Grenvillian Terrane Research Articles

Time constraints on Early Tonian Rifting and Cryogenian Arc terrane-continent convergence along the northern margin of the West African craton: Insights from SHRIMP and LA-ICP-MS zircon geochronology in the Pan-African Anti-Atlas belt (Morocco)

The Neoproterozoic Anti-Atlas belt (Morocco) is a key segment in tracing the history of the northern margin of the West African craton (WAC) from Rodinia breakup to Gondwana assembly. In order to constrain geodynamic events related to rifting and convergence and their stratigraphic records, a radiometric study was carried out on the volcano-sedimentary Tachdamt and Bleïda Formations. The volcaniclastic deposits of Tachdamt Fm. yield an age of ca. 883 Ma and provide a new constraint for the timing of the subaqueous volcanic eruption and the initiation of rifting in the Anti-Atlas. This age is ~100 my older than the previous age of 788 ± 10 Ma obtained from Rb/Sr isotope dating (Clauer, 1976). Available zircon data from the interbedded clastic deposits from Tachdamt Fm. indicate major shifts in provenance during the rifting with sediments sourced from the WAC and a nearby Grenvillian terrane. Detrital zircon ages from metasiltstones of Bleïda Fm. set a maximum depositional age at ca. 700 Ma. The age spectra discloses a major shift in sediment input with provenance from the WAC, Grenvillian terrane and an arc-related terrane (ca. 770–700 Ma) lying along the Anti-Atlas margin. The newly obtained results combined with existing radiometric data allowed the refinement of the stratigraphic and geotectonic framework of the pre-Pan-African strata in the WAC. The Early Tonian syn-rift volcanism has much in common with similar events in other cratons and provides a stratigraphic record for incipient Rodinia breakup. The overlying Bleïda Fm. deposited in a peripheral foreland basin chronicles the Cryogenian arrival of the ca. 770–700 Ma Bou Azzer-Siroua arc-related terrane at the Anti-Atlas margin, and correlates with initial accretion in West Gondwana. The new results indicate that the Grenvillian detrital input in the Anti-Atlas during both geodynamic stages points toward a possible Mesoproterozoic terrane existing along the western margin of the WAC.

Variable modification of continental lithosphere during the Proterozoic Grenville orogeny: Evidence from teleseismic P-wave tomography

Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for ≥300 Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from ∼1.5−0.25 Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere that underlies these Proterozoic regions is typically identified by elevated seismic velocities but lithospheric modification by fluid/melt-derived metasomatic enrichment above a subduction zone, can lead to a reduction in VP with little effect on VS and density. Absolute P-wavespeed constraints are therefore a vital complement to existing S-wave tomographic models of North America to investigate craton edge modification mechanisms in the Grenville orogen.New P-wave tomographic imaging of the North American continent, which benefits from recent developments in arrival-time processing of regional network deployments from the Canadian shield, reveals along strike wavespeed variation in the Grenville orogen. In the north, high seismic wavespeeds (to depths of 250 km) extend eastwards, from the Archean core of North America to beneath the Canadian Grenville Province. In contrast, below the southern U.S., high lithospheric wavespeeds are restricted to west of the Grenville Province, in particular at depths less than 150 km. We argue that subduction-derived metasomatism beneath eastern Laurentia modified the southern Grenville, prior to thermal stabilization and perhaps mantle keel formation. Beneath the northern Grenville, the thick, depleted Laurentian lithosphere resisted extensive metasomatism. Along strike age differences in Grenvillian terranes and their resulting metasomatic modification histories suggests that at least 250 Myr is required for Proterozoic lithosphere to gain resistance to modification.

