DOCSLIB.ORG

Intermediate-Depth Earthquakes Facilitated by Eclogitization-Related Stresses

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Intermediate-Depth Earthquakes Facilitated by Eclogitization-Related Stresses Geology, published online on 4 April 2013 as doi:10.1130/G33796.1 Intermediate-depth earthquakes facilitated by eclogitization-related stresses Junichi Nakajima1*, Naoki Uchida1, Takahiro Shiina1, Akira Hasegawa1, Bradley R. Hacker2, and Stephen H. Kirby3 1Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 2Earth Research Institute, University of California–Santa Barbara, Santa Barbara, California 93106-9630, USA 3U.S. Geological Survey, Menlo Park, California 94025, USA ABSTRACT skipping of waveforms due to polarity reversal, Eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume we calculated traveltime differences indepen- reduction of 10%–15%. One consequence of the negative volume change is the formation of dently for the reverse- and normal-fault groups. a paired stress fi eld as a result of strain compatibility across the reaction front. Here we use We applied hypoDD to the calculated differen- waveform analysis of a tiny seismic cluster in the lower crust of the downgoing Pacifi c plate tial traveltime data, and determined the relative and reveal new evidence in favor of this mechanism: tensional earthquakes lying 1 km above locations of the earthquakes in the reverse- and compressional earthquakes, and earthquakes with highly similar waveforms lying on well- normal-fault groups with the one-dimensional defi ned planes with complementary rupture areas. The tensional stress is interpreted to be velocity model (Ueno et al., 2002). caused by the dimensional mismatch between crust transformed to eclogite and underlying The relative locations of the reverse-fault untransformed crust, and the earthquakes are probably facilitated by reactivation of fossil earthquakes with respect to the normal-fault faults extant in the subducting plate. These observations provide seismic evidence for the role earthquakes were determined using S–P time of volume change–related stresses and, possibly, fl uid-related embrittlement as viable pro- differences. The polarity of waveform was cesses for nucleating earthquakes in downgoing oceanic lithosphere. reversed for the normal-fault earthquakes, and their coherencies with the reverse-fault earth- INTRODUCTION show that the cluster is located in the lowermost quakes were calculated in the same manner as Generation of intermediate-depth earth- part of the oceanic crust (Fig. 1B) and concen- for the earthquake classifi cation. Then, we iden- quakes is an ongoing enigma because high litho- trated in a volume no more than 2 km × 2 km × tifi ed the earthquake pair with the most-similar static pressures render ordinary dry frictional 5 km (Figs. 1C and 1D). The earthquakes in the waveform (an average coherency of 0.92) failure unlikely. Earthquakes in subducting oce- cluster have occurred not in swarms but fairly (Fig. DR2). For this pair, we calculated S–P anic crust have been linked to the transformation evenly over time (Fig. 1E). time differences at 14 common stations based of basalt to eclogite and concomitant dehydra- We selected 106 earthquakes (M ≥ 2) in the on waveform similarity, and determined the tion (e.g., Kirby et al., 1996; Preston et al., 2003; period from January 2003 to February 2011, and relative locations of the two earthquakes using Hacker et al., 2003). A major, untested predic- classifi ed the earthquakes into subgroups on the the S–P time differences. Finally, we shifted tion of this hypothesis is, however, that the large basis of waveform similarity. A time window of the other earthquakes with respect to that earth- (10%–15%) volume