South Shetland Trench Research Articles

A seismic swarm at the Bransfield Rift, Antarctica

The Brasfield rift and South Shetland trench are part of the active margin of the Antarctic continent. From the records of a seismic station at the King George Island installed by the Observatorio Geofísico del Uruguay, 3186 seismic events during August 26 and September 11, 2020 were retrieved with an average of 187 earthquakes per day and a maximum of 359 on August 30. 217 events with a local magnitude of ≥4 were well recognized and located by a single-station method. The events show that an earthquake swarm close to the South Shetland Islands and at a short hypocentral distance (~25 km) to the station occurred in August of 2020. The earthquakes in the swarm are near Orca seamount and the listric normal faults, with a ENE-WSW trend of the Bransfield Basin, suggesting their volcano and volcanotectonic origin. In addition to the swarm, several earthquakes in the same time period related to the Artigas Fault with NW-SE trend are also located, which indicates the reactivation of this structure. From all the earthquakes, a relatively high b-value (1.22) is estimated, which might suggest variable resistance for stress accumulation in the Bransfield rift.

Cretaceous termination of subduction at the Zealandia margin of Gondwana: The view from the paleo-trench

During the Permian to Cretaceous, Zealandia occupied a position on the proto-Pacific-facing, convergent margin of Gondwana. Subduction on this margin ceased somewhere between ~105 Ma and perhaps 70 Ma, but the timing of this tectonic transition remains controversial. Resolution of this uncertainty is important for tectonic reconstructions of the southwest Pacific and for global plate-tectonic models. Here, we revisit the problem by reference to new stratigraphic and geochemical data from the East Coast Basin of New Zealand, which occupied a position adjacent to and possibly superimposed on the relict Cretaceous subduction trench at the time subduction ceased; this basin is expected to preserve direct structural and stratigraphic evidence for or against Late Cretaceous subduction.In western parts of the East Coast Basin – the “Western Sub-belt” – a conspicuous unconformity separates undoubted accretionary prism rocks of the Torlesse Composite Terrane from younger Cretaceous “cover” units. Strata beneath and overlying this unconformity vary in age from place to place, but abundant paleontological data and published ages of detrital zircons (some reinterpreted herein), indicate that exposed Torlesse rocks are nowhere younger than ~100 Ma. Overlying Zealandia Megasequence “cover” strata are mostly younger than ~110 Ma. Between ~110 and ~85 Ma, the entire length of the Western Sub-belt reveals complex stratigraphic architecture of relatively small-scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion. There is widespread evidence for compression on the Western Sub-belt over this period, although the overall amount of shortening is likely to be relatively modest. In contrast, the Eastern Sub-belt preserves a record of near-continuous and apparently simple deposition over the same interval of time. We see little and somewhat equivocal evidence for extension in either sub-belt through the Late Cretaceous. Superimposed on this general pattern, we observe discrete, widespread or basin-wide, tectonic events at 95 Ma, and within the intervals ~86–83 Ma and ~83–81 Ma, indicated by the presence of unconformities and sedimentary facies changes. Importantly, all these events can be correlated with coeval events recorded elsewhere in Zealandia, suggesting that the East Coast Basin was subject to the same tectonic regime as the rest of Zealandia and shared a common history during the mid- to Late Cretaceous.Integrating these findings with data from elsewhere in Zealandia, we argue that there is diverse evidence to indicate that subduction finished along the New Zealand segment of the Gondwana margin by 100 Ma. That said, the causes of on-going, Late Cretaceous compression in the East Coast Basin and in other parts of Zealandia are not well constrained, although several possible explanations are plausible. There are few analogue, abandoned subduction margins described in the literature; however, the situation in the South Shetland Trench shows strong similarities with patterns and complexities observed in the East Coast Basin.

Lithospheric structure of an incipient rift basin: Results from receiver function analysis of Bransfield Strait, NW Antarctic Peninsula

