Younger Tectonic Events Research Articles

Satellite gravity data for mapping lithospheric structure of Precambrian tectonic blocks in Africa: The advantages and limitations

In this study, we use the Precambrian tectonic blocks in southern Africa to examine the utility of satellite gravity data in mapping the lithospheric structure beneath the Precambrian terrains in Africa. For this, we focus on the Archean – Paleoproterozoic Congo – Tanzania – Bangweulu, Zimbabwe – Kaapvaal – Niassa cratons, Paleoproterozoic – Neoproterozoic Trans-Southern Africa orogen, the Neoproterozoic early Cambrian Lufilian arc, and Paleoproterozoic – Neoproterozoic Irumide metacraton. We use the Bouguer gravity anomalies of the World Gravity Model 2012 (WGM 2012). Our goal is to estimate the thickness of the lithosphere (hence the depth to lithosphere – asthenosphere boundary (LAB)) beneath these Precambrian tectonic blocks. We use two approaches including the two-dimensional (2D) radially averaged power spectral analysis (henceforth the spectral analysis), and 2D forward gravity modeling (henceforth the 2D forward gravity model). Using the spectral analysis, we are able, to a large extent, image the Congo – Tanzania – Bangweulu craton, as been underlain by thick lithosphere with the LAB depth of ∼250 km. Differently, using this method, we find no anomalously thick lithosphere beneath the Zimbabwe – Kaapvaal craton, although some of the 2D forward gravity models show relatively thick lithosphere beneath this craton. The only segment of the Trans-Southern Africa orogen we find to have thinner lithosphere (∼150 km) using the spectral analysis is the Mesoproterozoic – Neoproterozoic Damara – Ghanzi – Chobe orogen. We find the “system” limitations in using satellite gravity data for imaging Precambrian lithospheric structure to be the selection of linear segments in the spectral curves, selection of the size of sub-windows during the analysis, and selection of density values during the 2D forward gravity modeling. We find the “environmental” limitations to be the modification of Precambrian lithospheric structure by younger tectonic events.

High‐Resolution Ps Receiver Function Imaging of the Crust and Mantle Lithosphere Beneath Southern New England and Tectonic Implications

AbstractSouthern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana‐derived terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P‐to‐S receiver function analysis on SEISConn data, including both single‐station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well‐defined, ∼15 km step‐like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana‐derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west‐dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere.

Documentation of the Sirjan Orocline in the southeast Sanandaj-Sirjan Zone, Iran

At the southeastern part of the Sanandaj-Sirjan Zone of Iran, a group of structural elements outline a large-scale arc curvature around a vertical axis. This curvature comprises several elongated structural elements and their dividing faults, axialfold traces, layering, and foliation. The most frequent lithological units include Paleozoic metamorphic rocks, Mesozoic-Paleogene sedimentary rocks, and Mesozoic magmatic-ophiolitic complex disposed in several anticlines and synclines, forming a horseshoeshaped structure with a 240-km arc length and a 90-km wavelength. We name this structure the Sirjan Orocline, and characterize this structure here through field observations and satellite image analyses. The Sirjan Orocline formed during the late Eocene-Oligocene related to the most significant deformation event after regional metamorphism. The final form of this structural arc is affected by a younger tectonic event that compressed and transected this structure.

