scholarly journals Structure of oceanic crust in back-arc basins modulated by mantle source heterogeneity

Geology ◽  
2020 ◽  
Author(s):  
Ingo Grevemeyer ◽  
Shuichi Kodaira ◽  
Gou Fujie ◽  
Narumi Takahashi
Keyword(s):  
Oceanic Crust ◽  
Mantle Source ◽  
Seismic Data ◽  
Lower Crust ◽  
Subduction Zones ◽  
P Wave ◽  
Spreading Ridge ◽  
Volcanic Arc ◽  
Depleted Mantle ◽  
Back Arc

Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4–5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5–6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ~7 km thick, and the lower crust supports seismic velocities of 7.2–7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge–type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust.

Lithospheric architecture of the Ligurian Basin from seismic travel time tomography

2021 ◽  
Author(s):  
Anke Dannowski ◽  
Heidrun Kopp ◽  
Ingo Grevemeyer ◽  
Grazia Caielli ◽  
Roberto de Franco ◽  
...  
Keyword(s):  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Central Basin ◽  
Uppermost Mantle ◽  
Mantle Boundary ◽  
Oceanic Spreading ◽  
Refraction Seismic ◽  
Back Arc ◽  
Ligurian Basin

<p>The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast retreat of the Apennines-Calabrian subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning little is known about the style of back-arc rifting. Today, seismicity indicates the closure of this back-arc basin. In the basin, earthquake clusters occur in the lower crust and uppermost mantle and are related to re-activated, inverted, normal faults created during rifting.</p><p>To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. An amphibious refraction seismic profile was shot across the Ligurian Basin in an E-W direction from the Gulf of Lion to Corsica. The profile comprises 35 OBS and three land stations at Corsica to give a complete image of the continental thinning including the necking zone.</p><p>The majority of the refraction seismic data show mantle phases with offsets up to 70 km. The arrivals of seismic phases were picked and used to generate a 2-D P-wave velocity model. The results show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The P-wave velocities in the crust reach 6.6 km/s at the base. The uppermost mantle shows velocities >7.8 km/s. The crust-mantle boundary becomes shallower from ~18 km to ~12 km depth within 30 km from Corsica towards the basin centre. The velocity model does not reveal an axial valley as expected for oceanic spreading. Further, it is difficult to interpret the seismic data whether the continental lithosphere was thinned until the mantle was exposed to the seafloor. However, an extremely thinned continental crust indicates a long lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. The distribution of earthquakes and their fault plane solutions, projected along our seismic velocity model, is in-line with the counter-clockwise opening of the Ligurian Basin.</p>

scholarly journals Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark: Four Episodes of magmatism on the margin of the Baltic Shield

2010 ◽  
Vol 58 ◽  
pp. 35-65
Author(s):  
Paul Martin Holm ◽  
L.E. Pedersen, ◽  
B Højsteen
Keyword(s):  
Mantle Plume ◽  
Mantle Source ◽  
Small Degree ◽  
Spinel Peridotite ◽  
Alkali Basalts ◽  
Enriched Mantle ◽  
Depleted Mantle ◽  
Proterozoic Basement ◽  
The Baltic ◽  
Back Arc

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb < 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

scholarly journals Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)

2010 ◽  
Vol 61 (4) ◽  
pp. 273-292 ◽  
Author(s):  
Damir Slovenec ◽  
Boško Lugović ◽  
Irena Vlahović
Keyword(s):  
Mantle Source ◽  
Spreading Ridge ◽  
Accretionary Wedge ◽  
Carbonate Sediments ◽  
Basaltic Rocks ◽  
Depleted Mantle ◽  
Unit Form ◽  
Ophiolite Mélange ◽  
Internal Dinarides ◽  
Ophiolite Melange