reduction during eclogite 30 s was set for the vertical component, begin- quake pair. formation should produce a paired stress regime ning 3 s before the onset of the P wave, and To estimate the extent of source areas, we along the eclogitization reaction front in which coherency was calculated using the cross spec- determined the source radii of the 30 earth- tension is underlain by compression (Hacker, tral method (Poupinet et al., 1984) for each pair quakes from corner frequencies. Corner fre- 1996; Kirby et al., 1996). This study presents of earthquakes in a frequency band of 2–12 Hz. quencies were estimated by applying the spectral the fi rst seismological evidence of such a paired When the calculated coherencies were larger ratio method to P-wave amplitude spectra for a stress fi eld associated with eclogite formation in than 0.9 at three or more stations, we regarded tapered 2.0 s time window of the vertical com- the oceanic crust. the earthquake pair as part of the same group. In ponent (e.g., Frankel and Wennerberg, 1989) this manner, we categorized 30 earthquakes into (Fig. DR3). We used an omega-square source DATA ANALYSIS nine groups with similar waveforms (Fig. 1F; model (Brune, 1970) to model the spectral The Pacifi c plate beneath northeastern Japan Fig. DR1 in the GSA Data Repository1): four ratio stacked for the available station pairs. As has a well-developed double seismic zone in groups (R1–R4) of 19 reverse-fault earthquakes a result, we obtained the corner frequencies of which earthquakes with downdip compression and fi ve groups (N1–N5) of 11 normal-fault eight earthquakes. The source radii of the eight dominate the upper seismic zone within the earthquakes. earthquakes were calculated using the circular crust, and earthquakes with downdip tension For precise relative earthquake relocations, crack model (Sato and Hirasawa, 1973). For the dominate the lower seismic zone 30–40 km traveltime differences of the 30 earthquakes 22 earthquakes without the estimates of cor- deeper within the mantle (e.g., Hasegawa et were calculated using the cross spectral method ner frequencies, we calculated the source radii al., 1978). Somewhat-isolated seismic clusters for both P and S waves. A time window of 3 s from the average stress drop (7 MPa) using the are observed locally in the upper seismic zone was set 0.5 s before the onset of each wave, and formula of Eshelby (1975). The radii of source at a depth of ~150 km (red and pink arrows in delay times were calculated from the phases of areas thus calculated range from 43 m to 216 m Fig. 1A). We investigate seismic features of the cross spectra in a frequency band of 2–12 Hz with stress drops of 3.5–33.3 MPa. Source most-active isolated seismic cluster (red arrow) with a coherency of >0.8. To avoid half-cycle parameters obtained for the 30 earthquakes are using high-quality waveform data. Hypocenters listed in Table DR1 in the Data Repository. of the cluster relocated by the double-difference method (hypoDD) (Waldhauser and Ellsworth, 1GSA Data Repository item 2013179, Figures RESULTS AND DISCUSSION DR1–DR3, and Table DR1, is available online at www 2000) with catalog-derived differential data .geosociety.org/pubs/ft2013.htm, or on request from The relocated hypocenters show important [email protected] or Documents Secretary, GSA, features in terms of earthquake-generating stress *E-mail: [email protected]. P.O. Box 9140, Boulder, CO 80301, USA. and seismic rupture areas. (1) The normal-fault GEOLOGY, June 2013; v. 41; no. 6; p. 1–4; Data Repository item 2013179 | doi:10.1130/G33796.