HYDROTERMÁLNÍ MINERALIZACE V HORNINÁCH SOLÁŇSKÉHO SOUVRSTVÍ V ZÁPADNÍ ČÁSTI CHŘIBŮ

Hydrothermal mineralization sporadically occurs in the western part of Chřiby Hills in a form of thin carbonate veins and veinlets hosted by sedimentary rocks of the Lukov Member of the Soláň Formation. These veins were found in old quarries and slope debris nearby Koryčany, Cetechovice, and Roštín. The mineral filling of studied veins is formed by calcite or rarely calcite and barite. Based on the investigation of fluid inclusions, hydrothermal calcite crystallized from low-temperature (Th = 70–187 °C) and on average low-salinity (0.2–5.6 wt. % NaCl equiv.) hydrothermal solutions. Hydrothermal veins filling crack systems with the direction NNW–SSE or W–E in quarries Holý kopec near Koryčany and Roštín-Chapel are probably post-tectonic. Their origin can be connected to young tectonic events after the termination of main phase of the Alpine Orogeny. Diagenetic origin cannot be excluded in case of calcite vein which fills a bed joint between layers of conglomerate in old quarry 3.5 km south–southeast from the town Koryčany. The source fluids can be derived from mixing of seawater with diagenetic waters. In case of post-tectonic veins, a contribution of meteoric waters is also possible. In addition, UV-fluorescence microscopy reveals a sporadic presence of inclusion with higher hydrocarbons which exhibits strong blue-white fluorescence. These inclusions document a migration activity of higher hydrocarbons in the northwestern part of the Rača Unit in Chřiby Hills.

The geological structure of the Muizenberg Block, Cape Peninsula, South Africa

The Muizenberg Block, is an outlier of the Table Mountain Group (Cape Supergroup) and lies to the west of the main occurrence of the Cape Fold Belt. Structural mapping of this outlier has identified a spaced cleavage in Peninsula Formation quartz arenites, and an exposure of dolerite that have not been previously documented. Monoclinal folding, an unrelated spaced cleavage, low-angle faults, and steeply dipping faults, that strike east-northeast – west-southwest, are similar to structures found in the syntaxis domain of the Cape Fold Belt and are therefore thought to have formed during the Late Permian – Early Triassic Cape Orogeny. However, as the Muizenberg Block lies outside the presently accepted boundaries of the syntaxis domain, those boundaries may deserve re-examination. Sub-vertical dilational quartz veining, jointing, cataclastic faulting and the intrusion of a sub-parallel dolerite dyke, that all strike northwest – southeast, are thought to represent a younger tectonic event. They are provisionally attributed to the Upper Jurassic to Cretaceous taphrogenesis that preceded the drifting apart of South America and Africa.

Crustal Architecture of the Inverted Central Lapland Rift Along the HUKKA 2007 Profile

We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km.

The Meso-Neoarchaean Belomorian eclogite province: Tectonic position and geodynamic evolution

The aim of this paper is to review the main features of the Meso-Neoarchaean Belomorian eclogite province (BEP) in the northeastern Fennoscandian Shield, including regional and local geology, geochemistry, petrology and geochronology and to compare the Belomorian eclogites with Precambrian eclogites elsewhere. Two eclogite associations have been recognized within Belomorian TTG gneisses: (1) the subduction-type Salma association and (2) Gridino eclogitized mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising gabbro, Fe–Ti gabbro and troctolites, formed at ~2.9Ga in a slow-spreading ridge setting (like the Southwest Indian Ridge). The main subduction and eclogite-facies events occurred between ~2.87 and ~2.82Ga. Injection of mafic magma into an active continental margin setting, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87Ga. Crustal delamination of the active margin and subsequent involvement of the lower crust in subduction between 2.87 and 2.82Ga ago caused high-pressure metamorphism of the Gridino dykes, culminating in eclogite-facies conditions between 2.82 and 2.78Ga and accompanying amalgamation of the Karelia, Kola and Khetolamba blocks and formation of the Mesoarchaean Belomorian accretionary–collisional orogen. The clockwise P–T paths of the Salma and Gridino associations cross the granulite-facies P–T field. Detailed metamorphic studies indicate a complicated post-eclogite history with thermal events and fluid infiltration, related to plume activity at 2.72–2.70, ~2.4 and ~1.9Ga. The eclogite assemblages were exhumed to mid-to-lower crustal depths at ~1.7Ga, while erosion or younger tectonic events were responsible for final exhumation to the surface. Comparison of P–T–t paths and data for peak metamorphic parameters demonstrates the general similarity of the Archaean and Palaeoproterozoic eclogites worldwide and their association with anomalously “hot” environments. The occurrence of high-T conditions during eclogite-facies metamorphism can be attributed to either subduction of a mid-ocean ridge (Archaean, BEP) or to interaction with mantle plumes (Proterozoic).