Bransfield Basin (BB), located northwest of the Antarctic Peninsula (AP) and southeast of the South Shetland Islands (SSI), is the most active section of the Antarctic continental margin. The region has long been (50 Ma) a convergent plate boundary where the Phoenix plate was subducting beneath the Antarctic Plate and is characterized by long-lived arc magmatism and accretion. However, the collision of the Antarctic-Phoenix spreading center with the subduction front near SSI (ca. 4 Ma) gave way to the opening of slab windows and dramatic decrease in the subduction rate of the Phoenix plate beneath AP and SSI. Consequently, the Phoenix slab began to rollback slowly along the South Shetland Trench (SST), giving way to slow extension in the back-arc region and rifting along the BB. Although there is consensus on the factors that control the current deformation and extension of the BB, the origin of the BB and the tectonic configuration of the basin are still unclear. Most of the controversy stems from uncertainties regarding the crustal thickness of the BB. Hence, we computed teleseismic receiver functions for 10 broadband stations in the region that belong to existing permanent and temporary deployments in order obtain robust constraints on the lithospheric structure and crustal thickness of the BB, as well as the AP and SSI. Our results indicate that the crust is thinning from 30 km to 26 km from the AP towards the South Shetland trench and Central BB showing the asymmetrical character of the rift basin. The crustal thickness and Vp/Vs variations are less pronounced along the AP but very significant across the SSB indicating the lithospheric scale segmentation of the South Shetland Block (SSB) and the incipient rift basin under the control of the opening of slab window and the roll-back of stalled Phoenix slab. High Vp/Vs ratios (∼1.9) beneath BB and SSI, agree well with the nascent rift character of BB, the presence of a steep Phoenix slab and consequently a wider mantle wedge characterized by the presence of underplating partial melts beneath SSI and BB.

Structure of the Bransfield strait crust

Data on the high heat flow, active volcanism, and extensional folding, in combination with modeling of the gravity and magnetic anomalies and earthquake focal mechanisms, indicate that the Bransfield strait floor represents a zone of lithosphere extension forming in the Antarctic Peninsula. The most important structural element of the strait floor is the neovolcanic zone near the South Shetland Islands and the diapirism zone near the Antarctic Peninsula. The discussed stages of rifting of the Bransfield floor reflect the propagating of the American-Antarctic Ridge into the continental lithosphere of the Antarctic Peninsula. The propagating causes thrusting of the South Shetland Island arc onto the peripheral part of the relict Phoenix plate, which is accompanied by the seismicity in the South Shetland trench.

Tectonomagmatic activity and ice dynamics in the Bransfield Strait back‐arc basin, Antarctica

An array of moored hydrophones was used to monitor the spatiotemporal distribution of small‐ to moderate‐sized earthquakes and ice‐generated sounds within the Bransfield Strait, Antarctica. During a 2 year period, a total of 3900 earthquakes, 5925 icequakes and numerous ice tremor events were located throughout the region. The seismic activity included eight space‐time earthquake clusters, positioned along the central neovolcanic rift zone of the young Bransfield back‐arc basin. These sequences of small magnitude earthquakes, or swarms, suggest ongoing magmatic activity that becomes localized along isolated volcanic features and fissure‐like ridges in the southwest portion of the basin. A total of 122 earthquakes were located along the South Shetland trench, indicating continued deformation and possibly ongoing subduction along this margin. The large number of icequakes observed show a temporal pattern related to seasonal freeze‐thaw cycles and a spatial distribution consistent with channeling of sea ice along submarine canyons from glacier fronts. Several harmonic tremor episodes were sourced from a large (∼30 km2) iceberg that entered northeast portion of the basin. The spectral character of these signals suggests they were produced by either resonance of a small chamber of fluid within the iceberg, or more likely, due to periodicity of discrete stick‐slip events caused by contact of the moving iceberg with the seafloor. These pressure waves appear to have been excited by abrasion of the iceberg along the seafloor as it passed Clarence and Elephant Islands.

Is subduction ongoing in the South Shetland Trench, Antarctic Peninsula?: new constraints from crustal structures of outer trench wall

The South Shetland Trench (SST) northwest of the South Shetland Islands is the only trench along the Pacific margin of the Antarctic Peninsula. Multichannel seismic reflection profile shows very narrow normal faulting zone (about 20 km) on the outer wall of the SST and thick trench-fill sediments (up to 1000 m) on two distinct crustal horst and graben structures in the trench. These structures are rare in the active subduction trenches. On the basis of the implications from these seismic structures and known tectonic history, the following scenarios of subduction activities of the former Phoenix plate are proposed: (1) normal faulting has not occurred since subduction rate in the SST sharply decreased after cessation of seafloor spreading at the West Scotia Ridge at about 6 Ma; and (2) subduction almost stopped after the cessation of spreading in Drake Passage at about 3.3 Ma. Recent contractional structures around the SST are indicative of current crustal shortening (not subduction) accommodating trenchward movement of South Shetland Islands caused by the ongoing extension of Bransfield Basin behind the South Shetland Islands.

