Plate Boundary Reorganization Research Articles

Late Miocene transition from extension to dextral transtension in the southern Coastal Sonora fault zone, Gulf of California rift, Mexico

The Coastal Sonora fault zone (CSFZ) in northwestern Mexico records the Pacific–North America plate boundary reorganization during the Miocene from subduction to a transform boundary. This study presents the structural architecture of the Sierra El Aguaje-San Carlos-Guaymas region along the southern part of the CSFZ. The study area shows a complex fault pattern divided into four tectonic domains: ENE–WSW left-lateral, normal, and oblique faults in the Sierra El Aguaje and Cerro El Vigía domains and N–S domino-type normal faults in the El Parral and San Carlos domains, which are separated by the NW-striking dextral San Carlos fault. Based on a new fault-slip dataset and K-Ar ages, we propose a structural evolution model in which deformation progressively evolved from extension to dextral transtension. The deformation history involved ∼E–W pure extension associated with N–S normal faults from ∼15.3 Ma to ∼11–10 Ma, followed by transtension and the development of strike-slip faults after ∼11–10 Ma. That change induced the reactivation of preexisting faults and local strain partitioning between normal, oblique, and strike-slip faults. Transtension became strike-slip dominated during the latest Miocene and the Pliocene, accumulating ∼8 km of post-6.3 Ma dextral slip along the San Carlos fault. Paleostress analysis demonstrates that during the extensional phase, σ2 and σ3 were horizontal and oriented ∼N–S and ∼E–W, respectively. The transition to transtension required that σ2 reached the lithostatic stress value, its magnitude approached σ1, and its orientation remained ∼N–S. These conditions implied a horizontal tectonic force directed towards the north began after ∼11–10 Ma in northwestern Mexico. We propose that this new stress regime was related to the end of the Farallon–North America convergence and the inception of the Gulf of California oblique rifting with σ1 oriented N–S.

Structure of the Lithosphere and Conditions of Formation of Oceanic Rises in the Sub-Antarctic Sector of the South Atlantic Using Density and Physical Modeling

The kinematic reorganization of plate boundaries, accompanied by the cessation of old spreading centers and the formation of new ones, and manifestations of plume magmatic activity in the southeastern part of the Antarctic sector of the South Atlantic led to the formation of a complex structural plan of the region. As a result of these processes, a system of ridges, rises, and plateaus with varying morphological expressions and different geophysical characteristics were formed. Results of density modeling of the crust and lithosphere structure along profiles extending from the Falkland Plateau to the Mozambique Ridge and crossing a series of rises and ridges separated by deepwater basins showed that rises have different crustal structures, indicating different origins. The conditions for the formation of different types of submarine rises were studied based on physical modeling. A new experimental model of lithosphere and submarine rises formation in the region was constructed, in which the fracture of the large Agulhas magmatic province into the Agulhas Plateau and the Northeast Georgia rise played an important role, as did the accretion of oceanic crust on the Agulhas spreading ridge and subsequent jump of the spreading axis, leading to the cessation of spreading on this ridge and the formation of the southern segment of the Mid-Atlantic Ridge and its associated Meteor and Islas Orcadas rises. Jumps of spreading axes, accompanied by the periodic activity of hotspots, played an important role in the formation of submarine rises of different genetic types, which determined the different structures of their crust.

Steady Long‐Term Slip Rate on the Blue Cut Fault: Implications for Strain Transfer Between the San Andreas Fault and Eastern California Shear Zone

AbstractThe Eastern Transverse Ranges (ETR) province of California contains a system of E‐W‐trending left‐lateral faults accommodating clockwise block rotation between the San Andreas Fault (SAF) system in the Coachella Valley and the Eastern California Shear Zone (ECSZ) in the Mojave Desert. Building upon established geometric relationships, we estimate that this rotation across the ETR may transfer right‐lateral strain from the SAF to the ECSZ at a time‐averaged rate of ∼4.3–7.7 mm/yr. Through geomorphic mapping and the analysis of 38 10Be surface exposure samples, we derive a long‐term slip rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, yet analysis of the displacement record indicates a rate decrease ∼71 kya. While a rate change on this fault could have implications for possible plate boundary reorganization in this area, we argue that this slip rate variability more likely reflects routine fluctuation about a steady lifetime slip rate.