Glimpses of East Antarctica: Aeromagnetic and satellite magnetic view from the central Transantarctic Mountains of East Antarctica

Aeromagnetic and satellite magnetic data provide glimpses of the crustal architecture within the Ross Sea sector of the enigmatic, ice‐covered East Antarctic shield critical for understanding both global tectonic and climate history. In the central Transantarctic Mountains (CTAM), exposures of Precambrian basement, coupled with new high‐resolution magnetic data, other recent aeromagnetic transects, and satellite magnetic and seismic tomography data, show that the shield in this region comprises an Archean craton modified both by Proterozoic magmatism and early Paleozoic orogenic basement reactivation. CTAM basement structures linked to the Ross Orogeny are imaged 50–100 km farther west than previously mapped, bounded by inboard upper crustal Proterozoic granites of the Nimrod igneous province. Magnetic contrasts between craton and rift margin sediments define the Neoproterozoic rift margin, likely reactivated during Ross orogenesis and Jurassic extension. Interpretation of satellite magnetic and aeromagnetic patterns suggests that the Neoproterozoic rift margin of East Antarctica is offset by transfer zones to form a stepwise series of salients tracing from the CTAM northward through the western margin of the Wilkes Subglacial Basin to the coast at Terre Adélie. Thinned Precambrian crust inferred to lie east of the rift margin cannot be imaged magnetically because of modification by Neoproterozoic and younger tectonic events.

40Ar– 39Ar dating of volcanic rocks of the Shyok suture zone in north–west trans-Himalaya: Implications for the post-collision evolution of the Shyok suture zone

40Ar– 39Ar geochronological studies carried out on the Khardung volcanics of Ladakh, India and our earlier Ar–Ar results from the volcanics of the Shyok suture along with the available geological and geochemical data provide good constraints for post-collision evolution of the Shyok suture zone. Whole-rock samples from the Shyok volcanics yielded disturbed age-spectra and we have demonstrated earlier that the youngest tectonic event in the Shyok suture zone responsible for the thermal disturbance of these samples is Karakoram fault activation at ∼14 Ma. Contrastingly whole-rock samples from the Khardung volcanics, which are in tectonic contact with these Shyok volcanics, and are exposed in the form of thick rhyolitic and ignimbritic flows, yielded undisturbed age-spectra and good plateau-ages. The whole-rock plateau-ages of two rhyolite samples are 52.8 ± 0.9 and 56.4 ± 0.4 Ma. We interpret these ages to be the time and duration of emplacement of these volcanics over thickened margin of the continental crust, which appears to be coeval with the initiation of the collision between the Indian and Asian plate. The lesser extent of post-emplacement isotopic re-equilibration in these samples unlike the Shyok volcanics indicate that these samples were present in different tectonic settings, away from the Karakoram fault, at the time of deformation in the Shyok suture zone. We propose that the two volcanic belts of contrasting nature were brought together in juxtaposition by the Karakoram strike slip faulting at ∼14 Ma.

Carbon isotope and magnetostratigraphy of the Cretaceous (Barremian – Aptian) Pabellón Formation, Chañarcillo Basin, Chile

New oxygen and carbon isotopic analyses of fine–fraction samples from the Pabellón Formation (Chañarcillo Basin, Chile), together with magnetostratigraphic data, are presented. The carbon isotope curve shows a distinctive pattern including a negative isotope excursion preceding a positive event. Within the Lower Aptian this chemostratigraphic event is widely used for correlation of marine and continental stratigraphic sections and confirms the global nature of the episode. The carbon isotope profile improves the stratigraphic correlation of the formation and, together with published ammonite data, the lower part of the succession is interpreted as early Aptian in age, whilst the upper part of the succession is of late Aptian age. The oxygen isotope data are consistent with a diagenetic and metamorphic overprint of the sediments. The palaeomagnetic data are consistent with the rocks having suffered a substantial clockwise rotation during a younger tectonic event, consistent with other units in the wider area and these data do not therefore help further constrain the age of deposition.