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the mélange, include Middle-Triassic (Illyrian-Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian-Lower Callovian) tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystallization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compositions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V= 62.2-82.4; (La/Lu)cn= 6.4-12.8] with Nd-Sr isotope signatures close resembling the Bulk Earth [εNd(T=235 Ma)= + 1.6 to + 2.5]. They are recognized as preophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the Meliata-Maliac ocean system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted mantle source (εNd(T=165 Ma) = + 6.01 vs. + 6.35) succesively metasomatized by expulsion of fluids from a subducting slab leading to a more pronounced subduction signature in the latter [Ti/V=31.6-44.8 and (Nb/La)n=0.67-0.90 vs. Ti/V=21.5-33.9 and (Nb/La)n=0.32-0.49]. These composite blocks indicate crust formation in an extensional basin spreading over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed at a spreading ridge and incorporated in the mélange of the accretionary wedge formed in the proto-arc-fore-arc region. The Mt Samoborska Gora ophiolite mélange represents the trailing edge of the Kalnik Unit as a discrete sector that records the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.

scholarly journals Crustal structure and evolution of the Niuafo'ou Microplate in the northeastern Lau Basin, Southwestern Pacific

2020 ◽  
Author(s):  
Florian Schmid ◽  
Heidrun Kopp ◽  
Michael Schnabel ◽  
Anke Dannowski ◽  
Ingo Heyde ◽  
...  
Keyword(s):  
Gravity Data ◽  
Seafloor Spreading ◽  
P Wave ◽  
Spreading Center ◽  
Plate Boundaries ◽  
Volcanic Arc ◽  
Lau Basin ◽  
Refraction Seismic ◽  
Wave Tomography ◽  
Back Arc

&lt;p&gt;The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is fairly well understood, the structure and evolution of the back-arc crust are not. We present refraction seismic, multichannel seismic and gravity data from a 300 km long east-west oriented transect crossing the Niuafo&amp;#8217;ou Microplate (back-arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17&amp;#176;20&amp;#8217;S. Our P wave tomography model shows strong lateral variations in the thickness and velocity-depth distribution of the crust. The thinnest crust is present in the Fonualei Rift and Spreading Center, suggesting active seafloor spreading there. In the much thicker crust of the volcanic arc we identify a region of anomalously low velocities, indicative of partial melts. Surprisingly, the melt reservoir is located at ~17 km distance to the volcanic front, supporting the hypothesis that melts are deviated from the volcanic arc towards the FRSC in sub-crustal domains. We identify two distinct regions in the back-arc crust, representing different opening phases of the northeastern Lau Basin. During initial extension, likely dominated by rifting, crust of generally lower upper-crustal velocities formed. During an advanced opening phase, likely dominated by seafloor spreading, crust of higher upper-crustal velocities formed and is now up to 11 km thick. This thickening is the result of magmatic underplating, which is supported by elevated upper mantle temperatures in this region.&lt;/p&gt;

scholarly journals Tectonic tremor and deep slow slip on the Alpine Fault

2021 ◽  
Author(s):  
A Wech ◽  
C Boese ◽  
Timothy Stern ◽  
John Townend
Keyword(s):  
Lower Crust ◽  
Subduction Zones ◽  
Plate Boundary ◽  
P Wave ◽  
Plate Boundaries ◽  
Depth Range ◽  
Slow Slip ◽  
High Attenuation ◽  
Alpine Fault ◽  
Tectonic Tremor

Tectonic tremor is characterized by persistent, low-frequency seismic energy seen at major plate boundaries. Although predominantly associated with subduction zones, tremor also occurs along the deep extension of the strike-slip San Andreas Fault. Here we present the first observations of tectonic tremor along New Zealand's Alpine Fault, a major transform boundary that is late in its earthquake cycle. We report tectonic tremor that occurred on the central section of the Alpine Fault on 12days between March 2009 and October 2011. Tremor hypocenters concentrate in the lower crust at the downdip projection of the Alpine Fault; coincide with a zone of high P-wave attenuation (low Q p) and bright seismic reflections; occur in the 25-45km depth range, below the seismogenic zone; and may define the deep plate boundary structure extending through the lower crust and into the upper mantle. We infer this tremor to represent slow slip on the deep extent of the Alpine Fault in a fluid-rich region marked by high attenuation and reflectivity. These observations provide the first indication of present-day displacement on the lower crustal portion of the Australia-Pacific transform plate boundary. © Copyright 2012 by the American Geophysical Union.