3D seismic datasets applied to mineral exploration: revisiting three Canadian case studies

The benefits of working with legacy seismic data are broad and valued in a wide range of geoscience applications and result from the comprehensive volume of industry and public seismic data acquired over several decades. Legacy seismic data exist in many parts of the world including areas that, due to stricter regulatory processes, environmental restrictions, demographic changes, or other factors would be challenging or even impossible to survey today. Examples are numerous but one includes 2D marine seismic data acquired in the 1980s with a large airgun array to provide a deep cross-section through the Appalachians beneath the now sen-sitive Gulf of St Lawrence, eastern Canada (Marillier et al., 1989). Sometimes benefits arise through the use of newly developed tech-niques to extract additional information from legacy seismic data. This idea is obviously not new and many examples can be found in the literature. One example is the use of extended Vibroseis cor-relation developed in 1980s (Okaya and Jarchow, 1989) applied to increase the depth range of marine Vibroseis seismic data acquired in 1971 in two of the Great Lakes of North America (Milkereit et al., 1992). Marine seismic data in the Great Lakes are sparse, but contain information about deep Grenvillian terranes that, in this case, could only be revealed using a technology developed more than a decade after data acquisition. Reflection seismology has been used for mineral exploration for more than 30 years (Reed, 1993). During that period, many case studies demonstrated the utility of the method for imaging deep lithological contacts and structures indirectly associated with mineralization, and for the direct imaging of deposits. Malehmir et al. (2012) provide a comprehensive overview of key accomplishments for various commodities and mining locations in Canada and around the world. In particular, many examples demonstrate the advantages of the 3D seismic method for providing high-resolution images of subsurface structures and deep-seated ore deposits in a variety of geological environ-ments. Several of these legacy 3D seismic data sets have been reprocessed with advanced and novel techniques to provide more reliable and detailed images of the subsurface. Here, we show three early examples of the application of 3D seismic data for mineral exploration in the Bathurst, Sudbury, and Val d’Or mining camps, Canada (Figure 1). Specifically, we demonstrate that reprocessing of 3D datasets from earlier applications of seismic reflection can refine the imaging of known ore deposits and can lead to the identification of new exploration targets. For the case studies presented here, most improvements resulted from processing and imaging strategies built from an understanding of the response of mineralized bodies on seismic data and improving specific steps that prevented their proper imaging on final volumes. The case studies are introduced according to their year of acquisition, starting from the oldest (i.e., the Trill 3D survey in the Sudbury mining camp) to the most recent (e.g., the Louvicourt 3D survey in the Val d’Or mining camp). All 3D seismic surveys are located in areas with limited access and difficult terrain and as a result, were acquired with explosives. Surface conditions varied for each survey area but included steep hills, extensive swamps, and thick glacial sediments. Geology also varied at the three sites. Host rocks are either nearly-transparent-to-seismic waves or are highly heterogeneous and generating seismic scattering potentially masking the response of the mineralized body. In all three areas, however, mineralized bodies scattered or reflected energy in a preferred direction, and were observed only on a limited number of prestack seismic traces. In some cases, such a sparse signature was buried in noise when stacking all traces and required particu-lar attention to be properly imaged.

Accretion of Grenvillian terranes to the southwestern border of the Río de la Plata craton, western Argentina

A comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Rio de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic–ultramafic rocks of 1,244–1,027 Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846–570 Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640–580 Ma) and Cambro-Ordovician (~550–470 Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480–450 Ma), late phase (440–420 Ma), and Chanic phase (400–360 Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt.

A Laurentian provenance for the Dalradian rocks of north Mayo, Ireland, and evidence for an original basement–cover contact with the underlying Annagh Gneiss Complex

