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Crustal Thickness and Geodynamics of the Western U.S. (NEW!!! Brought to You By EarthScope) Topography Gravity Seismic (Moho) Anthony R. Lowry Department of Geology, Utah State University [email protected] Heat Flow Thanks Also To: Philip Crotwell, University of South Carolina Jon Kirby, Curtin University, Perth, Australia Marta Pérez-Gussinyé, CSIC Barcelona, Spain Christine Puskas, University of Utah Joel Rackham, Utah State University Bob Smith, University of Utah Chris Swain, Curtin University, Perth, Australia 3D Viscosity • Just a bit on Isostatic Analysis • Where Geodynamics (& Rheology) come into play… • Crustal Thickness estimation from EarthScope Transportable Array (TA) seismometers • Some likely geodynamical implications of western U.S. crustal thickness… Isostasy Local Isostasy Flexural Isostasy gdz gdz column 1 column 2 Balance of vertical stress in a fluid “asthenosphere” Equilibrium of horizontal and vertical stress for an elastic plate (or “lithosphere”) over a fluid “asthenosphere” Isostatic Analysis: Uses easily available data (gravity, topography) Topography &/or Bathymetry Gravity or Geoid Parameterized by flexural rigidity ( rheology) Effective Elastic Thickness (Te) Provides information about loads ( mass flux processes) Loads Mass Flux Processes: Surface Loads Erosion Deposition Fault Displacements Volcanic Construction Subsurface Loads Thermal Variations Lithologic Variations Crustal Thickness (Lower Crustal Flow) Gravity & Topography reflect a complicated mix of all mass flux processes… But if we can separate the loads from their isostatic response, it narrows the field of candidate processes. METHOD: Using equations for observed topography h and geoid N including: the definition of surface load finite amplitude geoid calculation flexure of a thin elastic shell over a self-gravitating, viscous sphere Then search for Te (& perhaps other parameters) that minimize the difference between observed & predicted coherence Or equivalently, that minimize coherence of the load fields EXAMPLE The Tharsis Rise, Mars: Martian Topography: • Hemispheric “crustal dichotomy” • Tharsis rise ~ 5000 m elevation 20% of surface area Martian Geoid: • 2000 m Tharsis anomaly (largest in solar system!) Controversial Nature of Tharsis Rise: Volcanic Construction? ( Surface Load!) Thermal/chemical buoyancy of a single mantle plume? [e.g., Willemann & Turcotte, 1982; Solomon & Head, 1982] [e.g., Sleep & Phillips, 1979; Harder & Christensen, 1996; Harder, 2000] Probably some combination of both! “BEST ESTIMATE” (i.e., minimum load correlation) • ~ 17 km volcanic extrusives • ~ 12 km flexural deflection • Small (~ 5%) internal load Flexural Rigidity reflects rheology, and hence distributions of intraplate seismicity Te and Flow Rheology Reflects both the geotherm and composition… A Geodynamical Application: Flat Slab Subduction Many studies emphasize buoyancy of the down-going slab and/or velocity of the over-riding upper plate as controls on subduction geometry But these are poorly correlated with slab dip in South America Isostasy illuminates geodynamics… Correlation of South American flat slab subduction with high Te near the trench suggests thickness of viscous upper plate may control subduction geometry! The Problem: Recovery from synthetic data, from Pérez-Gussinyé et al. [2007]: Kirby & Swain [G3, 2008] found poor recovery where sub-sampled wavelet load fields exhibit chance correlation Possible Solution: Use seismic data to independently constrain internal mass fields E.g., EARS receiver function estimates of crustal thickness [Crotwell & Owens, SRL, 2005] http://www.seis.sc.edu/ears/index.html EarthScope sampling is promising, but Receiver function compilations such as EARS are prone to outliers & errors Receiver Function Estimates of Crustal Thickness: Delay Time P Ps Crust Mantle P Ps • Deconvolve source-time function to get impulse response of phases converted at impedance boundaries • Delay time between phase arrivals depends on crustal thickness and P- & S-velocity • EARS uses iterative time-domain deconvolution [Ligorria & Ammon, BSSA, 1999]: well-suited to automation Contribution of crustal thickness H versus Vp/Vs ratio K to delay time is ambiguous… P Ps PpPs PpSs PsPs PpSs PsPs P Ps PpPs Resolve using reverberations, which have differing sensitivity to H and K H–K Stacking: [Zhu & Kanamori, JGR, 2000] Ps PpSs & H-K parameters that predict the observed phase delay times intersect at a point in parameter space PsPs PpPs PpSs PsPs P Ps PpPs H–K Stacking: [Zhu & Kanamori, JGR, 2000] (EARS H–K stack for station COR) Ps PpSs & PsPs PpPs P Ps PpPs Method stacks observed amplitudes at delay times predicted for each phase, for all earthquakes. Max stack amplitude should reveal PpSs true crustal PsPs thickness & Vp/Vs ratio. The Problem: The Moho is not the only lithospheric impedance contrast… And crustal thickness is not constant (EARS H–K stack for station Y35) EARS can yield extreme crustal thickness or unlikely changes over short distances. Crustal Thickness H Vp/Vs Ratio K Despite outliers, H & K have properties consistent with a fractal surface… Station TA.P10A (Central Nevada) 2 1 The semivariance properties can be used to estimate a “most likely” crustal thickness and Vp/Vs ratio by optimal interpolation from nearby sites. Station TA.P10A (Central Nevada) Can also model gravity predicted by estimates… 2 1 1 2 And find a “most likely” model with uncertainties. Station TA.S16A (Central Utah) Gravity Model Likelihood Filter Optimal Interp. Likelihood Filter Combined Station TA.P10A (Central Nevada) Unlikely stack amplitude maxima are downweighted using likelihood statistics Method removes outliers, provides a map that makes sense (with some exceptions) The reliable (i.e. western) parts of the map confirm some alreadyknown aspects of western U.S. geodynamics & also suggest a few new interpretations… EARS-derived Old refraction data The reliable (i.e. western) parts of the map confirm some alreadyknown aspects of western U.S. geodynamics & also suggest a few new interpretations… Raw EARS estimates Weighted stack estimates And interesting patterns show up in Vp/Vs Perhaps the most intriguing implications arise from the gravity modeling however…