The Penokean orogeny in the Lake Superior region

The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine–Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine–Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. The Penokean orogen was strongly overprinted by younger tectonic and thermal events, some of which were previously ascribed to the Penokean. Principal among these was a period of vertical faulting in the Archean basement and overlying Paleoproterozoic strata. This deformation is now known to have post-dated the terminal Penokean plutons by at least several tens of millions of years. Evidence of the Penokean orogen is now largely confined to the Lake Superior region. Comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive Penokean orogen have been removed or overprinted by younger tectonic events.

Brittle deformation between the Ambin and Vanoise domes in the frame of the structural evolution of the internal Alpine belt

New geophysical data and geological observations emphasize the importance of brittle tectonics in the Neogene to present day evolution of the Western Alps. The oldest Neogene stress state (F1) is characterized by extension direction parallel to the orogen and shortening direction vertical or perpendicular to the Alpine belt whereas the youngest tectonic event (F2) is characterized by brittle extension perpendicular to the mountain belt. According to new brittle microtectonics data from the High Maurienne valley near Modane and a synthesis of the available geochronological and microtectonics data at the scale of the internal Alps, we discuss a timing of the brittle tectonics phases and their role in the formation of basement domes (Internal Crystalline massifs and Brianconnais domes). We propose that the F1 event spans from 32-30 Ma to 22 Ma south of the Simplon fault and was probably active up to 5 Ma on the Simplon fault. The F2 event started at about 6-5 Ma and is still active. In this scheme, basement domes observed in the Western Alps are the results of interference between an early E-W extension along NE-SW ductile to brittle faults and the two F1-F2 brittle tectonic events documented here.

Thermo-tectonic history of the Tethyan Himalayas deduced from the palaeomagnetic record of metacarbonates from Shiar Khola (Central Nepal)

Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM1) and magnetite (ChRM2) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM2.Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination (Iexp) by intersection of the remanence small-circle with the small-circle of constant Iexp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM1; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.

The Trans Mojave-Sierran shear zone and its role in Early Miocene collapse of southwestern North America

Abstract The relative motion between the Pacific and North American plates in early Miocene time was not parallel to the overall NW strike of the transform, but was instead oblique and transtensional. It has recently been proposed that in response to this divergence, the western edge of the North American plate east of the transform gravitationally collapsed and moved 100–150 km to the southwest (S50°–60°W). The region of collapse covered an area of nearly 10 6 km 2 and included what is now southern California, southwestern Arizona, and northwestern Mexico. A major structure facilitating collapse between 21 and 18 Ma was the Trans Mojave-Sierran shear zone (TMSSZ). This east-west shear zone linked the classic detachment fault terranes and metamorphic core complexes of the Mojave desert, southeastern California, southern Arizona, and Sonora, Mexico, to the transtensional plate boundary. To more fully understand the nature and kinematic significance of the TMSSZ and its role in facilitating early Miocene fragmentation of the North American plate, a palinspastic reconstruction of the Mojave desert was performed to remove the disruptive effects of the TMSSZ and younger tectonic events. Features formed just before movements along the TMSSZ were used as markers to assess the TMSSZ deformation. Our analysis indicates that TMSSZ deformation was distributed across a c. 90 km wide band; restoration of markers to their original positions implies that &gt;80 km of dextral shear occurred along the TMSSZ. First-order dextral shear deformation within the TMSSZ is expressed internally by clockwise vertical axis rotations of large areas that were facilitated by second-order zones of sinistral shear that separated the blocks. These second-order sinistral zones apparently exploited older transfer zones of the 24-21 Ma Mojave Extensional Belt.