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | June Society 2013 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 Geology, published online on 4 April 2013 as doi:10.1130/G33796.1 Longitude (E) Longitude (E) 2001), some special process must produce local 138.5° 139° 139.5° 140° 140.5° 141° 141.5° 139.2° 139.24° 139.28° 139.32° 39° 37.96° tension. What process can generate earthquakes R1 N1 A C R2 N2 with local tension in proximity to compressional R3 N3 150 125 R4 N4 earthquakes and produce clusters of seismic N5 175 km events on well-defi ned and separate planes? 38.5° Y Pacific 37.92° Progressive eclogitization of the oceanic plate ′ crust of subducting slabs is interpreted from Latitude (N) Y X gradual increases in seismic velocities and 38° TU.SBT 100 the disappearance of seismic activity (e.g., 75 37.88° 2 km Helffrich, 1996; Abers, 2005; Kita et al., 2006). Latitude (N) The upper, basaltic layer of the crust trans- N.MRMH X’ Y Y′ 37.5° forms to eclogite more easily because of fi ner D 150 grain size, increased alteration, and higher tem- seismic station volcano plate interface peratures (e.g., Hacker, 1996); eclogitization of 37° XX′ basalt beneath northeastern Japan is interpreted 100 B 154 to occur at depths of 70–130 km (Igarashi et al., 120 2001; Kita et al., 2006). Transformation of the mantle Depth (km) Moho lower, gabbroic layer to eclogite is, however, 140 wedge plate interface Pacific Plate 158 retarded because of coarser grain size, substan- 160 tially less alteration, and lower temperatures 0246 810 180 Distance (km) (e.g., Hacker, 1996); because of this, eclogitiza- Depth (km) 4 tion likely progresses from the top of the crust 200 Moho 80 100 120 140 160 E downward (Fig. 4). A low-velocity layer at the depth of hypocenters (km) 3 220 04080120 160 200 top of the Pacifi c slab suggests that eclogitiza- 2 Distance (km) tion of the gabbroic layer is incomplete even Magnitude 1 at depths down to 150 km (Matsuzawa et al., 0 1986) where the seismic cluster described here 2004 2006 2008 2010 Year is located. F TU.SBT Vertical component Band-pass filter : 2-12 Hz One possibility to explain the observed seis- Group R2 mic cluster is that the eclogitic and gabbroic 101531125 M2.3 layers in the slab have different strengths and 083321938 M2.2 respond differently to the slab stress state.
Load more

Recommended publications
  • Initiation of Plate Tectonics in the Hadean: Eclogitization Triggered by the ABEL Bombardment
    Accepted Manuscript Initiation of plate tectonics in the Hadean: Eclogitization triggered by the ABEL Bombardment S. Maruyama, M. Santosh, S. Azuma PII: S1674-9871(16)30207-9 DOI: 10.1016/j.gsf.2016.11.009 Reference: GSF 514 To appear in: Geoscience Frontiers Received Date: 9 May 2016 Revised Date: 13 November 2016 Accepted Date: 25 November 2016 Please cite this article as: Maruyama, S., Santosh, M., Azuma, S., Initiation of plate tectonics in the Hadean: Eclogitization triggered by the ABEL Bombardment, Geoscience Frontiers (2017), doi: 10.1016/ j.gsf.2016.11.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT MANUSCRIPT ACCEPTED P a g e ‐|‐1111‐‐‐‐ ACCEPTED MANUSCRIPT ‐ 1‐ Initiation of plate tectonics in the Hadean: 2‐ Eclogitization triggered by the ABEL 3‐ Bombardment 4‐ 5‐ S. Maruyama a,b,*, M. Santosh c,d,e , S. Azuma a 6‐ a Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1, 7‐ Ookayama-Meguro-ku, Tokyo 152-8550, Japan 8‐ b Institute for Study of the Earth’s Interior, Okayama University, 827 Yamada, 9‐ Misasa, Tottori 682-0193, Japan 10‐ c Centre for Tectonics, Resources and Exploration, Department of Earth 11‐ Sciences, University of Adelaide, SA 5005, Australia 12‐ d School of Earth Sciences and Resources, China University of Geosciences 13‐ Beijing, 29 Xueyuan Road, Beijing 100083, China 14‐ e Faculty of Science, Kochi University, KochiMANUSCRIPT 780-8520, Japan 15‐ *Corresponding author.