Abstract: Metasediments of the early Dalradian Grampian Group (Erris Group) and probable equivalents (Inishkea Division) structurally overlie Palaeoproterozoic to Neoproterozoic Annagh Gneiss Complex orthogneisses in NW Mayo, Ireland. Sm–Nd isotopic data suggest a Palaeoproterozoic source for the metasediments. K-feldspar and granitoid clasts from the Doonamo Formation, Erris Group, record U–Pb zircon ages of c . 1740 Ma and c . 980 Ma, respectively. These ages are within error of the c . 1730–1750 Ma Mullet gneisses and c . 990 Ma Grenvillian migmatitic leucosomes in the underlying Annagh Gneiss Complex. U–Pb detrital zircon data reveal that the Erris Group was deposited after c . 955 Ma, with predominant input from c . 1640, c . 1500 and c . 990 Ma interpreted Laurentian sources (Labradorian, Pinwarian and Grenvillian terranes, respectively). Limited detrital zircon data from the Inishkea Division yield similarly aged detritus and a tentative maximum depositional age of c . 1005 Ma. Correlation of both sequences with the Scottish Dalradian Grampian Group is considered valid based on detrital zircon U–Pb and whole-rock Sm–Nd data. The clast ages support the hypothesis that the Dalradian unconformably overlies the Annagh Gneiss Complex in north Mayo, whereas the detrital zircon data imply more distal Laurentian sources. Dalradian deposition is thereby tied to the margins of Laurentia. Supplementary material: U–Pb zircon (secondary ion mass spectrometry) data and descriptions of dated zircon grains are available at http://www.geolsoc.org.uk/SUP18414 .

Proterozoic southward accretion and Grenvillian orogenesis in the interior Grenville Province in eastern Labrador: Evidence from U–Pb geochronological investigations

This study is a fascinating example, from the eastern Grenville Province, of unraveling the triple challenges inherent in the geochronological investigation of any multi-orogenic region, namely inheritance, emplacement and subsequent metamorphism. The existence of hitherto unknown pre-Labradorian crust within allochthonous Grenvillian terranes is demonstrated; formerly unrecognized spatial relations between Labradorian and Pinwarian crust are outlined; and a new perception of mid-Grenvillian magmatic and metamorphic events is presented. The pre-Labradorian crust has an age range of 1800–1770 Ma and consists of calc-alkaline plutonic rocks intruding earlier supracrustal assemblages containing Archean and early Paleoproterozoic inherited zircons. Evidence is provided from dated enclaves and inheritance in younger rocks that similar-aged pre-Labradorian crust once existed farther south. The geochronologically defined geographic extent of early Labradorian crust having a 1680–1655 Ma age range is expanded. This crust is intruded by the mid-Labradorian Mealy Mountains Intrusive Suite, for which new results confirm its 1650–1630 Ma time of emplacement. Pinwarian orthogneiss, dated between 1513 and 1496 Ma and previously unknown in the region, is now recognized to be common. Sufficient data exist to define, for the first time, a boundary between Labradorian and Pinwarian orthogneiss (cryptic in the field), establishing the presence of Pinwarian-aged crust to the southwest, lacking Labradorian inheritance except close to the boundary. A belt of layered mafic, anorthositic and monzogranitic intrusions is situated close to the boundary. Current data favour a Pinwarian age for emplacement of these rocks. Mid-Grenvillian plutonism is demonstrated by 1043 Ma ages from two K-feldspar megacrystic granodiorite bodies. They are situated on either flank of a crustal-scale, northeast-trending, northeast-closing fold. The fold, which is 40 km wide and at least 160-km long and exposes Pinwarian orthogneiss in its core, post-dates emplacement of the 1043 Ma intrusions. The fold probably formed at the peak of Grenvillian tectonism in the region, between 1030 and 1015 Ma, and certainly before intrusion of weakly deformed granitoid rocks at 992 Ma. Final activity included the emplacement of late- to post-Grenvillian granitoid plutons between 964 and 951 Ma. A pluton having a 951 Ma age is the youngest known in the eastern Grenville Province. Cooling and stabilization was completed by ca. 940 Ma. Broader implications of the results, in conjunction with existing data, are that: (i) the northeasternmost part of the eastern Grenville Province was only peripherally affected by Grenvillian orogenesis, (ii) an east–northeast offshore extrapolation of the Grenville front as a major tectonic feature does not exist, (iii) high-grade metamorphic mineral assemblages in the Groswater Bay and Hawke River terranes are pre-Grenvillian, rather than an eastward expression of a Grenvillian high-pressure belt, and (iv) the north shore of the eastern St. Lawrence estuary represents across-orogen exposure, rather than orogen-parallel as previously supposed.