Microcontinent Breakup and Links to Possible Plate Boundary Reorganization in the Northern Gulf of California, México

AbstractFaults on microcontinents record the dynamic evolution of plate boundaries. However, most microcontinents are submarine and difficult to study. Here, we show that the southern part of the Isla Ángel de la Guarda (IAG) microcontinent, in the northern Gulf of California rift, is densely faulted by a late Quaternary‐active normal fault zone. To characterize the onshore kinematics of this Almeja fault zone, we integrated remote fault mapping using high‐resolution satellite‐ and drone‐based topography with neotectonic field‐mapping. We produced 13 luminescence ages from sediment deposits offset or impounded by faults to constrain the timing of fault offsets. We found that north‐striking normal faults in the Almeja fault zone continue offshore to the south and likely into the nascent North Salsipuedes basin southwest of IAG. Late Pleistocene and Holocene luminescence ages indicate that the most recent onshore fault activity occurred in the last ∼50 kyr. These observations suggest that the North Salsipuedes basin is kinematically linked with and continues onshore as the active Almeja fault zone. We suggest that fragmentation of the evolving IAG microcontinent may not yet be complete and that the Pacific‐North America plate boundary is either not fully localized onto the Ballenas transform fault and Lower Delfin pull‐apart basin or is in the initial stage of a plate boundary reorganization.

Evolution of Subduction Cusps From the Perspective of Trench Migration and Slab Morphology

The geometries of trenches vary worldwide due to continuous plate boundary reorganization. When two trenches intersect to generate a corner, a subduction cusp is formed. Although subduction cusps are frequently observed throughout historical plate movement reconstructions, few studies have been conducted to explore the controlling factors of trench migration and slab morphology along subduction cusps. Here, we use a 3-D dynamic subduction model to explore the influence of the overriding plate strength, initial slab-pull force, and initial cusp angle on the evolution of subduction cusps. Our numerical model results suggest the following: 1) subduction cusps have a tendency to become smooth and disappear during the subduction process; 2) the slab dip angle is smallest in the diagonal direction of the subduction cusp, and a larger cuspate corner angle leads to a larger slab dip angle; 3) the asymmetric distribution of the overriding plate strength and initial slab-pull force determine the asymmetric evolutionary pathway of subduction cusps. Our results provide new insights for reconstructing the evolution of subduction cusps from seismological and geological observations.

Juvenile hafnium isotopic compositions recording a late Carboniferous–Early Triassic retreating subduction in the southern Central Asian Orogenic Belt: A case study from the southern Alxa

AbstractTwo successive and parallel magmatic arcs within the southern Alxa provide an ideal area to examine the influence of tectonic switching on temporal and spatial distribution of magmatism within accretionary orogens. This study presents new geochronological and geochemical data for Yingen and Quagu plutons from the southern Alxa, located in the southern Central Asian Orogenic Belt. Late Permian Yingen granitic dikes (ca. 252 Ma) have depleted whole-rock Nd isotopic compositions, high Sr, low Y and Yb, and high Sr/Y ratios, all of which indicate they were generated by the partial melting of subducted young/hot oceanic slab. The Middle Permian (271 Ma) Yingen hosting granites contain elevated contents of Nb and Zr, and have high 10,000 × Ga/Al ratios, suggesting that they resulted from mixing between Neoproterozoic crust-derived felsic magmas and depleted mantle-derived mafic magmas. The Quagu pluton yields ca. 271–262 Ma zircon U-Pb ages and has an adakitic high-Mg diorite-like geochemical composition, suggesting that it originated from interaction between slab-derived melts and overlying peridotite material. Collectively, these data record the subduction of the Enger Us oceanic slab beneath Mesoproterozoic–Neoproterozoic sialic crust, generating a Japan-type arc within the southern Alxa during Middle–Late Permian. Temporal-spatial variations of zircon Hf isotope for plutons suggest tectonic switching from advancing to retreating subduction during Carboniferous–Early Triassic within the southern Alxa. An advancing subduction resulted from the subduction of the Paleo-Asian Ocean, and a retreating subduction was related to plate boundary reorganization during the assembly of Pangea.