    [Show full text]
  • Geologic Extremes of the NW Himalaya
    Louisiana State University LSU Digital Commons LSU Doctoral Dissertations Graduate School 2016 Geologic Extremes of the NW Himalaya: Investigations of the Himalayan Ultra-high Pressure and Low Temperature Deformation Histories Dennis Girard Donaldson Louisiana State University and Agricultural and Mechanical College Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_dissertations Part of the Earth Sciences Commons Recommended Citation Donaldson, Dennis Girard, "Geologic Extremes of the NW Himalaya: Investigations of the Himalayan Ultra-high Pressure and Low Temperature Deformation Histories" (2016). LSU Doctoral Dissertations. 3295. https://digitalcommons.lsu.edu/gradschool_dissertations/3295 This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Doctoral Dissertations by an authorized graduate school editor of LSU Digital Commons. For more information, please [email protected]. GEOLOGIC EXTREMES OF THE NW HIMALAYA: INVESTIGATIONS OF THE HIMALAYAN ULTRA-HIGH PRESSURE AND LOW TEMPERATURE DEFORMATION HISTORIES A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Geology and Geophysics by Dennis G. Donaldson Jr. B.S., Virginia State University, 2009 August 2016 Dedicated to Jaime, Eliya, Noah, my siblings and parents. ii ACKNOWLEDGEMENTS I am very pleased that I have had a rewarding journey during this chapter of my life with the Department of Geology and Geophysical at Louisiana State University. I would not be here right now if it wasn’t for Dr. Alex Webb, Dr.
    [Show full text]
  • The High Pressure Belt in the Grenville Province: 75 Architecture, Timing, and Exhumation1
    100 867 100 95 95 75 The High Pressure belt in the Grenville Province: 75 architecture, timing, and exhumation1 25 25 Toby Rivers, John Ketchum, Aphrodite Indares, and Andrew Hynes 5 5 0 0 Abstract: We propose that the Grenvillian allochthonous terranes may be grouped into High Pressure (HP) and Low Pressure (LP) belts and examine the HP belt in detail in the western and central Grenville Province. The HP belt is developed in Paleo- and Mesoproterozoic rocks of the pre-Grenvillian Laurentian margin and characterized by Grenvillian eclogite and co-facial HP granulite in mafic rocks. Pressure–temperature (P–T) estimates for eclogite-facies conditions in well-preserved assemblages are about 1800 MPa and 850°C. In the central Grenville Province, HP rocks formed at -1060–1040 Ma and underwent a single stage of unroofing with transport into the upper crust by -1020 Ma, whereas farther west they underwent two stages of unroofing separated by penetrative mid-crustal recrystallization before transport to the upper crust at -1020 Ma. Unroofing processes were comparable in the two areas, involving both thrusting and extensional faulting in an orogen propagating into its foreland by understacking. In detail, thrusting episodes preceded extension in the western Grenville Province, whereas in the central Grenville Province, they were coeval, resulting in unroofing by tectonic extrusion. In the central Grenville Province, the footwall ramp is well preserved, but any former ramp in the western Grenville Province was obliterated by later lower crustal extensional flow. Continuation of the HP belt into the eastern Grenville Province is not established, but likely on geological grounds.
    [Show full text]
  • Backarc Origin for Neoarchean Ultrahigh-Temperature Metamorphism, Eclogitization, and Orogenic Root Growth
    Backarc origin for Neoarchean ultrahigh-temperature metamorphism, eclogitization, and orogenic root growth Gregory Dumond1*, Michael L. Williams2, Julia A. Baldwin3, and Michael J. Jercinovic2 1Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003-9297, USA 3Department of Geosciences, University of Montana, Missoula, Montana 59812-1296, USA ABSTRACT modeling of Thompson et al. (2001) and provide evidence for crustal Contraction of continental crust during orogeny results in elevated root growth associated with high-P metamorphism of mid-oceanic ridge topography at the surface and a root at depth. Thermomechanical basalt (MORB)–like mafc intrusions. Intraplating of these intrusions models suggest that root growth is enhanced by thickening of ther- facilitated partial melting and UHT metamorphism of peraluminous supra- mally softened thin lithosphere. A >400 km2 region of Archean gneiss crustal host rocks (Dumond et al., 2015). We use Th–U–total Pb monazite in the Athabasca granulite terrane in the Canadian shield contains petrochronology to constrain the timing of high-P melting preserved in abundant mafc sills with mid-oceanic ridge basalt–like chemistry. felsic granulite paragneiss in contact with a previously dated eclogite Heat from the sills facilitated melting of supracrustal host rocks along sill that yielded U-Pb zircon dates of 2.54 Ga and 1.90 Ga (Baldwin et a prograde pressure-temperature (P-T) path culminating at P > 1.4 al., 2004). These results are combined with bulk-rock geochemistry for GPa and T > 950 °C in the Neoarchean. A basalt sill, converted to mafc granulites and eclogite near the base of the Upper Deck to infer a eclogite near the base of the domain, exhibits positive Eu anomalies backarc origin for a Neoarchean orogenic root.