An application of Nd isotope mapping in structural geology: delineating an allochthonous Grenvillian terrane at North Bay, Ontario

Fifty new Nd isotope analyses are presented from the North Bay area of the Grenville Province in Ontario. These data are used to map the extent of an allochthonous Grenvillian terrane which is an outlier of the Allochthonous Polycyclic Belt of the Grenville Province. Amphibolite facies orthogneisses from the allochthonous terrane have depleted mantle Nd model ages (TDM) below 1.8 Ga, whereas the gneisses of the structurally underlying parautochthon almost invariably have model ages above 1.8 Ga. The distribution of model ages is consistent with the distribution of distinct types of metabasic rock, used by other researchers as the criterion for recognizing rocks of the allochthonous and parautochthonous belts of the Grenville Province. The agreement between these different types of evidence demonstrates that Nd isotope mapping is a reliable and powerful tool for mapping terrane boundaries in high-grade metamorphic belts.

Petrology of sapphirine granulite and associated sodic gneisses from the Indian Head Range, Newfoundland

Migmatitic granulites from the Indian Head Range (IHR) are dominated by granoblastic, Opx-bearing (quartz) dioritic gneiss with subordinate garnet+orthopyroxene+biotite+albite (±quartz±microcline±cordierite±sillimanite) gneiss and comparatively biotite-rich, sapphirine+cordierite+orthopyroxene+albite (±microcline±sillimanite±corundum) gneiss. The latter contains at least two generations of sapphirine and cordierite, one apparently predating migmatization, the other associated with or following this event. Mineral thermobarometers yield temperatures up to ∼800 °C at 8 kbar. The IHR granulites have very high δ 18O values (+10.8 to +14.5) which most likely indicate a sedimentary precursor. Their REE and HFSE contents resemble the trace element signature of post-Archean Australian shale (PAAS). Lithological analogues from other Grenvillian inliers in western Newfoundland have similar oxygen isotopic and immobile-element signatures. They too are interpreted as metasedimentary rocks. The IHR rocks, however, have unusually sodic compositions (e.g., Na 2O/CaO and Na 2O/K 2O=2.1–22.5 and 0.93–13.8, respectively) and contain albite or sodic oligoclase despite their high metamorphic grade. The geochemical data indicate that these rocks were albitized prior to high-grade metamorphism. No counterpart of this event is recorded in granulites from the other inliers, indicating that the IHR may be highly allochthonous with respect to these other Grenvillian terranes.

Evidence for Paleoproterozoic, Grenvillian, and Pan-African age Gondwanan crust beneath northeastern Mexico

A number of previous studies including two recent reconstructions of Rodinia have considered the Grenvillian basement of Mexico to be continuous with the Laurentian basement of Texas. Isotopic studies and U–Pb dating of zircons from Precambrian granitoids that occur as cobbles and boulders in a Paleozoic conglomerate exposed in the state of Coahuila, northeastern Mexico, indicate the region is underlain by Gondwanan rather than Laurentian basement. Essentially nothing was known of the basement before this study because the region is blanketed with Phanerozoic sedimentary and volcanic deposits. Nine out of ten analyzed samples have Grenvillian crystallization ages (1232±7 to 1214±2 Ma). This phase of magmatic activity in northeastern Mexico was coeval with the construction of volcanic arcs in east-central and southern Mexico (∼1220–1150 Ma), but was later than the arc magmatism of Laurentia (e.g., Elzevirian arcs, >1230 Ma, in Texas and the Grenville Province). A subset of the Grenvillian samples contains xenocrystic ∼1.85 Ga zircons, which are the first evidence of Paleoproterozoic crust south of the Paleozoic Ouachita suture in Mexico. The remaining sample has a Pan-African crystallization age (580±4 Ma), and this is the first sample of Pan-African age basement found in Mexico. The Pan-African orogeny marked the assembly of Gondwana, and Laurentia was unaffected by that event. Two lines of evidence argue that some of the Grenvillian rocks of northeastern Mexico were affected by a Pan-African age event. First, two of the Grenvillian clasts have U–Pb lower intercept ages within error of 580 Ma. Second common Pb and Nd isotopic compositions of the Pan-African granite strongly suggest that it was derived either by melting of Grenvillian basement or by assimilating massive amounts of that basement. Thus far, no other Grenvillian terrane is known to have the exact combination of Nd–Pb isotopic compositions and absolute ages as the Coahuila samples although there are some similarities with the Santa Marta massif of Colombia and the Cordillera de Merida of Venezuela.