SHALLOW GEOLOGICAL STRUCTURES OF THE SOUTH SHETLAND TRENCH, ANTARCTIC PENINSULA

In order to study the differential passive subduction among the three oceanic segments of the Phoenix Plate remnant, the R/V OGS-Explora acquired new geophysical data in the South Shetland Trench (offshore to the Antarctic Peninsula). An integrated geophysical dataset was acquired in the trench across the incoming oceanic plate segmented by the “D” and “E” oceanic Fracture Zones, and along the frontal part of the Antarctic Peninsula accretionary prism. We present here a high resolution multibeam bathymetry map, Chirp sub-bottom profiles and marine magnetic data to study changes in the deformational style along strike and to better understand features and processes of passive subduction. The new multibeam image and Chirp data display in detail shallow structures affecting the seafloor as a response to crustal deformation. Normal faults, bounding horst and graben structures cutting the incoming plate, are detected and characterized by variable orientation with respect to the inferred “D” and “E” Fracture Zones. Normal faults, width and depth of the trench and morphology of the frontal prism are related to bending and roll-back of the Phoenix Plate, and to inherited structural discontinuities. Some of these oblique faults offset the seafloor, with variable throws and with local dip changes. The orientation of the oblique faults, the different depth of the three oceanic segments and narrowing and deepening of the trench indicate that differential coupling between the Hero, Shackleton, “E” and “D” Fracture Zones may locally modify the regional stress field orientation and the sinking of each oceanic segment, during bending and roll-back.

Seismicity and tectonics of the South Shetland Islands and Bransfield Strait from a regional broadband seismograph deployment

We investigate the tectonics of the South Shetland Trench and Bransfield Strait by performing a detailed study of local seismicity. During 1997–1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the South Shetland Island region. The data we obtained indicate a high level of local seismicity (mb 2–5), and we accurately located ∼150 earthquakes. Many of the earthquakes occur at locations and depths indicative of ongoing subduction in the South Shetland trench. A focal mechanism for the largest event in the forearc indicates shallow angle thrusting. The maximum depth of seismicity is ∼65 km, but the majority of the events are shallower than 30 km. These seismic results are consistent with recent magnetic, GPS, and multichannel seismic reflection data that suggest continued subduction at a very slow rate. The South Shetland trench thus represents an extreme end‐member of hot subduction resulting from slow convergence of young lithosphere, and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths. We have located many earthquakes associated with volcanism and rifting in Bransfield Strait. A swarm of events near a submarine volcano suggests current magmatic activity. A normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension. Earthquakes associated with rifting in the northeastern portion of the strait are clustered along well‐established rifts, but the seismicity is much more diffuse to the southwest. This observation is consistent with other evidence that extension has propagated from northeast to southwest.

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.

Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin

Research Article| November 01, 2000 Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin José M. González-Casado; José M. González-Casado 1Departamento de Química Agrícola, Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain Search for other works by this author on: GSW Google Scholar Jorge L. Giner Robles; Jorge L. Giner Robles 1Departamento de Química Agrícola, Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain Search for other works by this author on: GSW Google Scholar Jerónimo López-Martínez Jerónimo López-Martínez 1Departamento de Química Agrícola, Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (11): 1043–1046. https://doi.org/10.1130/0091-7613(2000)28<1043:BBAPNA>2.0.CO;2 Article history received: 06 Apr 2000 rev-recd: 17 Aug 2000 accepted: 21 Aug 2000 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 José M. González-Casado, Jorge L. Giner Robles, Jerónimo López-Martínez; Bransfield Basin, Antarctic Peninsula: Not a normal backarc basin. Geology 2000;; 28 (11): 1043–1046. doi: https://doi.org/10.1130/0091-7613(2000)28<1043:BBAPNA>2.0.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 The Bransfield Basin, a marginal basin located northwest of the Antarctic Peninsula, has been habitually considered as a backarc basin associated with the rollback process that took place along an inactive plate boundary, the South Shetland Trench, where the Antarctic and the Phoenix plates meet. New geophysical and structural data discussed in this paper show that the basin opening is related to a sinistral simple-shear couple between the Scotia and the Antarctic plates, and not to the previously suggested rollback mechanism. The widening of the Bransfield Basin and the lack of trench retreat are causing compression in the South Shetland Islands. Two different neotectonic stress directions, with interchanged stress axes, are found in the area of the South Shetland block. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula

