The relative paucity of recent post‐subduction environments globally has meant that, so far, little is known about tectonic processes that occur during and after subduction termination, as previously ...convergent tectonic plates adjust to the new stress regime. The region of Southeast Asia that now encompasses northern Borneo has been host to two sequential episodes of subduction—both now terminated—since the mid‐Paleogene. It is expected that these processes will have left signatures in the fabric of the upper mantle, which are manifest in the form of seismic anisotropy. We investigate the evidence for, and alignment of, anisotropic fabrics by measuring the splitting of a family of teleseismic shear phases. These observations provide a measure of the orientation of the effective anisotropic elastic tensor, in the form of the orientation of the fast shear‐wave polarization, ϕ, and add constraints on the strength of the anisotropic fabric, in the form of the delay time, δt. We observe two principal trends across northern Borneo that appear to be confined to the lithosphere. These patterns are likely related to tectonic processes associated with subduction, continental collision, and oceanic basin formation, events that can exert primary influence on the formation of post‐subduction settings.
Plain Language Summary
This study is concerned with understanding what happens to the upper 200 km of the Earth when subduction—the process by which one plate pushes beneath another and sinks into the Earth's interior—stops. We measure a property of the rock in the upper 200 km called seismic anisotropy, which tells us how fast earthquake waves move when traveling or polarized in one direction compared to another. Seismic anisotropy can inform us about both large‐scale plate tectonic events in recent geological history (10s of millions of years) and present‐day deformation. Northern Borneo has undergone two phases of active subduction followed by termination in the last 25 million years, making it one of the few places on Earth where we can explore this important stage of the subduction cycle. We find that tectonic compression and extension events related to termination and post‐subduction processes have left strong imprints in the upper 100 km of the Earth, leaving little‐to‐no remnants of signals we might have expected to observe from the active phase of subduction.
Key Points
New catalog of shear‐wave splitting measurements from a dense network in post‐subduction setting
Two trends in the fast orientations, corresponding to fabric generated by subduction termination and post‐subduction processes
Seismic anisotropy is limited to the lithosphere beneath northern Borneo, with no strong signal in radial anisotropy of simple asthenospheric flow
Summary Background Ecological and observational studies suggest that male circumcision reduces the risk of HIV acquisition in men. Our aim was to investigate the effect of male circumcision on HIV ...incidence in men. Methods 4996 uncircumcised, HIV-negative men aged 15–49 years who agreed to HIV testing and counselling were enrolled in this randomised trial in rural Rakai district, Uganda. Men were randomly assigned to receive immediate circumcision (n=2474) or circumcision delayed for 24 months (2522). HIV testing, physical examination, and interviews were repeated at 6, 12, and 24 month follow-up visits. The primary outcome was HIV incidence. Analyses were done on a modified intention-to-treat basis. This trial is registered with ClinicalTrials.gov , with the number NCT00425984. Findings Baseline characteristics of the men in the intervention and control groups were much the same at enrolment. Retention rates were much the same in the two groups, with 90–92% of participants retained at all time points. In the modified intention-to-treat analysis, HIV incidence over 24 months was 0·66 cases per 100 person-years in the intervention group and 1·33 cases per 100 person-years in the control group (estimated efficacy of intervention 51%, 95% CI 16–72; p=0·006). The as-treated efficacy was 55% (95% CI 22–75; p=0·002); efficacy from the Kaplan-Meier time-to-HIV-detection as-treated analysis was 60% (30–77; p=0·003). HIV incidence was lower in the intervention group than it was in the control group in all sociodemographic, behavioural, and sexually transmitted disease symptom subgroups. Moderate or severe adverse events occurred in 84 (3·6%) circumcisions; all resolved with treatment. Behaviours were much the same in both groups during follow-up. Interpretation Male circumcision reduced HIV incidence in men without behavioural disinhibition. Circumcision can be recommended for HIV prevention in men.
Kaposi sarcoma-associated herpesvirus (KSHV), also known as human herpesvirus 8 (HHV8), is a recent addition to the list of human viruses that are directly associated with lymphoproliferative ...disorders. KSHV was first shown to be involved in multicentric Castleman disease and primary effusion lymphoma (PEL). Subsequently, the virus was identified in solid lymphomas, often of extranodal sites, with morphological and immunophenotypic characteristics similar to those of PEL, and in other lymphoproliferative disorders with heterogeneous clinicopathological presentations. The recent advances in our understanding of the histology, immunophenotype and pathogenesis of these KSHV-associated lymphoproliferative disorders are reviewed.