Evolution of the Western Margin of Australia during the Rodinian and Gondwanan Supercontinent Cycles

The proto-Darling Fault zone and its successor, the Darling Fault, extend for 1, 000 km along the western continental margin of Australia and appear to have been active at several periods during the geological past. Deformation commenced at ∼2,570 Ma and affected Late Archaean granitoids along the western margin of the Yilgarn Craton. Much of the later activity reflects events related to the accretion and breakup associated with the Rodinia and Gondwanaland supercontinent cycles. In the north, rocks of the Northampton and Mullingarra Complexes form part of a high-grade Grenvillian orogenic belt lying to the west of the Darling Fault, referred to as the Pinjarra Orogen. They underwent granulite facies metamorphism ∼1080 Ma ago and form part of the global collisional event that resulted in the amalgamation of Rodinia. These rocks extend southward beneath Phanerozoic sedimentary cover (the Perth Basin), where they are constrained to the east by the Darling Fault and to the west by the Dunsborough Fault, the latter marking the eastern boundary of the Leeuwin Complex. The Leeuwin Complex is a fragment of Pan-African crust that has traditionally been considered part of the Pinjarra Orogen. It is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from A-type, anorogenic granitoids. Conventional and SHRIMP U-Pb zircon geochronology has established that the granitoids evolved between ∼780 Ma and ∼520 Ma and were metamorphosed at ∼615 Ma. These events are equated with rifting associated with the breakup of Rodinia. Sm-Nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre-existing Archaean continental material. During the Phanerozoic, the Dunsborough and Darling Faults were reactivated, as normal faults defining the inner arm of a major rift system within Eastern Gondwanaland and controlling sedimentation in the Perth Basin that now overlies the Grenvillian terrane. Major normal movement on the Darling Fault ceased by the Late Jurassic and it appears that continental breakup in the Early Cretaceous occurred along fractures closely related to the western boundary of the Leeuwin Complex that defined the eastern margin of the outer arm of the rift system. Breakup between Australia and Greater India commenced at ∼132 Ma and was followed by eruption of the Bunbury Basalt at 130 Ma and 123 Ma. This possibly resulted from hot spot activity beneath Eastern Gondwanaland and may have been a reflection of the Kerguelen plume, though the evidence is equivocal. It is argued from the petrographic, geochemical and isotopic characteristics, together with the likely contiguity of the Eastern Gondwanaland continents since the assembly of Rodinia, that the Leeuwin Complex evolved within an intracrustal rift and is not an exotic terrane. It is distinct from adjacent portions of the Pinjarra Orogen and should be considered a separate terrane. It is recommended that use of the term ‘Pinjarra Orogen’ be confined to rocks recording the Grenvillian events, thereby excluding those rocks (the Leeuwin Complex) that evolved during the later Pan-African orogeny.

Crustal‐scale shortening and extension across the Grenville Province of western Québec