SHRIMP UPb zircon geochronology of granites in the Arunta Inlier, central Australia: implications for Proterozoic crustal evolution

New SHRIMP UPb zircon isotopic ages are reported for ten granitic suites in the eastern Arunta Inlier. These data, combined with previous UPb zircon studies in the region, constrain the timing of major Palaeoproterozoic igneous events within the Arunta Inlier, and establish a geochronological framework for understanding lithostratigraphic relationships, tectonic events, and geochemical and isotopic evolution of the granites in the region. Most granites in the Arunta Inlier, including both syn-tectonic and post-tectonic types, yield zircon ages in the range of 1770-1710 Ma, which are significantly younger than the 1880-1850 Ma “orogenic” Barramundi Igneous Association recognised throughout many other Proterozoic terrains in northern Australia. No rocks of the Barramundi Association have been identified in this study. The previously proposed Barramundi-type suites, including the Jervois and Dneiper granites, yield ages of 1771 ± 6 and 1771 ± 15 Ma, respectively. Many of the granite suites contain inherited zircons with ages in the range of 1890-1830 Ma, similar to the age of the Barramundi Association. This, and the age of the Atnarpa Igneous Complex (Zhao and Cooper, 1992), which is coeval with, but geochemically distinct from, the Barramundi Association, suggest that the 1880-1850 Ma orogenic event affected the Arunta Inlier, but has been obscured by the younger tectonic events. The youth of tectonic events in the Arunta Inlier as compared with other Proterozoic terrains of northern Australia may have resulted from the inlier being located on the southern margin of the Northern Australian Orogenic Province during the Palaeoproterozoic, with the generation of granites in the Arunta Inlier possibly being related to subduction. The combined data indicate that major granitic activity occurred at: 1880-1850, 1820, 1770-1750, 1730-1710, 1660-1650 and 1615-1590 Ma. This result is broadly consistent with earlier summaries of the other Proterozoic terrains in northern Australia, although the intensity of each episode varies in different regions.

Paleostress stratigraphy: A new technique for analyzing tectonic control on sedimentary-basin subsidence

The authors propose a new technique, paleostress stratigraphic analysis, based on the orientation of minor structures, particularly striated fault surfaces, in the strata of sedimentary basins. These structures are interpreted to be the response to local or regional stress field controlling subsidence of tectonically active basins. A method is used that allows multiple stress tensors to be extracted from a fault population. By extracting the stress tensor from the youngest part of a stratigraphic sequence and by imposing that same tensor on the underlying rocks, one can, in principle, isolate the faults associated with the youngest tectonic event. By reiterating this procedure on progressively older units until the fault data are exhausted, a chronologic succession of paleostress tensors during basin subsidence emerges. The authors paleostress stratigraphy to the relatively undeformed Paleogene Central Basin of Spitsbergen and its underlying Cretaceous strata, and define five major paleostress tensors coeval with the earliest stages of the opening of the Norwegian-Greenland Sea. A Late Cretaceous dextral transpressive regime was probably responsible for the major pre-Tertiary unconformity and possibly initiated development of the Central Basin. During subsequent sedimentation, movement reversed, yielding well-constrained sinistral shearing during early Danian time. A composite compressional event followed that themore » authors interpret as being coeval with the culmination of folding and thrusting west of the Central Basin. The youngest strata studied record a final extensional event that they correlate with rifting of Spitsbergen from Greenland.« less

Stratigraphy and Structure of Southern Blake Plateau, Northern Florida Straits, and Northern Bahama Platform from Multichannel Seismic Reflection Data