Multichannel seismic data from Bransfield Strait, a young (&lt;4 Ma?) marginal basin located at the NW tip of the Antarctic Peninsula, reveal a complex array of rifting styles short of clearly defined seafloor spreading. The strait is undergoing basinwide extension. While enriched mid‐ocean ridge basalts have been dredged from topographic highs in the NE part of the basin, seismic evidence does not support the formation of oceanic crust there. Acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor; their distribution and style support propagation of volcanism and associated extension from NE to SW. Active extensional structures along both margins control a strong physiographic asymmetry. The South Shetland Islands margin is composed of a few, closely spaced, large‐offset normal faults; the gentler Antarctic Peninsula margin exhibits broader, distributed extension. To the NE, normal fault polarity reversal structures (NFPRs) dominate; listric faults sole into NW dipping, low‐angle reflectors interpreted as detachments. To the SW, larger NFPRs and convex‐up geometries up to ∼20 km across suggest broad uplift rather than detachment‐based extension. Such uplift may precedes seafloor volcanism and rapid associated deflation along detachments. NW‐SE trending accommodation zones also segment the basin along‐strike. Bransfield Strait extension may result both from plate boundary tectonics at the South Shetland Trench and at the evolving Antarctic‐Scotia plate boundary. Extension was probably initiated by onset of subduction rollback at the trench, as a slab window opened from the SW. Augmented extension in the NE part of the basin may be a response to left‐lateral transtension associated with reorganization of the triple junction at the Shackleton Fracture Zone‐South Scotia Ridge intersection.

Gravity models for the South Shetland Trench and the Shackleton Fracture Zone, Antarctica

Four deep crustal models of the South Shetland Trench (SST) and the Shackleton Fracture Zone (SFZ) off the northern Antarctic Peninsula are presented based on gravity data. The gravity models of the SST suggest that the dip of the subducting crust increases from southwest to northeast ranging from 25° in Line KSL93-5 to 30° in Line KSL93-6 as the age of crust increases along the trench axis. The gravity low observed near the island are is directly associated with the deep forearc basin bounded seaward by a large fault. In the SFZ, the thin crust is concentrated primarily beneath the ridge, where Moho shallows by 2.5 km. A low-density material (about 2.45 g/cm3) assigned to the SFZ ridge is presumably due to serpentinite intrusion. The gravity low associated with the trough is due to the relatively low-density sediments filled in the trough. The crustal thickening to the western slope of the ridge near the triple junction was probably caused by the collision of the SFZ ridge with the Shetland Platform.

Active margin processes along the Antarctic Peninsula

The Antarctic Peninsula has a remarkable record of active margin processes, which include subduction with progressive ridge-trench collisions, margin segmentation by major fracture zones, rifting in a hybrid back-arc and sheared plate margin context, fore-arc basin development and glacial-marine controlled trench fill processes. Several facets of these active margin processes both of internal (crustal dynamic) and external origin (climate-controlled) have been documented by a geophysical survey during the Antarktis VI/2 cruise of R.V. Polarstem(October-December 1987). Reflection seismic profiles have been shot over the rift basin of Bransfield Strait, over an elongated sediment-filled trough interpreted as a fore-arc basin, over accretional and progradational slopes, over recent and ancient trench environments and over the facing oceanic domain. In this oceanic domain, different fracture zones have highly contrasting morphological and geophysical expressions. The subduction of a fracture zone like Hero F.Z., characterized by a significant relief possibly related to the presence of buoyant (serpentinite) ridges, may have been a factor of subduction termination for the last segment of the Aluk (Drake) plate; it may also have played a role in the separation of a blueschist-bearing fragment (Smith Island) from the base of the accretionary plate margin and in its lift to the surface. The magnetic anomaly pattern of the oceanic slabs facing the northwestern Peninsula margin shows evidence of an intriguing spreading acceleration, which apparently preceded ridge-trench collision. The same anomaly pattern provides a clue to the stratigraphie interpretation of the oceanic sediment cover and of the frontal part of the prograding, now passive margin south of the South Shetland Island Arc. An apparently broken and tilted oceanic plate fragment, squeezed between the South Shetland Trench and Shackleton Fracture Zone, may argue for the role of transpression associated with the oblique convergence of the Antarctic and Scotia plates.

Preliminary results of seismic reflection investigations and associated geophysical studies in the area of the Antarctic Peninsula

A geophysical data set including some 1400 km of reflection seismic profiles and 17000 km of gravity lines was recorded in the waters north-west of the Antarctic Peninsula during the austral spring cruise of RV Polarstern (October to December 1987). These data form a contribution to the earlier and continuing studies dealing with the tectonics of Bransfield Strait, the structure of the South Shetland Trench, and the segmentation of the adjacent oceanic crust. The seismic profiles and gravity data illustrate among other features the differential extension of Bransfield Strait and the structure and morphology of the former converging plate boundary, which vary corresponding to the time of the last ridge trench collision. Observations have been made about the active character of several oceanic fracture zones in time which meet the former plate boundary.