Subduction polarity reversal (SPR) is a key subduction initiation mechanism often associated with arc‐continent collision zones. Northern Borneo has long been recognized as a location where ...sequential but opposing subduction zones were present in the Miocene, but has not been examined in the context of SPR. Here, we exploit teleseismic data from northern Borneo to investigate crustal thickness variations using Virtual Deep Seismic Sounding (VDSS). Our results reveal a thick crustal root beneath the Crocker Range and an area of relatively thin crust in the southeast, which appears to extend northeast into the Sulu Sea, where back‐arc rifting behind the younger subduction zone developed. Overall, our findings are consistent with predictions from numerical models of SPR involving arc‐continent collision, but with several important differences—including a substantial mountain range and more limited back‐arc rifting that can be attributed to northern Borneo being an example of SPR involving continent‐continent collision.
Plain Language Summary
Northern Borneo lies in the heart of Southeast Asia and was assembled by a complex series of tectonic events over the last 40 million years. Principle among these was the subduction of the Proto South China Sea plate beneath its northwest continental margin, which ended in continent‐continent collision. Subduction of the Celebes Sea plate in the southwest followed, which terminated ∼9 million years ago. In this study, we exploit seismic data collected in northern Borneo to construct a new crustal model of the region, that robustly constrains the crust‐mantle boundary. We find that crustal thickness variations are consistent with a subduction polarity reversal (SPR) event induced by continent‐continent collision, with thicker crust underlying the collision zone and thinner crust related to extension driven by retreat of the younger subduction zone. Our results have important implications for the study of SPR and subduction initiation and the processes that influence post‐subduction tectonic settings.
Key Points
Crustal thickness map of northern Borneo obtained from Virtual Deep Seismic Sounding
Evidence of crustal thinning indicates Sulu Sea extension propagated into northern Borneo during the late Miocene
Variations in crustal thickness consistent with subduction polarity reversal induced by continent‐continent collision
Rhyodacite tephra and three lavas erupted ∼27 ka, interpreted to be early leaks from the climactic magma chamber of Mount Mazama, contain ubiquitous resorbed crystals (antecrysts) that were recycled ...from young granodiorite and related plutonic rocks of the same magmatic system. The shallow composite pluton is represented by blocks ejected in the 7.7-ka climactic eruption that formed Crater Lake caldera. Plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented Fe-oxide needles exsolved at subsolidus conditions. At least 80% of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks. Other crystals in the rhyodacite, notably zircon, also were recycled. SIMS
238U–
230Th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between ∼20 ka and ≥
300 ka with concentrations of analyses near 50–70, ∼110, and ∼200 ka that correspond to periods of dacitic volcanism dated by K–Ar. U–Th ages of zircon from a rhyodacite sample yield similar results. No analyzed zircons from the granodiorite or rhyodacite are pre-Quaternary. Zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton. Survival of zircon in zircon-undersaturated hydrous magma and of Fe-oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the ∼27-ka eruption of the rhyodacite. The ∼27-ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high-level magma chamber. In contrast, the voluminous 7.7-ka climactic rhyodacite pumice is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the intervening period. Mineralogical evidence of assimilation may be destroyed in hot, vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of Mount Mazama.
The great diversity in plant genome size and chromosome number is partly due to polyploidization (i.e. genome doubling events). The differences in genome size and chromosome number among diploid ...plant species can be a window into the intriguing phenomenon of past genome doubling that may be obscured through time by the process of diploidization. The genus Hibiscus L. (Malvaceae) has a wide diversity of chromosome numbers and a complex genomic history. Hibiscus is ideal for exploring past genomic events because although two ancient genome duplication events have been identified, more are likely to be found due to its diversity of chromosome numbers. To reappraise the history of whole-genome duplication events in Hibiscus, we tested three alternative scenarios describing different polyploidization events. Using target sequence capture, we designed a new probe set for Hibiscus and generated 87 orthologous genes from four diploid species. We detected paralogues in > 54% putative single-copy genes. 34 of these genes were selected for testing three different genome duplication scenarios using gene counting. All species of Hibiscus sampled shared one genome duplication with H. syriacus, and one whole genome duplication occurred along the branch leading to H. syriacus. Here, we corroborated the independent genome doubling previously found in the lineage leading to H. syriacus and a shared genome doubling of this lineage and the remainder of Hibiscus. Additionally, we found a previously undiscovered genome duplication shared by the /Pavonia and /Malvaviscus clades (both nested within Hibiscus) with the occurrences of two copies in what were otherwise single-copy genes. Our results highlight the complexity of genomic diversity in some plant groups, which makes orthology assessment and accurate phylogenomic inference difficult.
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