    [Show full text]
  • Andean Flat-Slab Subduction Through Time
    Andean flat-slab subduction through time VICTOR A. RAMOS & ANDRE´ S FOLGUERA* Laboratorio de Tecto´nica Andina, Universidad de Buenos Aires – CONICET *Corresponding author (e-mail: [email protected]) Abstract: The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guan˜acos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat- slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to dela- mination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcan- ism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination.
    [Show full text]
  • Foreland Uplift During Flat Subduction Insights from the Peruvian Andes
    Tectonophysics 731–732 (2018) 73–84 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Foreland uplift during flat subduction: Insights from the Peruvian Andes and T Fitzcarrald Arch ⁎ Brandon T. Bishopa, , Susan L. Becka, George Zandta, Lara S. Wagnerb, Maureen D. Longc, Hernando Taverad a Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, AZ 85721, USA b Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road NW, Washington, DC 20015, USA c Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA d Instituto Geofísico del Perú, Calle Badajoz 169, Lima 15012, Peru ARTICLE INFO ABSTRACT Keywords: Foreland deformation has long been associated with flat-slab subduction, but the precise mechanism linking Basal shear these two processes remains unclear. One example of foreland deformation corresponding in space and time to Flat slab flat subduction is the Fitzcarrald Arch, a broad NE-SW trending topographically high feature covering an area Andes of > 4 × 105 km2 in the Peruvian Andean foreland. Recent imaging of the southern segment of Peruvian flat slab Crustal thickening shows that the shallowest part of the slab, which corresponds to the subducted Nazca Ridge northeast of the Lithosphere present intersection of the ridge and the Peruvian trench, extends up to and partly under the southwestern edge Fitzcarrald Arch of the arch. Here, we evaluate models for the formation of this foreland arch and find that a basal-shear model is most consistent with observations. We calculate that ~5 km of lower crustal thickening would be sufficient to generate the arch's uplift since the late Miocene.
    [Show full text]
  • Eclogites and Other High-Pressure Rocks in the Himalaya: a Review
    Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 Eclogites and other high-pressure rocks in the Himalaya: a review PATRICK J. O’BRIEN Department of Earth and Environmental Sciences, University of Potsdam, Karl-Liebknecht-Strasse 24/25, 14469 Potsdam-Golm, Germany 0000-0003-0235-9116 [email protected] Abstract: Himalayan high-pressure metamorphic rocks are restricted to three environments: the suture zone; close to the suture zone; and (mostly) far (>100 km) from the suture zone. In the NW Himalaya and South Tibet, Cretaceous-age blueschists (glaucophane-, lawsonite- or carpholite-bearing schists) formed in the accretionary wedge of the subducting Neo-Tethys. Microdiamond and associated phases from suture-zone ophiolites (Luo- busa and Nidar) are, however, unrelated to Himalayan subduction–collision processes. Deeply subducted and rapidly exhumed Indian Plate basement and cover rocks directly adjacent to the suture zone enclose eclogites of Eocene age, some coesite-bearing (Kaghan/Neelum and Tso Morari), formed from Permian Panjal Trap, con- tinental-type, basaltic magmatic rocks. Eclogites with a granulite-facies overprint, yielding Oligocene–Miocene ages, occur in the anatectic cordierite ± sillimanite-grade Indian Plate mostly significantly south of the suture zone (Kharta/Ama Drime/Arun, north Sikkim and NW Bhutan) but also directly at the suture zone at Namche Barwa. The sequence carpholite-, coesite-, kyanite- and cordierite-bearing rocks of these different units dem- onstrates the transition from oceanic subduction to continental collision via continental subduction. The gran- ulitized eclogites in anatectic gneisses preserve evidence of former thick crust as in other wide hot orogens, such as the European Variscides.