Preliminary results of seismic reflection investigations and associated geophysical studies in the area of the Antarctic Peninsula

A geophysical data set including some 1400 km of reflection seismic profiles and 17000 km of gravity lines was recorded in the waters north-west of the Antarctic Peninsula during the austral spring cruise of RV Polarstern (October to December 1987). These data form a contribution to the earlier and continuing studies dealing with the tectonics of Bransfield Strait, the structure of the South Shetland Trench, and the segmentation of the adjacent oceanic crust. The seismic profiles and gravity data illustrate among other features the differential extension of Bransfield Strait and the structure and morphology of the former converging plate boundary, which vary corresponding to the time of the last ridge trench collision. Observations have been made about the active character of several oceanic fracture zones in time which meet the former plate boundary.

The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest–trench interactions

New magnetic anomaly identifications W of the Shackleton Fracture Zone show 5 spreading sections, separated by fracture zones. In the 2 most southerly, the ridge crest collided with a trench at the margin of the Antarctic Peninsula only 6.5 and 4 Ma ago, the latest of a series of collisions starting at the base of the peninsula SO Ma ago. Following each collision, spreading and subduction both stopped. Opposite the South Shetland trench and actively extending Bransfield Strait, spreading also stopped 4 Ma ago, but before the spreading centre reached the trench. The tendency of the subducting plate to continue sinking probably initiated the opening of the Bransfield Strait. The parallelism between fracture zone orientation on the descending plate and convergence direction, previously thought responsible for the tectonic segmentation of the peninsula, was effective for only 10–30 Ma before collision. Before that, either the fracture zones did not extend to the trench, or the subducting ocean floor formed at the Farallon–Phoenix (Nazca–Aluk) boundary, with fracture zones oblique to the subduction direction. The Aleutian Arc–Kula Ridge model, in which arc magmatism virtually ceases when ocean floor younger than 25–30 Ma occurs beneath the arc, fits the distribution of Antarctic Peninsula radiometric dates, explaining a 50–60 Ma gap between collisions and youngest ages in the S, and possibly the migration of the youngest activity towards the trench. Thus, gaps in the geologic record of arc magmatism need not imply cessation of subduction. The progressive steepening of the peninsular margin towards the 4 Ma collision site suggests tectonic erosion of the fore-arc as the ridge crest approached.

A Discussion on volcanism and the structure of the Earth - The evolution of the Scotia Ridge and Scotia Sea

Marine geophysical surveys over the Scotia Ridge show it to be composed of blocks mainly of continental origin. Major structures found on the blocks are in many cases truncated at block margins and their existence is also inconsistent with the present isolated situation of the blocks. The evidence suggests post-Upper Cretaceous fragmentation of a continuous continental area. Complementary marine geomagnetic studies over the deep water of the Scotia Sea have dated two areas as younger than 22 million years (Ma) and have indicated the direction of spreading in others. A model of present plate motions, based on the magnetic anomalies, explains the active volcanism of the South Sandwich Islands as being caused by consumption of Atlantic crust at the associated trench at a rate of 5.5 cm/year for the past 7 to 8 Ma at least. An Upper Tertiary episode of plate consumption at 5 cm/year at the South Shetland trench, suggested by the magnetic lineations, with a secondary slow extensional widening of Bransfield Strait is used to explain similarly the contemporaneous volcanism of the South Shetland Is. Making the reasonable assumption of a Tertiary formation of the undated parts of the Scotia Sea by spreading in the directions indicated by the magnetic lineations, a tentative reconstruction of the component blocks of the Scotia Ridge is made. The attempt is only partly successful in matching structural patterns across adjacent margins of reconstructed blocks, South Georgia being most obviously wrongly situated. It is suggested that the misfits result from minor errors in the initial assumptions and the modification of structures during fragmentation and drift. South Georgia may have formed on the Atlantic rather than the Pacific side of the compact continental region which is thought to have joined South America and west Antarctica for much of the Mesozoic at least. A Gondwanaland reconstruction is presented which is consistent with the Scotia Ridge reconstruction, in which the Antarctic Peninsula lies alongside the Caird Coast of east Antarctica. Upon break-up of Gondwanaland, the Antarctic Peninsula remained rigidly attached to South America, east Antarctica rotating clockwise to open the Weddell Sea, until early Tertiary times when the Peninsula transferred to east Antarctica which continued rotating clockwise to open the Scotia Sea.