Tectonic and magmatic evolution of the Aqishan-Yamansu belt: A Paleozoic arc-related basin in the Eastern Tianshan (NW China)

2020 ◽  
Author(s):  
Shuanliang Zhang ◽  
Huayong Chen ◽  
Pete Hollings ◽  
Liandang Zhao ◽  
Lin Gong
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Crust ◽  
Early Carboniferous ◽  
Late Carboniferous ◽  
Igneous Rocks ◽  
Calc Alkaline ◽  
Eastern Tianshan ◽  
Depleted Mantle ◽  
T Values

The Aqishan-Yamansu belt in the Chinese Eastern Tianshan represents a Paleozoic arc-related basin generally accompanied by accretionary magmatism and Fe-Cu mineralization. To characterize the tectonic evolution of such an arc-related basin and related magmatism and metallogenesis, we present a systematic study of the geochronology, whole-rock geochemistry, and Sr-Nd isotopes of igneous rocks from the belt. New zircon U-Pb ages, in combination with published data, reveal three phases of igneous activity in the Aqishan-Yamansu belt: early Carboniferous felsic igneous rocks (ca. 350−330 Ma), late Carboniferous intermediate to felsic igneous rocks (ca. 320−305 Ma), and Permian quartz diorite and diorite porphyry dikes (ca. 280−265 Ma). The early Carboniferous felsic rocks are enriched in large ion lithophile elements (LILEs) and depleted in Nb, Ta, and Ti, showing arc-related magma affinities. Their positive εNd(t) values (3.3−5.9) and corresponding depleted mantle model ages (TDM) of 0.83−0.61 Ga, as well as high MgO contents, Mg# values, and Nb/Ta ratios, suggest that they were derived from lower crust with involvement of mantle-derived magmas. The late Carboniferous intermediate igneous rocks show calc-alkaline affinities, exhibiting LILE enrichment and high field strength element (HFSE) depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg# values with positive εNd(t) values (3.9−7.9), and high Ba/La and Th/Yb ratios, implying a depleted mantle source metasomatized by slab-derived fluids and sediment or sediment-derived melts. The late Carboniferous felsic igneous rocks are metaluminous to peraluminous with characteristics of medium-K calc-alkaline I-type granites. Given the positive εNd(t) values (6.3−6.6) and TDM ages (0.56−0.53 Ga), we suggest the late Carboniferous felsic igneous rocks were produced by partial melting of a juvenile lower crust. The Permian dikes show characteristics of adakite rocks. They have relatively high MgO contents and Mg# values, and positive εNd(t) values (7.2−8.5), which suggest an origin from partial melting of a residual basaltic oceanic crust. We propose that the Aqishan-Yamansu belt was an extensional arc−related basin from ca. 350 to 330 Ma; this was followed by a relatively stable carbonate formation stage at ca. 330−320 Ma, when the Kangguer oceanic slab subducted beneath the Central Tianshan block. As the subduction continued, the Aqishan-Yamansu basin closed due to slab breakoff and rebound during ca. 320−305 Ma, which resulted in basin inversion and the emplacement of granitoids with contemporary Fe-Cu mineralization. During the Permian, the Aqishan-Yamansu belt was in postcollision extension stage, with Permian adakitic dikes formed by partial melting of a residual oceanic crust.

scholarly journals Workshop report on hard-rock drilling into mid-Cretaceous Pacific oceanic crust on the Hawaiian North Arch

