Dense Oceanic Crust Research Articles

Role of continental margin architecture in models of ophiolite emplacement

Ophiolite obduction, the process during which part of the oceanic crust overlaps the continental margin, presents a challenge in geodynamic reconstructions of lithospheric processes. The difference in buoyancy between the dense oceanic crust and the relatively buoyant continental crust makes obduction of the oceanic crust difficult, if not impossible, if we only consider the buoyancy forces. The initial configuration of the oceanic basins must have specific thermal and geometric constraints to overcome the difficulties posed by the negative buoyancy. Here, we present a systematic investigation of the geometric and geodynamic parameters controlling the process of ophiolite emplacement. We show which parameters are the most important during ophiolite emplacement and the optimum geometries favouring this emplacement. We focus on ‘Tethyan’ ophiolites, which are characterized by a relatively small inferred basin size and are commonly found in the Mediterranean region. Based on a combination of parameters, we identify the configurations most susceptible to ophiolite obduction. Our models, in agreement with the geological data, show that to achieve ophiolite obduction, the obducted lithosphere must be young and the length of the ocean–continent transition zone must be relatively sharp. Supplementary material: Supplementary figures S1–S5, that are mentioned in the main text, are available at https://doi.org/10.6084/m9.figshare.c.6922526 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Transient Injection of Flow: How Torn and Bent Slabs Induce Unusual Mantle Circulation Patterns Near a Flat Slab

AbstractTorn and bent slabs are usually associated with flat‐slab subduction where the descending plate develops a horizontal geometry beneath the overlying continent. How such slab dynamics modify the surrounding mantle flow and the overriding plate remains enigmatic. Here, we conduct three‐dimensional subduction numerical experiments to investigate the flat slab to steep‐angle slab transition region and examine the impact of slab geometry changes on mantle flows. The results show that the along‐strike change due to flattening a segment of slab induces oblique flow toward the mantle corner at the transition region where flat slab bends to steep‐subducting slab. Slab tears can occur due to the buoyancy contrast between an oceanic ridge and the surrounding dense oceanic crust and/or presence of weak zones. A vertical tear at the side edge of a flat slab causes toroidal flow around the steep‐angle slab where sub‐slab mantle floods rapidly through the tear. Flow through the tear spreads to all directions including upwelling toward the continental base that may trigger slab and partial melting and thus affect arc magmatism. A horizontal tear that occurs ahead of the flat portion where slab resumes its steep‐angle results in enhanced plate‐motion parallel flow. Notably, the tear‐induced flow behaves as a rapid injection through the slab breach with the peak velocity up to an order of magnitude higher than plate motion, lasting for 1–2 million years after initially tearing. This rapid pulse flow might be recorded in surface tectonics as distinct transient events of topography or metamorphism.

The Stability of Dense Oceanic Crust Near the Core‐Mantle Boundary

AbstractThe large low‐shear‐velocity provinces (LLSVPs) are thought to be thermo‐chemical in nature, with recycled oceanic crust (OC) being a contender for the source of the chemical heterogeneity. The melting process which forms OC concentrates heat producing elements (HPEs) within it which, over time, may cause any collected piles of OC to destabilize, limiting their suitability to explain LLSVPs. Despite this, most geodynamic studies which include recycling of OC consider only homogeneous heating rates. We perform a suite of spherical, three‐dimensional mantle convection simulations to investigate how buoyancy number, geochemical model and heating model affects the ability of recycled OC to accumulate at the core‐mantle boundary. Our results agree with others that only a narrow range of buoyancy numbers allow OC to form piles in the lower mantle which remain stable to present day. We demonstrate that heterogeneous radiogenic heating causes piles to destabilize more readily, reducing present day CMB coverage from 63% to 47%. Consequently, the choice of geochemical model can influence pile formation. Geochemical models which lead to high internal heating rates can cause more rapid replenishment of piles, increasing their longevity. Where piles do remain to present day, first order comparisons suggest that old (hot) OC material can produce seismic characteristics, such as Vs anomalies, similar to those of LLSVPs. Given the range of current density estimates for lower mantle mineral phases, subducted OC remains a contender for the chemical component of thermo‐chemical LLSVPs.