Approximately 2,100 km of 24-fold multichannel seismic reflection data reveal much about the subsurface geology for a large part of the continental margin east of Florida. Discordance between the westward-dipping pre-breakup sediments and the eastward-sloping basement along the edge of the Blake Plateau is interpreted as an effect of a splinter of continental margin derived from the African plate by a spreading-center jump in the Middle Jurassic. Early rifting centered under the main part of the Blake Plateau became inactive, as a spreading-center jump shifted the active rift to east of the present Blake Escarpment along the Blake Spur magnetic anomaly. In the northern Florida Straits the data reveal that the breakup unconformity, underlain by Triassic-Lower Jurassic(?) arkosic volcaniclastics, extends from southern Florida to the western Bahama Banks. These volcaniclastics are associated with the rift-crust of intermediate nature formed just prior to and during breakup of the North American and African continental plates. Back-reef platform deposits of limestones, dolomites, and evaporites of Late Jurassic to Albian age extend from the Blake-Bahama Escarpment westward beneath Florida. These deposits formed what once was a megabank extending over a wider area than the present smaller isolated Bahama Banks. The formation of the Florida Straits and Bahama channels occurred during the Cenomanian transgression. Only on the present Bahama Banks and Florida platform did shallow-water carbonate deposition persist to maintain a shallow-bank environment. Evidence of recurring scour by current erosion is found in the Florida Straits. Erosional events apparently occurred in the middle Cenomanian, middle Paleocene, early-middle Eocene, and Eocene-Oligocene, which coincidentally are times of lower eustatic sea level according to Vail et al (1977a). This evidence of Florida current scour indicates that the current was present as far back as the Cenomanian. Major faulting appears to have dropped the Northeast Providence Channel relative to the western Bahamas after the Albian. Submarine erosion and bank buildup created the channels and smaller relief features like Great Abaco Knoll beginning in about the Cenomanian. A carbonate bank margin and reef complex was present along the Bahama Escarpment since the Middle Jurassic. Apparently these organic buildups seeded on originally shallow structural relief on oceanic basement created during the spreading-center jump to the position of the Blake Spur magnetic anomaly. The bank margin apparently has retreated at least 15 km from a Late Jurassic-earliest Cretaceous position now marked by a bench below the Au unconformity. Active faulting occurred along the Great Abaco fracture zone at least through the Late Cretaceous and perhaps into the Tertiary. These relatively young tectonic events, together with the post-Albian faults in Providence Channel, indicate interactions between the Atlantic and Caribbean plates and that extensions of faulting have taken place far to the northeast of Cuba and the Greater Antilles.

Stratigraphy and Structure of Atlantic Continental Margin in Bahama Platform Area: ABSTRACT

Approximately 2,100 km of 24-fold multichannel seismic reflection data reveal the subsurface geology of a large part of the continental margin in the Bahamas region. Discordance between the westward-dipping prebreakup sediments and the eastward-sloping basement along the edge of the Blake Plateau is interpreted as an effect of a splinter of continental margin derived from the African plate by a spreading-center jump in Middle Jurassic. Early rifting centered under the main part of the Blake Plateau became inactive, as a spreading-center jump shifted the active rift to east of the present Blake Escarpment along the Blake Spur magnetic anomaly. In the northern Florida Straits the data reveal that the prebreakup unconformity, underlain by Triassic-Upper Jurassic(?) arkosic volcaniclastics, extends from southern Florida to the western Bahama Banks. These volcaniclastics are associated with the intermediate nature rift-crust formed just prior to and during breakup of the North American and African continental plates. Back-reef platform deposits of limestones, dolomites, and evaporites of Late Jurassic to Albian age extend from the Blake-Bahama Escarpment under Florida. These covered what once was a huge megabank extending over a wider area than the present smaller isolated Bahama Banks. The formation of the modern Florida Straits and Bahama channels occurred in the Cenomanian transgression when the overall area deepened. Only on the present Bahama Banks and Florida platform did shallow-water carbonate deposition persist to maintain the End_Page 1670------------------------------ shallow-bank environments. Evidence of recurring scour by current erosion is found in the Florida Straits. Erosional events apparently occurred in middle Cenomanian, middle Paleocene, early-middle Eocene, and middle Oligocene, which are times of lower eustatic sea level. This evidence of Florida current scour indicates that the current was present as far back as the Cenomanian. A carbonate bank margin and reef complex has been present along the Bahama Escarpment since Middle Jurassic. Apparently these organic buildups seeded on structural relief on oceanic basement created during the spreading-center jump along the Blake Spur anomaly. The bank margin apparently has retreated at least 15 km below the Au unconformity. Active faulting at least through the Late Cretaceous and perhaps into the Tertiary, occurred along the Great Abaco fracture zone. These relatively young tectonic events, along with the post-Albian faults in Providence Channel, indicate interactions between the Atlantic and Caribbean plates and extensions of faulting far to the northeast of Cuba and the greater Antilles. End_of_Article - Last_Page 1671------------