2019 ◽  
Vol 26 ◽  
pp. 47-58
Author(s):  
Tomoaki Morishita ◽  
Susumu Umino ◽  
Jun-Ichi Kimura ◽  
Mikiya Yamashita ◽  
Shigeaki Ono ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Lower Crust ◽  
Velocity Structure ◽  
Spreading Rate ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Uppermost Mantle ◽  
Magma Intrusion ◽  
Workshop Report ◽  
Geophysical Surveys

Abstract. The architecture, formation, and modification of oceanic plates are fundamental to our understanding of key geologic processes of the Earth. Geophysical surveys were conducted around a site near the Hawaiian Islands (northeastern Hawaiian North Arch region; Hawaiian North Arch hereafter), which is one of three potential sites for an International Ocean Discovery Program mantle drilling proposal for the Pacific plate that was submitted in 2012. The Hawaiian North Arch site is located in 78–81 Ma Cretaceous crust, which had an estimated full spreading rate of 7–8 cm yr−1. This site fills a major gap in our understanding of oceanic crust. Previously drilling has been skewed to young or older crust (<15 or >110 Ma) and slow-spread crust. P-wave velocity structure in the uppermost mantle of the Hawaiian North Arch shows a strong azimuthal anisotropy, whereas Moho reflections below the basement are variable: strong and continuous, weak, diffuse, or unclear. We assume that the strength of the Moho reflection is related to the aging of the oceanic plate. The Hawaiian volcanic chain (200 km to the southwest of the proposed drill site) and the nearby North Arch magmatism on the proposed Hawaiian North Arch sites might also have affected recognition of the Moho via deformation and/or magma intrusion into the lower crust of the uppermost mantle. This workshop report describes scientific targets for 2 km deep-ocean drilling in the Hawaiian North Arch region in order to provide information about the lower crust from unrecovered age and spreading rate gaps from previous ocean drillings. Other scientific objectives to be achieved by drilling cores before reaching the target depth of the project are also described in this report.

scholarly journals A Crustal Control on the Fe Isotope Systematics of Volcanic Arcs Revealed in Plutonic Xenoliths From the Lesser Antilles

2022 ◽  
Vol 9 ◽  
Author(s):  
G. F. Cooper ◽  
E. C. Inglis
Keyword(s):  
Trace Element ◽  
Mantle Source ◽  
Subduction Zones ◽  
Primary Objective ◽  
Chemical Diversity ◽  
Radiogenic Isotopes ◽  
Lesser Antilles ◽  
Volcanic Arc ◽  
Magmatic Processes ◽  
Isotope Systematics

Lavas produced at subduction zones represent the integration of both source heterogeneity and an array of crustal processes, such as: differentiation; mixing; homogenisation; assimilation. Therefore, unravelling the relative contribution of the sub-arc mantle source versus these crustal processes is difficult when using the amalgamated end products in isolation. In contrast, plutonic xenoliths provide a complementary record of the deeper roots of the magmatic plumbing system and provide a unique record of the true chemical diversity of arc crust. Here, we present the δ56Fe record from well characterised plutonic xenoliths from two distinct volcanic centres in the Lesser Antilles volcanic arc–the islands of Martinique and Statia. The primary objective of this study is to test if the Fe isotope systematics of arc lavas are controlled by sub-arc mantle inputs or during subsequent differentiation processes during a magma’s journey through volcanic arc crust. The Fe isotopic record, coupled to petrology, trace element chemistry and radiogenic isotopes of plutonic xenoliths from the two islands reveal a hidden crustal reservoir of heavy Fe that previously hasn’t been considered. Iron isotopes are decoupled from radiogenic isotopes, suggesting that crustal and/or sediment assimilation does not control the Fe systematics of arc magmas. In contrast to arc lavas, the cumulates from both islands record MORB-like δ56Fe values. In Statia, δ56Fe decreases with major and trace element indicators of differentiation (SiO2, Na2O + K2O, Eu/Eu*, Dy/Yb), consistent with fractionating mineral assemblages along a line of liquid descent. In Martinique, δ56Fe shows no clear relationship with most indicators of differentiation (apart from Dy/Yb), suggesting that the δ56Fe signature of the plutonic xenoliths has been overprinted by later stage processes, such as percolating reactive melts. Together, these data suggest that magmatic processes within the sub-arc crust overprint any source variation of the sub-arc mantle and that a light Fe source is not a requirement to produce the light Fe isotopic compositions recorded in volcanic arc lavas. Therefore, whenever possible, the complimentary plutonic record should be considered in isotopic studies to understand the relative control of the mantle source versus magmatic processes in the crust.