Pacific‐Panthalassic Reconstructions: Overview, Errata and the Way Forward

AbstractWe have devised a new absolute Late Jurassic‐Cretaceous Pacific plate model using a fixed hot spot approach coupled with paleomagnetic data from Pacific large igneous provinces (LIPs) while simultaneously minimizing plate velocity and net lithosphere rotation (NR). This study was motivated because published Pacific plate models for the 83.5‐ to 150‐Ma time interval are variably flawed, and their use affects modeling of the entire Pacific‐Panthalassic Ocean and interpretation of its margin evolution. These flaws could be corrected, but the revised models would imply unrealistically high plate velocities and NR. We have developed three new Pacific realm models with varying degrees of complexity, but we focus on the one that we consider most realistic. This model reproduces many of the Pacific volcanic paths, modeled paleomagnetic latitudes fit well with direct observations, plate velocities and NR resulting from the model are low, and all reconstructed Pacific LIPs align along the surface‐projected margin of the Pacific large low shear wave velocity province. The emplacement of the Shatsky Rise LIP at ~144 Ma probably caused a major plate boundary reorganization as indicated by a major jump of the Pacific‐Izanagi‐Farallon triple junction and a noteworthy change of the Pacific‐Izanagi seafloor spreading direction at around chron M20 time.

Oblique seafloor spreading across intermediate and superfast spreading centers

We show that oblique seafloor spreading occurs in several regions where obliquity, α, was not recognized before. These include the slow spreading centers of the Red Sea (α≈20°), intermediate spreading centers of the Cocos–Nazca plate boundary between 91°W and 94°W (α≈9°), and superfast spreading centers of the East Pacific Rise at the Nazca–Pacific plate boundary between 29°S and 32°S (α≈10°) and perhaps between ≈16°S and ≈22°S (α≈4°). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across intermediate and superfast spreading centers, oblique spreading does not occur in staircase-mode plate geometries, but does occur in oblique-mode plate geometries. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. Using prior methods, we estimate the ratio of overpressure to remote tectonic stress for several spreading centers. In particular we show along a segment of the Cocos–Pacific plate boundary that magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate.

Styles and rates of deformation in the frontal accretionary wedge of the Calabrian Arc (Ionian Sea): controls exerted by the structure of the lower African plate

The Calabrian Arc is a narrow subduction-rollback system resulting from Africa/Eurasia plate convergence. We analysed the structural style of the frontal accretionary wedge through a multi-scale geophysical approach. Pre-stack depth-migrated crustal-scale seismic profiles unravelled the overall geometry of the subduction complex; high-resolution multi-channel seismic and sub-bottom CHIRP profiles, together with morpho-structural maps, integrated deep data and constrained the fine structure of the frontal accretionary wedge, as well as deformation processes along the outer deformation front.We identified four main morpho-structural domains in the western lobe of the frontal wedge: the proto-deformation area at the transition with the abyssal plain; two regions of gentle and tight folding; a hummocky morphology domain with deep depressions and intervening structural highs; a highstanding plateau at the landward limit of the salt-bearing accretionary wedge, where the detachment cuts through deeper levels down to the basement. Variation of structural style and seafloor morphology in these domains are related to a progressively more intense deformation towards the inner wedge, while abrupt changes are linked to inherited structures in the lower African plate. Our data suggest focusing of intense shallow deformation in correspondence of deeply rooted faults and basement highs of the incoming plate.Back-arc extension in the Southern Tyrrhenian Sea has recently ceased, producing a slowdown of slab rollback and plate-boundary re-organization along trans-tensional lithospheric faults segmenting the continental margin. In this complex setting, it is not clear if the accretionary wedge is still growing through frontal accretion. Our data suggest that shortening is still active at the toe of the wedge, and uplift rates along single folds are in the range of 0.25-1.5 mm/yr. An unconformity within the Plio-Quaternary sediments suggests a discontinuity in sedimentation and tectonic processes, i.e. a slowdown of shortening rate or an increase in sedimentation rate, but not a real inactivation of frontal accretion, which still contributes to the migration of the outer deformation front towards the foreland.

