Stern, Robert J.
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/4141
Dr. Robert Stern serves as a Professor of Geosciences. His research interests include:
- Volcanism associated with convergent plate margins (subduction zones)
- How new subduction zones form
- How Earth's continental crust is generated and destroyed
- When Plate Tectonics started
- The geologic evolution of the Middle East (especially the region between Egypt and Iran)
- The formation of the Gulf of Mexico
- The geology of the Dallas- Fort Worth Metroplex.
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Recent Submissions
Item Age, Geochemistry, and Emplacement of the ~40-Ma Baneh Granite-Appinite Complex in a Transpressional Tectonic Regime, Zagros Suture Zone, Northwest Iran(Taylor & Francis Inc, 2018-01-12) Azizi, Hossein; Hadad, Sepideh; Stern, Robert J.; Asahara, Yoshihiro; 0000-0002-8083-4632 (Stern, RJ); 284885305 (Stern, RJ); Stern, Robert J.The Baneh plutonic complex is situated in the Zagros suture zone of northwest Iran between the Arabian and Eurasian plates. This complex is divided into granite and appinite groups. Zircon U-Pb dating shows that granites crystallized 41-38 million years ago but appinites experience more protracted magmatic evolution, from at 52 to 38 Ma. Whole-rock chemical compositions show significant major and trace element variations between the two lithologies. Granitic rocks are more evolved, with high contents of SiO₂ (62.4-77.0 wt%), low contents of TiO₂ (0.25 wt%), MgO (0.05-1.57 wt%), and Fe₂O₃ (0.40-4.06 wt%) and high contents of Na₂O + K₂O (approximate to 10 wt%). In contrast, appinites have low contents of SiO₂ (51.0-57.0 wt%) and K₂O (<2.1 wt%) and high Fe₂O₃ (6.4-9.35 wt%), MgO (2.0-9.9 wt%), and Mg number (Mg# = 35-76). The concentration of rare earth elements in the appinites is higher than in granitic rocks, making it difficult to form granites solely by fractionation of appinite magma. (⁸⁷Sr/⁸⁶Sr)ᵢ and ε_{Nd(40 Ma)} in both groups are similar, from 0.7045 to 0.7061 and -1.2 to +2.6, except for a primitive gabbroic dike with ε_{Nd(40 Ma)}) = +9.9. Appinites show mainly typical I-type characteristics, but granites have some S-type characteristics. The sigmoidal shape of the Baneh pluton and its emplacement into deformed Cretaceous shales and limestone showing kink bands, asymmetric and recumbent folds in a broad contact zone, with pervasive ductile to brittle structures in both host rocks and intrusion, indicate that magma emplacement was controlled by a transpressional tectonic regime, perhaps developed during early stages in the collision of Arabia and Eurasian plates.Item Commentary on JGR-Sold Earth Paper “Deep Seismic Structure Across the Southernmost Mariana Trench: Implications for Arc Rifting and Plate Hydration” By Wan et al.(Blackwell Publishing Ltd, 2019-05-06) Stern, Robert J.; 0000-0002-8083-4632 (Stern, RJ); 284885305 (Stern, RJ); Stern, Robert J.No abstract available.Item Jurassic Igneous Rocks of the Central Sanandaj–Sirjan Zone (Iran) Mark a Propagating Continental Rift, Not a Magmatic Arc(Blackwell Publishing Ltd, 2019-04-29) Azizi, H.; Stern, Robert J.; 0000-0002-8083-4632 (Stern, RJ); 284885305 (Stern, RJ); Stern, Robert J.Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro-diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction-related modifications. The arc-like features of S-type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time. © 2019 John Wiley & Sons LtdItem Eocene Initiation of the Cascadia Subduction Zone: A Second Example of Plume-Induced Subduction Initiation?