    [Show full text]
  • Foundered Lithospheric Segments Dropped Into the Mantle Transition Zone Beneath Southern California, USA Youqiang Yu1, Stephen S
    https://doi.org/10.1130/G46889.1 Manuscript received 20 August 2019 Revised manuscript received 23 October 2019 Manuscript accepted 29 October 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 9 December 2019 Foundered lithospheric segments dropped into the mantle transition zone beneath southern California, USA Youqiang Yu1, Stephen S. Gao2, Kelly H. Liu2 and Dapeng Zhao3 1 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China 2 Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA 3 Department of Geophysics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan ABSTRACT most likely due to the low number of receiver The diverse range of active tectonics occurring in southern California, USA, offers an functions (RFs) used in the studies (Gurrola opportunity to explore processes of continental deformation and modification in response to and Minster, 1998; Ramesh et al., 2002; Lewis the instability of the Pacific and Farallon plates. Here, we present a high-resolution receiver- and Gurrola, 2004; Vinnik et al., 2010). On the function image of the mantle transition zone (MTZ). Our result reveals significant lateral other hand, more recent RF studies using USAr- heterogeneities in the deep mantle beneath southern California. Both seismic tomography ray data are focused on the western or the entire and MTZ discontinuity deflections reveal foundered lithospheric segments that have dropped United States with resolutions that are not high into the MTZ beneath the western Transverse Ranges, the Peninsular Ranges, and part of the enough for identifying finer MTZ features in southern Sierra Nevada.
    [Show full text]
  • Eclogite Formation and the Rheology, Buoyancy, Seismicity, and H2O
    Eclogite Formationand the Rheology,Buoyancy, Seismicity,and H20 Contentof OceanicCrust BradleyR. Hacker1 Departmentof Geologicaland EnvironmentalSciences, Stanford University, Stanford, California A broad spectrumof variably altered igneous rocks with a wide range of grain sizes are compressedand heated over a wide range of pressure-temperaturepaths in subductionzones. Although experimentalkinetic data cannotbe extrapolatedto predict the rates of blueschistand eclogite formation in nature, textural data from rocks indicate that transformationbelow temperaturesof 150øCis minimal. Completetransformation of volcanicrocks occurs by •-250øC, but incompletetransformation of gabbroicrocks heatedto 800øC has been observed.There are important consequencesto the rapid transformation of volcanic rocks and the metastable persistenceof gabbroicrocks into the blueschistand eclogite stability fields. Fast seismic velocities shouldbe evident first in the upper oceaniccrust and may be substantiallyretarded in the lower oceaniccrust. The upper oceaniccrust will be denserthan asthenospherebefore the lower oceanic crust.Early in the processof eclogiteformation, volcanic rocks will be placedin deviatorictension and the underlyingcoarser grained rocks in compression;with furtherreaction, the stateof stressin gabbroicrocks will changefrom compressiveto tensile.Earthquakes at shallowdepths should be extensional in basalt and contractionalin gabbro, changing at deeper levels to extensional throughoutthe crust. INTRODUCTION This paper summarizes the rates and
    [Show full text]
  • Seismic Properties of a Unique Olivine-Rich Eclogite in the Western Gneiss Region, Norway
    minerals Article Seismic Properties of a Unique Olivine-Rich Eclogite in the Western Gneiss Region, Norway Yi Cao 1,*, Haemyeong Jung 2 and Jian Ma 1 1 State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China; [email protected] 2 Tectonophysics Laboratory, School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Korea; [email protected] * Correspondence: [email protected] Received: 24 July 2020; Accepted: 29 August 2020; Published: 31 August 2020 Abstract: Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0–2.5%, max. AVs = 0.6–2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist.