A global review and digital database of large-scale extinct spreading centers

Extinct mid-ocean ridges record past plate boundary reorganizations, and identifying their locations is crucial to developing a better understanding of the drivers of plate tectonics and oceanic crustal accretion. Frequently, extinct ridges cannot be easily identified within existing geophysical data sets, and there are many controversial examples that are poorly constrained. We analyze the axial morphology and gravity signal of 29 well-constrained, global, large-scale extinct ridges that are digitized from global data sets, to describe their key characteristics. Additionally, the characteristics of a representative collection of active spreading centers are analyzed to review the present-day variation in the bathymetry and gravity signal of ridges in different tectonic settings such as backarc basin ridges, microplate ridges, and large-scale plate boundaries with varied spreading rates. Uncertain extinct ridge-like structures are evaluated in comparison with the signals of well-defined extinct ridges, and we assess whether their morphology and gravity signals are within the range seen at extinct (or active) ridges. There is significant variability in extinct ridge morphology; yet we find that the majority of well-defined extinct ridges have a trough form and a negative free-air gravity anomaly. We compile available data on the spreading characteristics of extinct ridges prior to cessation, such as their spreading rates and duration of spreading, and find significant differences between ridge subtypes and between oceans. Large-scale extinct mid-ocean ridges persist much longer than extinct microplate spreading ridges and extinct backarc basin spreading ridges before cessation. Extinct fragmented plate and microplate spreading centers have the highest pre-extinction spreading rates, and they have greater median relief at their axial segments, suggesting that different crustal accretion styles could lead to different morphology after spreading cessation. Backarc basin ridges have more pronounced relief when they have been active for longer before cessation, which supports theories of reduced magmatic supply as the basin width increases. Extinct ridges in the Atlantic Ocean have the lowest spreading rates prior to cessation and tend to persist for twice as long as those in the Pacific before extinction. There are a larger number of extinct ridges preserved within marginal basins than expected for their combined area; these ridges may relate to the complexity of the plate boundaries in these regions. Our review of a large number of controversial extinct ridge locations offers some insight into which proposed locations are more likely to have been former spreading centers, and our analysis further leads to the discovery of several previously unidentified structures in the south of the West Philippine Basin that likely represent extinct ridges and a possible extinct ridge in the western South Atlantic. We make available our global compilation of data and analyses of individual ridges in a global extinct ridge data set at the GPlates Portal webpage.

Multibeam investigation of the active North Atlantic plate boundary reorganization tip

The previous orthogonal ridge/transform staircase geometry south of Iceland is being progressively changed to the present continuous oblique Reykjanes Ridge spreading geometry as North America–Eurasia transform faults are successively eliminated from north to south. This reorganization is commonly interpreted as a thermal phenomenon, caused by warmer Iceland plume mantle progressively interacting with the ridge, although other diachronous seafloor spreading reorganizations are thought to result from tectonic rift propagation. New marine geophysical data covering our reinterpretation of the reorganization tip near 57°N show successive transform eliminations at a propagation velocity of ∼110 km/Myr, ten times the spreading half rate, followed by abrupt reorganization slowing at the Modred transform as it was converted to a migrating non-transform offset. Neither the simple thermal model nor the simple propagating rift model appears adequate to explain the complicated plate boundary reorganization process.

Australia going down under: Quantifying continental subduction during arc-continent accretion in Timor-Leste

