The Yellowstone plateau volcanic field is less than 2 million years old, lies in a region of intense tectonic and hydrothermal activity, and probably has the potential for further volcanic activity. The youngest of three volcanic cycles in the field climaxed 600,000 years ago with a voluminous ashflow eruption and the collapse of two contiguous cauldron blocks. Doming 150,000 years ago, followed by voluminous rhyolitic extrusions as recently as 70,000 years ago, and high convective heat flow at present indicate that the latest phase of volcanism may represent a new magmatic insurgence. These observations, coupled with (i) localized postglacial arcuate faulting beyond the northeast margin of the Yellowstone caldera, (ii) a major gravity low with steep bounding gradients and an amplitude regionally atypical for the elevation of the plateau, (iii) an aeromagnetic low reflecting extensive hydrothermal alteration and possibly indicating the presence of shallow material above its Curie temperature, (iv) only minor shallow seismicity within the caldera (in contrast to a high level of activity in some areas immediately outside), (v) attenuation and change of character of seismic waves crossing the caldera area, and (vi) a strong azimuthal pattern of teleseismic P-wave delays, strongly suggest that a body composed at least partly of magma underlies the region of the rhyolite plateau, including the Tertiary volcanics immediately to its northeast. The Yellowstone field represents the active end of a system of similar volcanic foci that has migrated progressively northeastward for 15 million years along the trace of the eastern Snake River Plain (8). Regional aeromagnetic patterns suggest that this course was guided by the structure of the Precambrian basement. If, as suggested by several investigators (24), the Yellowstone magma body marks a contemporary deep mantle plume, this plume, in its motion relative to the North American plate, would appear to be "navigating" along a fundamental structure in the relatively shallow and brittle lithosphere overhead. The concept that a northeastwardpropagating major crustal fracture controls the migration path of the major foci of volcanisim is at least equally favored by existing data, as Smith et al. (19) noted.

https://science.sciencemag.org/content/sci/188/4190/787.full.pdf

Magma Beneath Yellowstone National park.

The question of the structure of the Wind River uplift, a Laramide foreland structure in Wyoming, has been answered by Consortium for Continental Reflection Profiling (Cocorp) deep crustal reflection profiling and by gravity interpretation. Wyoming uplifts are asymmetrical anticlines whose steep limb is commonly cut by a reverse fault or thrust. If the thrust continues into the crust with low dip, the uplift is caused by horizontal movements; and if the fault steepens into a high-angle reverse fault in depth, the uplifts are caused by vertical movement. The Wind River uplift is the largest of the Laramide structures in Wyoming, and the thrust fault has 14 km of vertical separation and 26 km of horizontal separation. About 168 km of crustal reflection profiling has been completed by Cocorp across the uplift. The Wind River thrust fault generates a continuous reflection from near the surface to a depth of about 24 km, and the thrust appears as a complex zone of faults. Apparent dip of the thrust is 30°–38°, and true dip may be up to 48°. Interpretation of deep crustal reflections is complicated by the presence of multiple reflections. A structure whose complexity approaches that of basement outcrops is found in the deep crust. Gravity modeling also suggests that the fault dips moderately and that dense rocks are uplifted in the deep crust. The Moho does not seem to be presently displaced by faulting. The crust in the Wind River uplift began to deform by large-scale folding and then broke and faulted as a rigid slab. Faulting is caused by crustal shortening from compression that is related to plate movements.

Structure of the Laramide Wind River Uplift, Wyoming, from Cocorp deep reflection data and from gravity data

Nd and Sr isotopic compositions and Rb, Sr, Sm and Nd concentrations are reported for madupites, wyomingites and orendites from the Pleistocene volcanic field of the Leucite Hills, Wyoming. All Leucite Hills rocks have negative eNd signatures, indicating derivation or contribution from an old light rare earth element (LREE) enriched source. In this respect they are similar to all occurrences of high potassium magmas so far investigated. But Sr isotopic variations are comparatively small and 87Sr/ 86Sr ratios are unusually low for high-K magmas (0.7053–0.7061, one sample excluded). These values suggest that the light REE enrichment of the source was not accompanied by a strong increase in Rb/Sr. Wyomingites and orendites are isotopically indistinguishable which is consistent with chemical and petrographic evidence for their derivation from a common magma series depending on emplacement conditions. Basic to ultrabasic madupites and more silicic wyomingites/orendites are distinct in their Nd isotopic variations (madupites: eNd= −10.5 to −12.3; wyomingites/orendites: eNd= −13.7 to −17.0) despite similar Sm/Nd ratios and complete overlap in 87Sr/86Sr. Selective or bulk assimilation of crustal material is unlikely to have significantly affected the Nd and Sr isotopic compositions of the magmas. The measured isotopic ratios are considered to reflect source values. The distinct isotopic characteristics of madupite and wyomingite/orendite magmas preclude their derivation by fractional crystallization, from a common primary magma, by liquid immiscibility or by partial melting of a homogeneous source. Two isotopically distinct, LREE enriched and slightly heterogeneous sources are required. Heterogeneities were most pronounced between magma sources from each volcanic centre (butte or mesa). The relationship between the madupite and wyomingite/orendite sources and their evolution is discussed on the basis of two simple alternative sets of models:

/pdf/nd-and-sr-isotopes-in-ultrapotassic-volcanic-rocks-from-the-3ghzrizqq3.pdf

Nd and Sr isotopes in ultrapotassic volcanic rocks from the Leucite Hills, Wyoming

An analysis of free-air, Bouguer, Pratt-Hayford (D = 113.7 km), and Airy-Heiskanen (T = 30 km) anomalies visa vis elevation in India indicates that on an over-all basis isostatic equilibrium prevails in India. Regression study of Bouguer anomalies versus elevation made by grouping the data according to tectonic and geologic units shows that the Bouguer anomaly decreases rather slowly with the increase in elevation over the Himalayas, Aravallis, Nilgiris, Eastern Ghats and the Assam plateau, all of which are post-Mesozoic rejuvenated blocks of the Archaean shield. Small, and sometimes even positive regression coefficients characterize these blocks; the isostatic anomalies are found to be near zero or positive. It is concluded that the uplifting of these shield blocks has probably taken place through movement and incorporation of material from the upper mantle into the crust.

Relation of gravity to elevation and rejuvenation of blocks in India

Complete chemical analyses, including ferric and ferrous iron, H2O contents and dD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc, (2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite nodules entrained in potas- sic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava in the Leucite Hills, Wyoming. The Fe 2 O 3 contents in the mi- cas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe 31 per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate mineral for- mulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe 31 yFe T ,a nd overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjust- ed totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western Mexico and eastern California have the highest Fe 31 yFe T ratios (56-87%), reflecting high magmatic oxygen fugacities (DNNO 51 2t o 15), in contrast to those from Uganda (25-40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe 31 yFe T ratio in the micas. Values of K MgyFe 21 D (+ 2s errors) were calculated for three phlogo- pite-olivine pairs (0.12 + 0.12, 0.26 + 0.14, 0.09 + 0.12), two biotite-hornblende pairs (0.73 + 0.08 and 1.22 + 0.10) and a single phlogopite-augite pair (1.15 + 0.12). Values of K FyOH D for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (, 0.2 wt%) of the four phases. The dD values obtained in this study encompass a large range (2137 to 243‰). The phlogo- pite and biotite separates from Uganda have dD values of 270 to 249‰, which overlap those believed to represent "primary" mantle. There is a larger range in dD values (2137 to 243‰) for phlogopite phenocrysts from west- ern Mexico minette lavas, although their range in d 18 O values (5.2-6.2‰) is consistent with "normal" mantle. It is unlikely, therefore, that the variable dD values reflect heterogeneity in the mantle source region of the minette magmas. Nor can the extremely low dD values reflect degassing of H2 or H2O since almost 100% loss of dis- solved water in the magma is required, an unrealistic scenario given the stability of the hydrous phenocrysts. The very low dD values of the Mascota minette phlogo- pites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the pro- cess responsible for the observed hydrogen isotope com- position, it had no effect on the d 18 O value, fO2 , aH2O or bulk composition of the host magmas.

/pdf/ferric-ferrous-ratios-h-2-o-contents-and-d-h-ratios-of-19nqj3ii66.pdf

Ferric-ferrous ratios, H 2 O contents and D/H ratios of phlogopite and biotite from lavas of different tectonic regimes

A density model of the upper crust of Wyoming, computed on structural and petrographic evidence without prior reference to gravity data, when compared with the observed gravity field, demonstrates that: (1) Existing measurements and theory of change of sedimentary rock density with depth of burial are satisfactory for gravity interpretation for local rock types and the slower rates of sedimentation in the area. (2) Gravity stations established over sedimentary basins are generally as satisfactory as those on basement rock outcrops for depicting regional Bouguer gravity trends. (3) The Laramide deformation was mainly germanotype. The regional gradient in Bouguer gravity across Wyoming has increasingly negative values to the southwest. Local variation in the Bouguer anomaly field is related mainly to the low density Cretaceous and Cenozoic sediments of the basins, and to a lesser degree to the Laramide structural blocks. Interpretation of the gravity field indicates that the blocks are bounded by near‐verti...

Ralph Moberly

Papers

Effects of structure on the gravity field of Wyoming