    [Show full text]
  • Mantle Earthquakes in the Himalayan Collision Zone Vera Schulte-Pelkum1, Gaspar Monsalve2, Anne F
    https://doi.org/10.1130/G46378.1 Manuscript received 22 May 2018 Revised manuscript received 20 May 2019 Manuscript accepted 22 May 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 10 June 2019 Mantle earthquakes in the Himalayan collision zone Vera Schulte-Pelkum1, Gaspar Monsalve2, Anne F. Sheehan1, Peter Shearer3, Francis Wu4, and Sudhir Rajaure5 1Cooperative Institute for Research in Environmental Sciences and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA 2Facultad de Minas, Universidad Nacional de Colombia, Medellin, Colombia 3Institute for Geophysics and Planetary Physics, University of California, San Diego, La Jolla, California 92093, USA 4Department of Geology, Binghamton University, State University of New York, Binghamton, New York 13902, USA 5Department of Mines and Geology, Lainchaur, Kathmandu 44600, Nepal ABSTRACT ity located using the network (Monsalve et al., Earthquakes are known to occur beneath southern Tibet at depths up to ~95 km. Whether 2006), deep events are seen in Nepal just south these earthquakes occur within the lower crust thickened in the Himalayan collision or in of the Lesser Himalaya (Fig. 1, cluster C) and the mantle is a matter of current debate. Here we compare vertical travel paths expressed as under Tibet (Fig. 1, clusters A and B). Figure delay times between S and P arrivals for local events to delay times of P-to-S conversions from 2B shows the same set of events on a previ- the Moho in receiver functions. The method removes most of the uncertainty introduced in ous structural depth profile from receiver func- standard analysis from using velocity models for depth location and migration.
    [Show full text]
  • Geophysical Journal International
    Geophysical Journal International Geophys. J. Int. (2013) doi: 10.1093/gji/ggt004 Geophysical Journal International Advance Access published February 5, 2013 Distinguishing eclogite from peridotite: EBSD-based calculations of seismic velocities James R. Worthington,1,∗ Bradley R. Hacker1 and George Zandt2 1Department of Earth Science, University of California, Santa Barbara, California, USA. E-mail: [email protected] 2Department of Geosciences, University of Arizona, Tucson, Arizona, USA Accepted 2013 January 4. Received 2012 October 31; in original form 2012 August 3 SUMMARY Seismic velocities were calculated for 11 eclogites from the Western Gneiss Region, Norway, based on electron-backscatter diffraction (EBSD). The P-wave velocities are 8.0–8.5 km s–1 –1 and the S-wave velocities are 4.5–4.8 km s ; VP/VS1 (the ratio of P-wave to fast S-wave veloc- Downloaded from ities) is 1.74–1.81. All the eclogites are relatively isotropic, with the higher anisotropies (3– 4 per cent) in micaceous samples. Peridotite is comparatively more anisotropic (4–14 per cent more for P waves and up to 10 per cent more for S waves), and can have anomalously low VP/VS1, which may be useful means of distinguishing it from eclogite. Micaceous eclogite may be modelled using hexagonal anisotropy with a slow unique axis, whereas peridotite is most http://gji.oxfordjournals.org/ robustly modelled using orthorhombic anisotropy. Key words: Composition of the mantle; Body waves; Surface waves and free oscillations; Seismic anisotropy; Acoustic properties; Dynamics of lithosphere and mantle. per quantifies the seismic signature of eleven eclogite samples.
    [Show full text]