Models of arc-continent accretion often assume that the period of subduction of continental lithosphere before plate boundary reorganization is fairly short lived, yet the timescale of this period is poorly constrained by observations in the geologic record. The island of Timor is the uplifted accretionary complex resulting from the active collision of the Banda volcanic arc with the Australian continental margin. The exposure of underplated and exhumed Australian strata on Timor allows for the characterization of the structural history of accretion of uppermost Australian crust and the quantification of subduction of its original continental lithospheric underpinnings. New structural mapping in East Timor (Timor-Leste) reveals that duplexing of a 2-km-thick package of Australian continental strata has built the majority of the structural elevation of the Timor orogen. Coupling new structural observations with previous thermochronology results reveals the sequence of deformation within the orogen, the presence of subsurface duplexing below the hinterland slate belt, and motion along a foreland subsurface thrust ramp. Construction of balanced cross sections allows for the quantification of the amount of shortening in the orogen, and from that, the length of the subducted Australian continental lithosphere. Two balanced cross sections in East Timor reveal 326–362 km of shortening and that 215–229 km of Australian continental lithosphere have been subducted below the Banda forearc. These results highlight the fact that considerable amounts of continental lithosphere can be subducted while accreting only a thin section of uppermost crust. Continental subduction may have been favorable at Timor because of fast subduction rates, old oceanic crust at the consumed Australian margin, and subduction of some length of transitional crust. These results provide quantitative constraints for future numerical modeling of the geodynamics of continental subduction and arc-continent collision.

Tectonic and magmatic evolution of the Bismarck Sea, Papua New Guinea; review and new synthesis

The Bismarck Sea, located north of Papua New Guinea and just south of the equator, formed during the final stages of a long, complex geological development of the Melanesian Borderland, which resulted from Cenozoic convergence between the Australian and Pacific-Caroline Plates and the opening of back-arc basins. The sea, which straddles two oppositely facing trenches, the inactive Manus trench and the active New Britain trench, covers two basins, the New Guinea Basin to the west and the Manus Basin to the east. These basins are separated by the shallow Willaumez-Manus Rise, trending roughly from WNW to ESE. The origin of these major structural units and their relationship with the present-day zone of major seismicity along the Bismarck Sea Seismic Lineation remains unclear. A detailed examination of geophysical and geochemical data, combined with geologic and geodetic information from surrounding regions, attests that the Bismarck Sea went through some rather unusual events of back-arc development beginning in the middle Pliocene. Around 3.5 Ma, the northern tip of New Guinea came into contact with the Finisterre-Huon Range. The event triggered a back-arc opening that eventually divided the Bismarck seafloor into North and South Bismarck Plates. The rapid opening also caused an upwelling of anomalously hot upper mantle beneath the Bismarck Sea. A large volcanic outflow during this period may have contributed to the formation of the rise west of Manus Island. Both the North and South Bismarck Plates interacted with surrounding plates, and small and large changes may have occurred throughout their history. A sudden shift in local plate motion, perhaps caused by locking such as plate collision, may have caused the overlying lithosphere to decouple from the mantle upwelling. The Willaumez-Manus Rise was probably created under such circumstances when a large volume of rising magma leaked out along a strike-slip plate boundary linking the spreading centers in the New Guinea and Manus basins. An alternative view is that the Willaumez-Manus Rise represents a magmatically robust segment of a single spreading axis that linked the two basins. The further docking of Finisterre-Huon Range with the New Guinea Highlands allowed the South Bismarck Plate to open faster toward the east in the Manus Basin and eventually to jump to the present-day Bismarck Sea Seismic Lineation. The Willaumez Transform was initiated by this more recent plate-boundary reorganization. Seafloor spreading commenced in the Manus Spreading Center <0.78 Ma, and the overall pattern suggests that rifting in the Bismarck Sea is propagating eastwards in the Southeastern Rift and westwards in the Western Spreading Ridges. One feature that distinguishes the Bismarck Sea from most other recently opened back-arc spreading systems in the western Pacific, except for the northern North Fiji and Lau basins, is the anomalously large distance (more than 400 km) between the arc and the spreading axis. This reflects the complex, combined geodynamic processes in this region, i.e., arc-continent collision, mantle upwelling, and rapid back-arc opening specific to this region. Central Bismarck Sea lavas are also unusual as they contain chemical components characteristic of the lower mantle. This may reflect proximity to the Melanesian Borderland, long a site of major subduction, and therefore of upper mantle enrichment by subduction-derived components, compared to other spreading systems.