Petrogenesis of the Ulungur Intrusive Complex, NW China, and Implications for Crustal Generation and Reworking in Accretionary Orogens

2020 ◽  
Vol 61 (2) ◽  
Author(s):  
Gong-Jian Tang ◽  
Qiang Wang ◽  
Derek A Wyman ◽  
Wei Dan ◽  
Lin Ma ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Lower Crust ◽  
Mineral Chemistry ◽  
Isotopic Signature ◽  
Source Rocks ◽  
Intrusive Complex ◽  
Calc Alkaline ◽  
Depleted Mantle ◽  
Heavy Rare Earth Elements ◽  
Pressure Crystallization

Abstract Accretionary orogens are characterized by voluminous juvenile components (recently derived from the mantle) and knowing the origin(s) of such components is vital for understanding crustal generation. Here we present field and petrological observations, along with mineral chemistry, zircon U–Pb age and Hf–O isotope data, and whole rock geochemical and Sr–Nd isotopic data for the c.320 Ma Ulungur intrusive complex from the Central Asian Orogenic Belt. The complex consists of two different magmatic series: one is characterized by medium- to high-K calc-alkaline gabbro to monzogranite; the other is defined by peralkaline aegirine–arfvedsonite granitoids. The calc-alkaline and peralkaline series granitoids have similar depleted mantle-like Sr–Nd–Hf isotopic compositions, but they have different zircon δ18O values: the calc-alkaline series have mantle-like δ18O values with mean compositions ranging from 5·2 ± 0·5‰ to 6·0 ± 0·9‰ (2SD), and the peralkaline granitoids have low δ18O values ranging from 3·3 ± 0·5‰ to 3·9 ± 0·4‰ (2SD). The calc-alkaline series were derived from a hydrous sub-arc mantle wedge, based on the isotope and geochemical compositions, under garnet peridotite facies conditions. This study suggests that the magmas underwent substantial differentiation, ranging from high pressure crystallization of ultramafic cumulates in the lower crust to lower pressure crystallization dominated by amphibole, plagioclase and minor biotite in the upper crust. The peralkaline series rocks are characterized by δ18O values lower than the mantle and enrichment of high field strength elements (HFSEs) and heavy rare earth elements (HREEs). They likely originated from melting of preexisting hydrothermally altered residual oceanic crust in the lower crust of the Junggar intra-oceanic arc. Early crystallization of clinopyroxene and amphibole was inhibited owing to their low melting temperature, leading to HFSEs and HREEs enrichment in residual peralkaline melts during crystallization of a feldspar-dominated mineral assemblage. Thus, the calc-alkaline and peralkaline series represent episodes of crust generation and reworking, respectively, demonstrating that the juvenile isotopic signature in accretionary orogens can be derived from diverse source rocks. Our results show that reworking of residual oceanic crust also plays an important role in continental crust formation for accretionary orogens, which has not previously been widely recognized.

scholarly journals Seismic investigation of an active ocean–continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

2019 ◽  
Vol 219 (1) ◽  
pp. 159-184 ◽  
Author(s):  
C Peirce ◽  
A H Robinson ◽  
A M Campbell ◽  
M J Funnell ◽  
I Grevemeyer ◽  
...  
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Fault Zone ◽  
Lower Crust ◽  
P Wave ◽  
Fault System ◽  
Transform Fault ◽  
Apparent Lack ◽  
The North ◽  
Transform Margin

SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at &gt;2 km subseabed, by a ∼20-km-wide region of density &gt;3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.

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