Serpentinite Diapirs and the Evolution of Oceanic Core Complexes

Serpentinites and peridotites are the predominant lithological components of Oceanic Core Complexes (OCCs), located commonly at triple junctions of slow-spreading oceanic accreting ridges, fracture zones and transform faults. These lithologies differ from the surrounding oceanic brittle lithosphere, built of basalt and gabbro, and the structural evolution of these OCCs is enigmatic. The present investigation suggests that the tectonics of OCCs is derived from the ascent of serpentinic diapirs generated by the unique proximity of shallow asthenosphere, faulted lithosphere and the juxtaposition of oceanic crusts of contrasting densities. Such setting initiated two structural stages in the evolution of the OCC, the first is spontaneous subduction of old and dense oceanic crust under the fresh and lighter basalt at the edge of the MOR across its intersection with transform fault - fracture zone. The sub ducted slab would be affected by the combined effect of the steep thermal gradient and the availability of volatiles there to enhance the alteration of pyroxene into serpentine. Analog and numeric experiments show that spontaneous subduction can initiate spontaneously if the density contrast between the juxtaposed slabs is significant, as is the case between fresh basalt (ρ~2.7 g/cm3 ) and older basalt (ρ~2.9 g/cm3 ). Since the average thermal gradient under the MOR is ca. 130o /km, then at depth of 4-5 km the pyroxene and plagioclase would start their alteration, mostly to serpentine. The abundant faults of MOR normal rifting and strike-slip transform faulting could enable the light and malleable serpentinite to form diapirs, which would ascent to the seafloor at the rift-fracture zone intersections. The friction at the top of the diapirs during their ascent would probably generate breccia at their tops. Highlights a) OCCs are built mostly of serpentinite diapirs that carry peridotite inclusions. b) Serpentinization occurs under low pressure-high temperature conditions. c) OCC source rock is slab of cold oceanic crust sub ducted under fresh crust. d) Such subduction occurs mostly where large density contrast between slabs exists. e) The link between OCCs and detachment normal faults requires support.

Burying Earth's Primitive Mantle in the Slab Graveyard

AbstractThe evolution of mantle composition can be viewed as a process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically “primitive” signature. A question that remains is how this primitive signature has survived four and a half billion years of vigorous convection. We hypothesize that some of Earth's primitive mantle is buried within a slab graveyard at the core‐mantle boundary. We explore this possibility using high‐resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad, robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model's evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material—summing up to ∼30% their total mass. The dense oceanic crust dominating the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike other theories for the survival of Earth's primitive mantle, this one does not require the early Earth to have large‐scale domains of anomalously high density and/or viscosity.

Subducted oceanic crust as the origin of seismically slow lower-mantle structures

Mantle tomography reveals the existence of two large low-shear-velocity provinces (LLSVPs) at the base of the mantle. We examine here the hypothesis that they are piles of oceanic crust that have steadily accumulated and warmed over billions of years. We use existing global geodynamic models in which dense oceanic crust forms at divergent plate boundaries and subducts at convergent ones. The model suite covers the predicted density range for oceanic crust over lower mantle conditions. To meaningfully compare our geodynamic models to tomographic structures, we convert them into models of seismic wavespeed and explicitly account for the limited resolving power of tomography. Our results demonstrate that long-term recycling of dense oceanic crust naturally leads to the formation of thermochemical piles with seismic characteristics similar to the LLSVPs. The extent to which oceanic crust contributes to the LLSVPs depends upon its density in the lower mantle for which accurate data is lacking. We find that the LLSVPs are not composed solely of oceanic crust. Rather, they are basalt rich at their base (bottom 100–200 km) and grade into peridotite toward their sides and top with the strength of their seismic signature arising from the dominant role of temperature. We conclude that recycling of oceanic crust, if sufficiently dense, has a strong influence on the thermal and chemical evolution of Earth’s mantle.

Incipient subduction at the contact with stretched continental crust: The Puysegur Trench

A seismic Benioff zone and plate kinematics show Puysegur Trench south of New Zealand transitioning to subduction. Because the local structure and its influence on subduction initiation is poorly understood, we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles. Our early results show that the overriding Pacific Plate beneath the Solander Basin is composed of block-faulted and thinned continental crust, and the inner trench wall of northern Puysegur Ridge is composed of folded and faulted sediment. The megathrust interface has been imaged and shows ∼500 m of downgoing, undisturbed sediments. Combining plate kinematic history with seismic velocity-inferred density, we show that the density difference across the plate boundary changed as oblique strike-slip plate motion juxtaposed dense oceanic crust with thinned continental crust. The density difference rapidly increased 18 to 15 Ma, coincident with subduction initiation, suggesting that compositional differences have a large influence on subduction initiation.