Structural evolution of the northern East China Sea Shelf Basin interpreted from cross-section restoration

The northern East China Sea Shelf Basin consists of three depressions (the Domi, Jeju, and Socotra Depressions), separated by basement highs or rises. Reconstruction of depth-converted seismic reflection profiles from these depressions reveals that the northern East China Sea Shelf Basin experienced two phases of rifting, followed by regional subsidence. Initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the plate margin. Major extension (~15 km) took place during the early phase of basin formation. The initial rifting was terminated by regional uplift in the Late Eocene-Early Oligocene, which was probably due to reorganization of plate boundaries. Rifting resumed in the Early Oligocene; the magnitude of extension was mild (<1 km) during this period. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 m of sediments and basement rock were removed by erosion during and after the second phase of uplift. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Subsequent erosion removed about 900 m of sediments. The regional subsidence has dominated the area since the Late Miocene.

Pacific‐North America plate boundary reorganization in response to a change in relative plate motion: Offshore Canada

The transition from subduction in Cascadia to the transform Queen Charlotte fault along western Canada is often drawn as a subduction zone, yet recent studies of GPS and earthquake data from northern Vancouver Island are not consistent with that model. In this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of (1) microplate subduction and (2) reorganization of the preexisting strike‐slip plate boundary. We focus on the critical region of outer Queen Charlotte Sound and the adjacent offshore. On much of the continental shelf, several million years of subsidence above thin crust are a counterindicator for subduction. An undated episode of compression uplifted the southernmost shelf, but subsidence patterns offshore show that recent subduction is unlikely to be responsible. Previously unremarked near‐vertical faults and a mix of extensional and compressional faults offshore indicate that strike‐slip faulting has been a significant mode of deformation. Seismicity in the last 18 years is dominantly strike‐slip and shows large amounts of moment release on the Revere‐Dellwood fault and its overlap with the Queen Charlotte fault. The relative plate motion between the Pacific and North American plates rotated clockwise ∼6 Ma and appears to have triggered formation of an evolving array of structures. We suggest that the paleo‐Queen Charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed Revere Dellwood fault propagated to the northwest.

Episodic uplift and exhumation along North Atlantic passive margins: implications for hydrocarbon prospectivity

Abstract We present observations that demonstrate that the elevated passive margins around the North Atlantic were formed by episodic, post-rift uplift movements that are manifest in the high-lying peneplains that characterize the coastal mountains, in the unconformities in the adjacent sedimentary basins and in accelerated subsidence in the basin centres. Results from West Greenland show that subsidence of the rifted margin took place for c . 25 Myr after rifting and breakup in the Paleocene, as predicted by classical rift theory, but that this development was reversed by a series of uplift movements (starting at c . 35, 10 and 5 Ma) that remain unexplained. East Greenland and Scandinavia seem to have had a similar evolution of post-rift subsidence followed by uplift starting at c . 35 Ma. There was no notable fall in sea-level at this time, so the subsiding basins must have been inverted by tectonic forces. We speculate that the forces causing this phase were related to the plate boundary reorganization in the North Atlantic around Chron 13 time. One feature that these areas have in common is that uplift took place along the edges of cratons where the thickness of the crust and lithosphere changes substantially over a short distance. It may be that the lateral contrasts in the properties of the stretched and unstretched lithosphere make the margins of the cratons unstable long after rifting. These vertical movements have profound influence on hydrocarbon systems, not only in frontier areas such as West and East Greenland, where Mesozoic basins are deeply truncated and exposed onshore, but also for the understanding of near-shore hydrocarbon deposits in mature areas such as the North Sea Basin, where low-angular unconformities may represent episodes of deposition and removal of significant sedimentary sections.

Treitel Ridge: A unique inside corner hogback on the west flank of extinct Aegir spreading ridge, Norway basin