(Geological Society of America, 2019-04-15) Stern, Robert J.; Dumitru, T. A.; Stern, Robert J.The existing paradigm for the major ca. 56-48 Ma subduction zone reorganization in the Pacific Northwest of North America is that: (1) the Siletzia large igneous province erupted offshore to the west of North America, forming an oceanic plateau; (2) Siletzia then collided with North America, clogging the Pacific Northwest segment of the Cordilleran subduction zone; and (3) the oceanic lithosphere west of Siletzia then ruptured to initiate the new Cascadia subduction zone. Oceanic lithosphere is strong and difficult to rupture, so this would represent a rare example of such a rupture initiating a new subduction zone. This paper explores an alternative hypothesis for the reorganization, a plume-induced subduction initiation (PISI) mechanism, which has previously been applied to the initiation of Caribbean plate subduction zones in the Cretaceous. In this PISI hypothesis, a newly formed, ~1200-km-diameter Yellowstone mantle plume head rose at ca. 55 Ma beneath western North America, generating Siletzia in situ on the North American margin, as well as generating the ~1700-km-long Challis-Kamloops volcanic belt ~600 km to the east of Siletzia. This destroyed the existing Cordilleran subduction zone and allowed the new Cascadia subduction zone to form by collapse of thermally weakened oceanic lithosphere over the hot western margin of the plume head. This PISI hypothesis provides an integrated framework for understanding Siletzia, the Challis-Kamloops belt, Eocene core complexes from Idaho (U.S.) to British Columbia (Canada), underplated mafic rocks beneath Oregon and Washington (U.S.), post-17 Ma manifestations of the Yellowstone plume, and geophysical characteristics of the lithosphere beneath the Pacific Northwest. © 2019 The Authors.Item The Robustness of Sr/Y and La/Yb as Proxies for Crust Thickness in Modern Arcs(Geological Society of America, 2019-03-29) Lieu, Warren K.; Stern, Robert J.; Lieu, Warren K.; Stern, Robert J.Trace element (TE) ratios of convergent-margin magmas have been found to vary systematically with arc crustal thicknesses. Here we use statistical smoothing techniques along with Sr/Y and La/Yb trace element Moho depth proxies to determine crustal thickness along the volcanic front for three arc segments: the Central Volcanic Zone of the Andes arc, the Central America arc at Nicaragua and Costa Rica, and segments of the Alaska-Aleutian Islands arc (northwesternmost USA). The results are comparable to those from seismic surveys. TE depth proxies give ~70 km crust thickness beneath the Central Volcanic Zone's Altiplano region and show thinner crust (60 km for La/Yb, 43 km for Sr/Y) as the volcanic line crosses into the Puna region. In Central America, the proxy analyses show crustal thickness changes between the Chorotega block and the Nicaragua depression, with both proxies agreeing for Nicaragua (~27 km) but with La/Yb giving considerable thicker (~45 km) crust than Sr/Y (~30 km) for Chorotega. For these two arc segments, the La/Yb proxy approximated the seismically inferred Moho depth to within 10 km for the entire profile, but the Sr/Y proxy-estimated crustal thicknesses diverge from those of the La/Yb proxy and seismic methods in the thin-crust regions. For the Alaska-Aleutian arc, both TE proxies indicate that crust varies from thick (~35 km) for the western Aleutian segment (175°E to 175°W), to thin (~22 km) for the transitional segment (175°W to 158°W), to thick (35+ km) for the eastern Alaska Peninsula (158°W to 150°W). Geophysical estimates favor a crustal thickness of 30-40 km for the same region. We propose that statistically treated geochemistry-based proxies can provide useful estimates of crustal thickness when estimates from Sr/Y and La/Yb agree. We investigated the disagreement in the Alaska-Aleutian case in more detail. Alaska-Aleutian crustal thickness was found to correlate with calc-alkaline (CA) versus tholeiitic (TH) segments of the arc, as represented by along-arc smoothing of the volcanoes' CA-TH indices. The thin crust of the transitional segment trends TH while the thicker crust of the flanking segments trend CA. We find that crustal thickness also plays a role in inferred magma flux (here approximated by volcano volume), with greater