Origins of the terrestrial Hf-Nd mantle array: Evidence from a combined geodynamical-geochemical approach

The formation and segregation of oceanic and continental crust from the mantle, and its return to the mantle via subduction and/or delamination, leads to the development of distinct geochemical reservoirs in the terrestrial mantle. Fundamental questions remain regarding the location, nature, and residence time of these reservoirs, as well as the respective roles of oceanic and continental crust in the development of the mantle's geochemical endmembers. The Lu-Hf and Sm-Nd isotope systems behave similarly in magmatic systems and together form the terrestrial mantle Hf-Nd isotopic array. Here we combine a geodynamic model of mantle convection with isotope and trace element (TE) geochemistry to investigate the evolution of the Hf-Nd mantle array. This study examines the sensitivity to: TE partition coefficients used in the formation of oceanic crust; density contrasts between subducting oceanic crust and the mantle; and the formation and recycling of continental crust. We show that the fractionation between the parent (Lu and Sm) and daughter (Hf and Nd) species needs to be higher than is indicated by partition coefficients determined from the present-day melting environment. This is consistent with the suggestion of deeper mantle melting earlier in Earth history and an increased role for residual garnet. Subduction and accumulation of dense oceanic crust produces a large mass of incompatible TE enriched material in the deep mantle. This deep mantle enrichment appears to play a more significant role than the extraction and recycling of continental crust in developing the Hf and Nd isotope and TE compositions of the mid-ocean ridge mantle source. The corollary of this result is that the formation of the continental crust plays a secondary role, contrary to the currently accepted paradigm. Nevertheless, the inclusion of continental crust formation and recycling produces a broader model mantle array, which better reproduces the spread in the natural data set. This model also produces the Hf and Nd isotope and TE compositions of the upper mantle and continental crust, as well as deep mantle compositions similar to those of plume-fed ocean island basalts. Our model is consistent with continental growth models based on the Lu-Hf isotopic composition of zircon, which suggest that 50–70% of the present-day mass of the continental crust is produced prior to 3 Ga, and that the recycling of continental crust becomes more prevalent after this time.

The tectonic transition from oceanic subduction to continental subduction: Zirconological constraints from two types of eclogites in the North Qaidam orogen, northern Tibet

In the plate tectonics theory, continental subduction is pulled by subduction of dense oceanic crust. In practice, however, it is not easy to demonstrate that preceding oceanic crust exposes as oceanic-type eclogite together with continental-type eclogite in collisional orogens. The North Qaidam orogen in northern Tibet is an ultrahigh-pressure (UHP) metamorphic belt that contains the two types of eclogites, providing us with an excellent opportunity to study the tectonic transition from oceanic subduction to continental subduction. In order to constrain the protolith nature and metamorphic evolution of eclogites, we performed a combined study of zircon U–Pb ages, trace elements, mineral inclusions and O–Hf isotopes for various eclogites from the orogen. We discriminate the two types of eclogites by their differences in zircon U–Pb ages and O–Hf isotopes. CL-dark zircon domains exhibit high Th/U ratios, steep HREE patterns and significantly negative Eu anomalies, indicating that they are protolith zircons of magmatic origin with different extents of metamorphic recrystallization. Relict magmatic zircon domains in Type I eclogites yield Neoproterozoic protolith ages of >830Ma and Hf model ages of 850–1100Ma, whereas those in Type II eclogites yield Cambrian protolith U–Pb ages of >489Ma and Hf model ages of 500–650Ma. Most of the CL-bright zircon domains show low Th/U ratios, flat HREE patterns and no negative Eu anomalies, and contain mineral inclusions of garnet, omphacite and rutile, indicating their growth under eclogite-facies metamorphic conditions. These metamorphic domains have consistent eclogite-facies metamorphic ages of 433–440Ma throughout the North Qaidam orogen, regardless of the eclogite types and locations. The metamorphic zircon domains in Type I eclogites mostly exhibit δ18O values higher than normal mantle values, whereas Type II eclogites mostly have δ18O values lower than the normal mantle values. The difference in the δ18O values indicates that their protoliths underwent different temperatures of hydrothermal alteration at different tectonic settings. Combining zircon U–Pb ages and O–Hf isotope compositions with local tectonics, it is inferred that Type I eclogites were metamorphosed from Neoproterozoic continental mafic rocks, whereas Type II eclogites were metamorphosed from oceanic mafic rocks that were subducted prior to the continental subduction. The consistent eclogite-facies metamorphic ages for the two types of eclogites indicate that the exhumed oceanic-type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust. Therefore, the coexistence of oceanic- and continental-type eclogites in the North Qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic.

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

abstractAn integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.

