for comparison with field recordings and they suggest that velocity transition zones within the deep crust are less than one kilometer in vertical extent. Along one profile an expanding spread was utilized and a strong reflection at 11.6 sec was continuously correlated over nearly 25 km. A least‐squares analysis of the X2, T2 plot gives an average vertical velocity o...

LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada

Abstract LITHOPROBE, Canadas national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the North American continent, is a multidisciplinary program spearheaded by seismic reflection studies. Five recently recorded seismic lines, discussed in this paper, are located in Precambrian regions: the Mesoproterozoic Grenville Province, the Paleoproterozoic Trans-Hudson orogen (THO), the Paleoproterozoic/Archean basement of Alberta, and the Archean Superior Province. Data acquired across the Grenville orogen in eastern Quebec show strong reflectivity throughout the crust; upper crustal reflections can be correlated with exposed structural elements, including extensional shear zones and packages of deformed high-pressure rocks (eclogites). In a marine survey across the Grenville orogen off southeastern Labrador, seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly. Data acquired across central Alberta show crustal-scale thrust stacking and imbrication of the Archean Hearne craton. To the east across the Trans-Hudson orogen, images of similar collisional features are observed. Geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1.8 Ga, suggesting that collisional tectonic activity was coeval over a broad crustal region, ca. 1000 km across strike. In the Superior Province, seismic data across a collision zone involving the northern Abitibi greenstone belt and the arc-related Opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s (∼30 km) into the mantle. The latter feature is interpreted as representing a relict 2.69-Ga-old suture associated with subduction, providing the first direct evidence that plate tectonics was active in the late Archean. These five examples, supported by other LITHOPROBE results, refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies, combined with other geoscience studies, can lead to a better understanding of Precambrian tectonics. Reflectivity persists throughout the crust; there is no general separation into a poorly reflective upper crust and a reflective lower crust. Crustal reflectivity in Archean and Proterozoic regions is as pervasive as that in areas of more recent tectonism. The Precambrian reflection Moho is generally well defined but shows a range of characteristics. Relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity.

Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada☆

Abstract In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite.

Crustal structure of NW British Columbia and SE Alaska from seismic wide‐angle studies: Coast Plutonic Complex to Stikinia

Crustal structure beneath the transition from the Coast belt to the Intermontane superterrane of the northern Canadian Cordillera is interpreted from the inversion of refraction and wide-angle reflection seismic data. The profile traverses an accretionary suture zone (Coast Plutonic Complex) to continental crust deformed by the transpressive collision (Stikine terrane). Using data acquired by the Accrete onshore/offshore experiment and by a partially overlapping Lithoprobe onshore experiment, P wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure. The model exhibits a well-defined transition throughout the crust that distinguishes the Coast Plutonic Complex (CPC) from Stikinia. Average crustal velocities beneath the CPC (6.45 km/s) are considerably faster than those beneath Stikinia (6.25 km/s). Crustal thickness also changes across the transition; thin crust beneath the Coast belt (30–32 km) thickens beneath Stikinia (35–37 km). The observations within the Coast belt are consistent with a tectonic history most recently dominated by extensional deformation. Primary structural control could be associated with either Neogene extension and/or the processes that are responsible for exhuming the Coast belt during the early Paleogene and have been inferred from geological studies. Slow mantle velocities (7.8–7.9 km/s) beneath the entire profile are indicative of high upper mantle temperatures. Comparison with the southern Cordilleran Coast belt reveals similar velocity structure within the massive plutonic complexes. However, substantial differences between the northern and southern Coast belts emphasize along-strike variations in terranes, orogen geometry and postorogenic tectonics.

Seismic and potential-field imaging of the Guichon Creek batholith, British Columbia, Canada, to delineate structures hosting porphyry copper deposits

The Guichon Creek batholith (GCB), located in south‐central British Columbia, contains several large, low‐grade copper deposits of considerable economic importance. The surface geology of the Guichon batholith and its surrounding region have been well mapped; however, little information about subsurface features is available. The batholith consists of four major phases, emplaced radially outward, which can be separated on the basis of their texture and composition. Previous interpretation of gravity data suggests a mushroom‐shaped structure for the batholith. Data from Lithoprobe seismic reflection line 88-11, acquired across the batholith in 1988, reveal weakly coherent east‐dipping reflections on the west side and west‐dipping reflections on the east in the upper 10 km. To determine if these are related to structures associated with the batholith, we reprocessed the upper 6 s with particular emphasis on applications of signal enhancement techniques (e.g., pattern recognition methods, refraction statics,...

Evidence for extensive tabular intrusions in the Precambrian shield of western Canada: A 160-km-long sequence of bright reflections

The Trans-Hudson orogen is the preserved Early Proterozoic collisional zone between the Archean Superior and Hearne-Rae cratons and the focus of Lithoprobes Trans-Hudson orogen transect where seismic reflection line S2b was recorded. The most intriguing structure in the data is a continuous band of strong reflections in the upper crust, herein named the Wollaston Lake reflector, which extends for 160 km across the northwestern hinterland of the Trans-Hudson orogen. The reflections range from 2.0 to 4.5 s two-way traveltime (6.0–13.5 km depth) and show a heterogeneous internal structure with multicyclic reflected arrivals and numerous diffractions. Our studies indicate that a series of high-velocity-high-density reflectors, having lengths of 800 m to a few kilometres and thicknesses from 50 to 150 m, most likely form the reflecting elements. Geologic mapping and dating document diabase intrusions associated with the Middle Proterozoic Mackenzie igneous event. Sills drilled in the region have thicknesses coinciding with those of our seismic model. We conclude that the Wollaston Lake reflector represents a series of diabase sheetlike intrusions emplaced during Mackenzie (i.e., post-Hudsonian) magmatic activity. Regionally, it provides constraints for the Middle Proterozoic paleo–stress field and for the late-stage tectonic history of the northwestern hinterland of the Trans-Hudson orogen. More generally, the detection of the reflector emphasizes the importance of large-scale horizontal injection of tabular bodies into the crust, over long distances and across tectonic boundaries.

Lithospheric structure in southern Sweden—results from FENNOLORA

Abstract The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Smaland-Varmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0–150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a transition zone of limited lateral extent, less than a few tens of kilometers. We suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the Smaland-Varmland Granite Belt and the Svecofennides.