flux associated with thinner crust. Thin crust beneath the Alaska-Aleutian transitional segment may reflect continuing loss of cumulates from the lower crust and/or lithospheric mantle into the asthenosphere, leading to enhanced melting beneath this region. © 2019 The Authors.Item Subduction Initiation Dynamics along a Transform Fault Control Trench Curvature and Ophiolite Ages(Geological Society of America) Zhou, X.; Li, Z. -H; Gerya, T. V.; Stern, Robert J.; Xu, Z.; Zhang, J.; 284885305 (Stern, RJ); Stern, Robert J.Understanding how new subduction zones form is essential for complete articulation of plate tectonic theory. Formation of new subduction zones by collapse of oceanic transform faults or fracture zones is suggested on the basis of empirical evidence. This process has heretofore been investigated with two-dimensional (2-D) numerical models, which thus ignore its intrinsic three-dimensional (3-D) geometry, lateral propagation, and dynamics. Here, we investigate a 3-D thermomechanical model, in which old and thick oceanic lithosphere (plate) is separated by a transform fault from a thinner and younger oceanic plate containing a transform-orthogonal spreading ridge. The results suggest that the older plate starts to sink spontaneously at the ridge-transform fault junction, and then subduction initiation laterally propagates along the transform away from the ridge. Two key factors control the 3-D subduction initiation (SI) dynamics in nature: (1) the age of the sinking plate, which controls its negative buoyancy; and (2) the thermal structure of the overriding plate, which reflects its spreading history. Our numerical models not only shed new light on the SI dynamics of Cenozoic subduction zones (e.g., the Izu-Bonin-Mariana zone in the Pacific Ocean), but also have implications for fossil convergent plate margins (e.g., the Bitlis-Zagros suture zone, west of the Strait of Hormuz). In the latter case, systematic variations in ages of supra-subduction zone ophiolites may reflect diachronous SI and its lateral propagation. ©2018 Geological Society of America.Item Eruption of South Sarigan Seamount, Northern Mariana Islands: Insights into Hazards from Submarine Volcanic Eruptions(Oceanography Society, 2014-06) Embley, R. W.; Tamura, Y.; Merle, S. G.; Sato, T.; Shizuka, O. I.; Chadwick Jr., W. W.; Wiens, D. A.; Shore, P.; Stern, Robert J.; 0000 0003 5076 6787 (Stern, RJ); 284885305 (Stern, RJ); Stern, Robert J.The eruption of South Sarigan Seamount in the southern Mariana arc in May 2010 is a reminder of how little we know about the hazards associated with submarine explosive eruptions or how to predict these types of eruptions. Monitored by local seismometers and distant hydrophones, the eruption from ~ 200 m water depth produced a gas and ash plume that breached the sea surface and rose ~ 12 km into the atmosphere. This is one of the first instances for which a wide range of preand post-eruption observations allow characterization of such an event on a shallow submarine volcanic arc volcano. Comparison of bathymetric surveys before and after the eruptions of the South Sarigan Seamount reveals the eruption produced a 350 m diameter crater, deeply breached on the west side, and a broad apron downslope with deposits > 50 m thick. The breached summit crater formed within a pre-eruption dome-shaped summit composed of andesite lavas. Dives with the Japan Agency for Marine-Earth Science and Technology Hyper-Dolphin remotely operated vehicle sampled the wall of the crater and the downslope deposits, which consist of andesite lava blocks lying on pumiceous gravel and sand. Chemical analyses show that the andesite pumice is probably juvenile material from the eruption. The unexpected eruption of this seamount, one of many poorly studied shallow seamounts of comparable size along the Mariana and other volcanic arcs, underscores our lack of understanding of submarine hazards associated with submarine volcanism.