Geodynamic reconstructions of the South America–Antarctica plate system

The South America–Antarctica plate system shows many oceanic accretionary systems and subduction zones that initiated and then stopped. To better apprehend the evolution of the system, geodynamic reconstructions (global) have been created from Jurassic (165 Ma) to present, following the techniques used at the University of Lausanne. However, additional synthetic magnetic anomalies were used to refine the geodynamics between 33 Ma and present. The reconstructions show the break up of Gondwana with oceanisation between South America (SAM) and Antarctica (ANT), together with the break off of ‘Andean’ geodynamical units (GDUs). We propose that oceanisation occurs also east and south of the Scotian GDUs. Andean GDUs collide with other GDUs crossing the Pacific. The west coast of SAM and ANT undergo a subsequent collision with all those GDUs between 103 Ma and 84 Ma, and the Antarctic Peninsula also collides with Tierra del Fuego. The SAM–ANT plate boundary experienced a series of extension and shortening with large strike-slip component, culminating with intra-oceanic subduction leading to the presence of the ‘V-’ and ‘T-’ anomalies in the Weddell Sea. From 84 Ma, a transpressive collision takes place in the Scotia region, with active margin to the east. As subduction propagates northwards into an old and dense oceanic crust, slab roll-back initiates, giving rise to the western Scotia Sea and the Powell Basin opening. The Drake Passage opens. As the Scotian GDUs migrate eastwards, there is enough space for them to spread and allow a north–south divergence with a spreading axis acting simultaneously with the western Scotia ridge. Discovery Bank stops the migration of South Orkney and ‘collides with’ the SAM–ANT spreading axis, while the northern Scotian GDUs are blocked against the Falkland Plateau and the North-East Georgia Rise. The western and central Scotia and the Powell Basin spreading axes must cease, and the ridge jumps to create the South Sandwich Islands Sea. The Tierra del Fuego–Patagonia region has always experienced mid-oceanic ridge subduction since 84 Ma. Slab window location is also presented (57–0 Ma), because of its important implication for heat flux and magmatism.

What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles

Abstract Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as ‘emplacement’. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the world’s ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) ‘Tethyan’ ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) ‘Cordilleran’ ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ‘ridge-trench intersection’ (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.

Where are the missing faults in translated terranes?

A central dilemma in understanding terranes that have translated long distances is that modern plate tectonics and paleomagnetic studies indicate translations of thousands of kilometers, while the faults along which the terranes moved these great distances are not found. Relations long known from geology and from recent studies of modern transform-plate boundaries suggest that preserving strike-slip faults that record 1000-km-scale translations is unlikely, and therefore, in many cases, paleomagnetism may be our only reliable indicator of how far terranes translated. Four reasons are explored here for why large-offset strike-slip faults are likely to be ‘missing’ in translated terranes. (1) Strike-slip faults in continents commonly are partially or wholly destroyed, or reactivated. Only ∼400–500 km of over 1000 km of offset on Cretaceous–early Tertiary strike-slip faults in northern British Columbia are accounted for in southern British Columbia. The missing strike-slip faults probably continued south into the hinterland of the Rocky Mountain thrust belt as oblique-thrust faults, much like the modern Alpine–Marlborough fault system of New Zealand. The major strike-slip faults in southeastern British Columbia were obliterated by Eocene extensional faulting, uplift, and erosion. (2) Offsets of 1000 km scale are likely to occur on many anastomosing faults with offsets of 100 km scale, many of which are ‘left behind’ and not found in the translated terrane. The greater San Andreas plate boundary has accumulated ∼1200 km of transform motion that is distributed on numerous faults from the offshore borderland to eastern California. There is a narrow ‘terrane’ in the offshore borderland of California that has moved ∼1200 km relative to North America, but no single fault has nearly that much offset. (3) Major faults in narrow oceans formed by oblique-divergent plate boundaries have a low potential for preservation. For example, the deep, dense oceanic crust of the Gulf of California, where the major transform faults of the oblique rift lie, has a low potential for preservation. In contrast, Baja California is currently a continental terrane that has translated ∼300 km relative to North America along the plate boundary in the Gulf of California. Because of the relative buoyancy of continental crust, Baja California is likely to be preserved in any future accretion event. (4) Faults in the margins of oblique-divergent plate boundaries are dominantly normal faults and not strike-slip faults. There is little strike-slip faulting in central Baja California, even though the plate boundary has a rift angle ( α) of only 20°. Recent field data and modeling show that at α=0–20° strike-slip faults will dominate the secondary faults. Surprisingly, at α=∼20 to ∼35°, there are both strike-slip and normal faults, and at α>∼35–45°, there are few or no strike-slip faults, even though the relative motion of these types of oblique rifts is dominated by transcurrent motion over extensional motion.