A deep seismic reflection survey shot in 1993 crosses almost the full width of the Grenville Province in western Québec. The seismic transect provides a very clear image of the crust‐mantle boundary and the most precise definition to date of the various Grenvillian terranes. The crust is around 44 km thick beneath the Grenville Front but thins rapidly to 36 km some 60 km to the southeast; it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect, far from the inferred location of the main Grenvillian collision. The Grenville Front zone, in which NW directed thrusting at about 1 Ga was followed by SE directed extension, is defined by discontinuous, SE dipping reflections, which extend down to the Moho. To the southeast, the overlying migmatitic Archean parautochthon, almost half of the transect, is characterised instead by NW dipping reflectors extending into the lower crust. These reflectors are in turn truncated to the southeast by a 12‐km‐thick zone of intense SE dipping reflections, the Baskatong crustal ramp. The base of the allochthonous terranes (Allochthon Boundary Thrust) is likely located at this ramp, which flattens out at around 30‐km depth into the base of the relatively transparent intermediate crust. A highly reflective upper crustal deck corresponding to rocks of the Mont‐Laurier terrane was thrust over the Baskatong crustal ramp and is represented further to the northwest by the klippelike Cabonga allochthon. The synformal, transparent Morin anorthosite‐charnockite complex belongs to the same upper crustal level. Ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the NW directed propagation of tectonic transport during the Grenvillian orogeny, involving deformation and displacement along the Baskatong ramp with a relay into the Grenville Front zone. Postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the Grenville Front and affecting the crust up to 350 km away from the front.

The significance of various garnet types in surficial materials in southeastern Gaspé Peninsula, Quebec

The distribution patterns of four types of garnet in surficial materials of southeastern Gaspé Peninsula are interpreted in terms of the glacial history of the area, the lithostratigraphy of the bedrock, and mineral occurrences. Each type is derived from a distinct source, and all have undergone at least some southeastward glacial dispersion. Type 1 garnets, anhedral Ca-garnet fragments, are from the McGerrigle granite metamorphic aureole and were brought into the region by the same ice flow that caused the well-documented southward dispersal of granite debris. Their distribution within the study area depicts the southern part of this important dispersal train with an improved degree of precision. Type 2 garnets, Ca-garnet crystals, seem to be related to skarns within the boundaries of the study area; some patterns cannot be linked to any known source, thus providing challenging exploration opportunities. Type 3 garnets, colored pyralspite garnet crystals, are related to a tectonic breccia within the Maquereau–Mictaw contact zone. Type 4 garnets, Grenville-type anhedral garnets, were not brought into the region by Laurentide ice, as one might suspect, but rather seem to be related to specific horizons within the Silurian Lower Chaleurs Group. On the basis of a palinspastic reconstruction of southern Gaspé Peninsula and a plate tectonic model for the Paleozoic of eastern North America, we propose that these garnets were derived from the erosion of a metamorphic Grenvillian terrane, which presently forms the Northwestern Highlands Zone of Cape Breton Island. The distribution data for this garnet type also lends support to the concept put forth in a previous study whereby a local Late Wisconsinan ice mass penetrated some 5–6 km into the region from the southeast near Saint-Godefroi.

Modeling wide‐angle seismic data for crustal structure: Southeastern Grenville Province

A modeling methodology for obtaining two‐dimensional (2‐D) crustal structure from wide‐angle seismic data is applied to data from the southeastern Grenville Province. Pre‐modeling steps include (1) assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal‐to‐noise ratio, (2) using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity, and (3) identifying data unsuitable for 2‐D modeling. The goal of the travel time inversion‐amplitude modeling approach is to obtain a minimum‐structure and minimum‐parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide‐angle data. Each step of a layer‐stripping procedure involves a series of inversions in which a one‐dimensional or simple starting model is improved with additional velocity and/or interface nodes until a satisfactory trade‐off between travel time fit, parameter resolution and complete ray coverage of all source‐receiver pairs is achieved. Using zero vertical‐velocity gradient layers and head waves during preliminary first‐arrival inversion can (1) decrease the number of intermediate models, (2) allow greater lateral heterogeneity to be imaged, and (3) simplify incorporation of amplitude modeling constraints into the final model. Using amplitude‐distance curves allows quantitative modeling of the relative amplitude and offset variations of phases. Discrepancies between observed and calculated reflection amplitudes are used to infer fundamental, non step‐like velocity changes at layer boundaries. Later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that “float” within the model without an associated velocity structure. These reflectors provide a spatial image like that obtained from vertical‐incidence reflection data, as opposed to a velocity image. The model of Grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data. A crustal‐scale zone of wide‐angle reflectors with an average easterly apparent dip of 13° defines a major Grenvillian terrane boundary.