Detailed shipborne mapping (sidescan sonar, swath bathymetry, seismic reflection, 3.5 kHz profiling, gravity and magnetics) of the extinct Aegir spreading axis (ca.55–25 Ma) in the Norway Basin revealed a highly unusual (in a MOR setting), narrow, 130 km long, asymmetrical basement hogback (Treitel Ridge; TR) on the southern west flank of the extinct rift, close to its junction with the Iceland–Faeroe aseismic ridge and the putative paleo-Iceland hotspot center. The ridge seems to continue north another 130 km in the form of a wider, more irregular basement rise, for a combined length of ca. 260 km. The southern, narrow part of TR, approximately following or slightly younger than chron 18n (ca. 39 Ma), attains a basement relief of ca. 1000 m in the south, its summit descending gradually (1.7:100) towards the NE. Although an east-flank conjugate appears generally lacking along both northern and southern parts of TR, asymmetry of the southern portion (steeper slope facing towards the extinct rift valley) suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary. The possibility of continental crust extending close to the southern end of TR may also be important, because some of this crust may have been remelted at depth, contributing magma to feed TR eruptions. Southern TR may record a brief (0.1–1 m.y.) inside-corner tectonic event (probably a flexurally compensated normal-faulted offset), in effect a “mega-abyssal hill”. We suggest TR was probably not an expression of change in Iceland hotspot activity or somehow related to nearby continental crust, but rather represents the volcano-tectonic response of the Aegir accretion axis–at the time it became the eastern edge of the new Jan Mayen micro-plate–to extinction of the Mid-Labrador Sea spreading center. As proposed by Nunns (1983), the annexation of the Greenland plate by the North American plate forced a plate boundary reorganization east of Greenland—with northward propagation of the Mid-Atlantic Ridge rift along the Greenland margin to form the Jan Mayen micro-plate. Simultaneously, two ca. 107° trending transforms developed from the previously 146° trending Faeroes Fracture Zone. Both TR formation and fracture zone trend changes may record Aegir Ridge response to this major plate kinematic reorganization. Because TR was formed during a short interval, spreading along the Mid-Labrador Sea ridge may have ceased abruptly as well.

Palaeocene–Recent plate boundaries in the NE Atlantic and the formation of the Jan Mayen microcontinent

Abstract: Breakup and sea-floor spreading between Greenland and Eurasia established a series of new plate boundaries in the North Atlantic region since the Late Palaeocene. A conventional kinematic model from pre-breakup to the present day assumes that Eurasia and Greenland moved apart as a two-plate system. However, new regional geophysical datasets and quantitative kinematic parameters indicate that this system underwent several adjustments since its inception and suggest that additional short-lived plate boundaries existed in the NE Atlantic. Among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented Jan Mayen microcontinent and subsequent deformation of its margins and surrounding regions. The major Oligocene plate boundary reorganization (and microcontinent formation) might have been precluded by various ridge propagations and/or short-lived triple junctions NE and possibly SW of the Jan Mayen microcontinent from the inception of sea-floor spreading (54 Ma) to C18 (40 Ma). Our model implies a series of failed ridges offshore the Faeroe Islands, a northern propagation of the Aegir Ridge NE of the Jan Mayen microcontinent, and a series of triple junctions and/or propagators in the southern Greenland Basin.

Plate boundary reorganization in the active Banda Arc–continent collision: Insights from new GPS measurements

New GPS measurements reveal that large sections of the SE Asian Plate are progressively accreting to the edge of the Australian continent by distribution of strain away from the deformation front to forearc and backarc plate boundary segments. The study was designed to investigate relative motions across suspected plate boundary segments in the transition from subduction to collision. The oblique nature of the collision provides a way to quantify the spatial and temporal distribution of strain from the deformation front to the back arc. The 12 sites we measured from Bali to Timor included some from an earlier study and 7 additional stations, which extended the epoch of observation to ten years at many sites. The resulting GPS velocity field delineates at least three Sunda Arc–forearc regions around 500 km in strike-length that shows different amounts of coupling to the Australian Plate. Movement of these regions relative to SE Asia increases from 21% to 41% to 63% eastward toward the most advanced stages of collision. The regions are bounded by the deformation front to the south, the Flores–Wetar backarc thrust system to the north, and poorly defined structures on the sides. The suture zone between the NW Australian continental margin and the Sunda–Banda Arcs is still evolving with more than 20 mm/yr of movement measured across the Timor Trough deformation front between Timor and Australia.

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

The Blanco Transform Fault Zone (BTFZ) forms the ∼350 km long Pacific–Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long‐term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along‐fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull‐apart basin. The longest transform (∼150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone (∼9 km) than other BTFZ transform segments and accommodates the largest (Mw 6.4–6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull‐apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional (≤15%) compared to transform areas (35–100%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.