Model for the formation of large, transtensional basins in zones of tectonic escape

Research Article| March 01, 1997 Model for the formation of large, transtensional basins in zones of tectonic escape Paul Mann Paul Mann 1Institute for Geophysics, University of Texas at Austin, 8701 North Mopac Expressway, Austin, Texas 78759-8397 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (3): 211–214. https://doi.org/10.1130/0091-7613(1997)025<0211:MFTFOL>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul Mann; Model for the formation of large, transtensional basins in zones of tectonic escape. Geology 1997;; 25 (3): 211–214. doi: https://doi.org/10.1130/0091-7613(1997)025<0211:MFTFOL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract “Tectonic escape” is strike-slip-dominated motion of colliding continental or arc crust in the direction of subductable oceanic crust. I describe four examples of large transtensional basins in Cenozoic zones of tectonic escape and propose that these basins formed along newly formed, transtensional faults formed between the collided and escaping parts of the plate. The reoriented direction of the escaping plate, split along the new transtensional fault zone, is determined by the presence of a “free face,” or subductable oceanic crust. Continued subduction toward the free face is driven by the force of gravity acting on dense oceanic crust (“slab pull”). The four examples presented here show that a variety of transtensional deformational styles can be generated within this specific type of divergent tectonic setting. These transtensional styles include pull-apart basins at fault stepovers, more asymmetrical, rift-like basins thought to be formed under conditions of fault-normal extensional stresses, and single or multiple short spreading ridges. Transtensional styles may be controlled more by the type and age of crust present than the angle of reorientation of the escaping plate. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Translation of terranes: Lessons from central Baja California, Mexico

Research Article| November 01, 1997 Translation of terranes: Lessons from central Baja California, Mexico Paul J. Umhoefer; Paul J. Umhoefer 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Rebecca J. Dorsey Rebecca J. Dorsey 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (11): 1007–1010. https://doi.org/10.1130/0091-7613(1997)025<1007:TOTLFC>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paul J. Umhoefer, Rebecca J. Dorsey; Translation of terranes: Lessons from central Baja California, Mexico. Geology 1997;; 25 (11): 1007–1010. doi: https://doi.org/10.1130/0091-7613(1997)025<1007:TOTLFC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Baja California is one of the best modern examples of a continental block, or terrane, that is translating hundreds of kilometers along a highly obliquely divergent plate. The central domain of Baja California is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late Neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the North American continent. In contrast, the deep, dense oceanic crust of the Gulf of California, where the main transform faults of the plate boundary lie, has little preservation potential. It is surprising that there is little strike-slip faulting in the central domain of Baja California, even though the plate boundary has a rift angle (angle between rift trend and azimuth of plate motion), alpha, of only 20°. Recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type, fault orientation, and extension direction with changing rift angle. That is, from secondary structures alone, one can predict the approximate obliquity of rifting and the orientation of the plate motion. With rift angles of 0°–20°, strike-slip faults at low angles to the rift trend will dominate the secondary structures; with rift angles from ∼ 20° to ∼ 35°, there are strike-slip faults subparallel to the rift trend and normal faults 20°–40° clockwise from the rift trend in dextral shear; above ∼ 35°, normal faults are dominant, and there are few or no strike-slip faults as secondary structures. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismotectonics of the northern Aegean area

Data concerning the focal mechanism and the spatial distribution of earthquakes have been used to investigate the active tectonics of the northern Aegean and the surrounding area. A thrust region, which includes the northernmost part of the Aegean and at least part of the Marmara Sea, has been defined. An amphitheatrical Benioff zone dipping towards the thrust region from south, east and probably from west, at a mean angle of about 30°, has been detected. The thrust region is surrounded by a region of normal faulting. An eastward progression of the seismic activity in this normal faulting region between 1954 and 1971 has been observed. A correspondence between the earthquake occurrence in the thrust and normal faulting regions has also been observed. Each large shock produced by tensional mechanism in the region of normal faulting is preceded or followed by one or more shocks of compressional mechanism in the thrust region. The focal mechanism, the distribution of the earthquake foci with intermediate focal depth, as well as some magnetic and gravimetric observations can be interpreted by assuming that dense oceanic crust sinks in the northern part of this area and that the adjacent lithosphere moves by segmentation to fill the void with the consequence of producing tensile stresses associated with normal faulting. Such a mechanism of lithospheric interaction suggests that accretion probably takes place in this area.