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H. G. Wilshire

Publications and source records attributed to H. G. Wilshire.

46 records · Page 3Linked to original sources
USGS1972

Geologic setting of the Apollo 15 samples

The samples and photographs returned from the Apollo 15 site show that Hadley Delta is largely underlain by breccias whose clasts are mainly fragments of coarse-grained feldspathic rocks and nonmare-type basalt. Conspicuous sets of lineaments, visible in surface and orbital photographs of Mount Hadley and Hadley Delta, may represent systematic layering or fracture sets. The mare surface, with regolith about 5 meters thick, is underlain by two major basalt types, at least one of which has extensive lateral continuity and is exposed in the upper wall of Hadley Rille. Gradual erosional recession of the edges and filing of the interior of the rille by talus have contributed to the present cross sectional profile.

Science↗
USGS1972

Structure of Sierra Madera, Texas, as a guide to central peaks of lunar craters

Like hundreds of other lunar craters of probable impact origin, Copernicus contains central peaks presumed to expose rocks uplifted from beneath the crater floor. A possible analog of these peaks on Earth is the central uplift of the Sierra Madera cryptoexplosion structure, a probable impact scar (astrobleme) in stratified Permian and Cretaceous rocks of west Texas. The most conspicuous part of this 12-km-wide structure is its central uplift, 6 to 8 km across, in which the oldest rocks have been raised 1,200 m above their normal position. The uplift is surrounded by a structural depression, beyond which is a concentric rim containing some folds and circumferential normal faults with downthrow toward the center. The intensity of folding and faulting increases inward from the flanks of the central uplift toward a central zone about 2 km across where dips and fold plunges are near vertical or overturned. As in salt domes, the beds moved inward as well as upward to occupy their present positions, causing faulting and radial folding due to crowding in the center. Repetition by folding due to inward movement of upper beds over lower appears to have been followed by upward movement of the core of the uplift pulling away from the flanks by faulting. Minor outward-directed thrusting may have been caused by gravitational spreading of the rising dome. The structural depression surrounding the uplift resulted from tectonic thinning accompanying the inward movement of rocks in the uplift. Analogy with experimental craters and with other cryptoexplosion structures indicates that the uplift at Sierra Madera protruded into a crater (since destroyed by erosion) that was about 12 km across. Inasmuch as the rocks forming such uplifts are derived from below the crater floors, analogous peaks in lunar craters such as Copernicus may offer samples of lunar crust uplifted from distances below the crater floor on the order of one-tenth of the crater diameter.

Texas↗
USGS1972

Preliminary geologic investigation of the Apollo 15 landing site

The Apollo 15 lunar module (LM) landed at longitude 03°39'20'' E, latitude 26°26'00'' N on the mare surface of Palus Putredinis on the eastern edge of the Imbrium Basin. The site is between the Apennine Mountain front and Hadley Rille. The objectives of the mission, in order of decreasing priority, were description and sampling of three major geologic features—the Apennine Front, Hadley Rille, and the mare.

Book chapter↗
USGS1972

Preliminary geologic investigation of the Apollo 16 landing site

The Apollo 16 landing site in the lunar central highlands encompassed terra plains and adjacent mountainous areas of hilly and furrowed terra. These morphologic units, representing important terrane types in the lunar highlands, had been interpreted as volcanic on most premission geologic maps. However, it became apparent during the mission that there are indeed few or no volcanic rocks or landforms at the site but rather that the area is underlain by a wide variety of impact-generated breccias.

Book chapter↗
USGS1971

Kaersutite - A product of reaction between pargasite and basanite at Dish Hill, California

Paragasitic amphibole, occurring interstitially and as veins in peridotite inclusions in basanite, has reacted with the host basanite to form kaersutitic amphibole. The amphibole compositions vary with respect to distance from the edge of the xenolith; iron, titanium, and potassium contents are higher and magnesium, silicon, sodium, and chromium contents are lower closer to the basanite. Pargasite was exposed to the basanite when peridotite blocks broke open along amphibole veins during transport to the surface. Small amphibole fragments isolated in the basanite show the most reaction; compositional gradients in interstitial and vein amphibole are steep into peridotite inclusions where the amphibole was shielded from reaction. The compositions of amphiboles so modified have no direct bearing on high pressure fractionation trends if the amphibole is cognate, or on the bulk composition of the upper mantle if it is accidental.

California↗
USGS1971

Impact breccias in carbonate rocks, Sierra Madera, Texas

Two main types of deformational breccia occur in the Sierra Madera cryptoexplosion structure: monolithologic breccias composed of shattered rock of a single lithology and mixed breccias composed of rocks of several lithologies. Monolithologic breccias generally show no mineralogic signs of shock deformation, but a few samples are shatter-coned in a manner suggesting simultaneous formation of breccias and shatter cones. Mixed breccias, forming irregular, cross-cutting bodies, consistently contain moderately to highly shocked material, with mineralogic evidence of shock pressures of 50 kb to more than 200 kb, which, with evidence from the structural geometry of Sierra Madera and orientation of shatter cones, indicate an impact origin of the breccias. The mode of occurrence of the breccias, petrographic characteristics, and association with shock features are shared by breccias in many other cryptoexplosion structures in both carbonate and crystalline rock terranes, suggesting that such breccias have a common origin.

Texas↗
USGS1969

Mineral layering in the Twin Lakes granodiorite, Colorado

The Twin Lakes intrusion is composed mainly of coarse-grained porphyritic granodiorite, and is zoned from a felsic core to a slightly more mafic border. Steeply dipping mineral layers, typically a few inches to 5 feet thick and several tens of feet long, occur in discontinuous marginal zones as wide as 5000 feet. Four main types of layers are defined by increased abundances of orthoclase, quartz, plagioclase, and mafic minerals. The characteristic minerals of each type of layer differ markedly in size (orthoclase, average length about 10 cm; quartz, average diameter about 1 cm; plagioclase, average length .45 mm; and mafic minerals, average length .15 mm). Textural evidence from fine-grained granodiorite porphyry and deformed mafic layers indicates that the magma contained 50 to 60 volume percent suspended crystals during emplacement. Structures in the mafic layers such as size and concentration grading normal to the plane of layering, wedge layering, and cross layering superficially resemble sedimentary structures. Inspection of these structures, however, reveals a number of features that are difficult to explain by a process of sedimentation, but which are consistent with a flow sorting process accompanied by deformation. The layering probably formed by size sorting of the suspended crystals in marginal zones of the intrusion by essentially vertical shear flow during emplacement.

Colorado↗
USGS1968

Structural pattern in central uplifts of cryptoexplosion structures as typified by Sierra Madera

The pattern of deformation in central uplifts of Sierra Madera and other well-known cryptoexplosion structures indicates that inward as well as upward movement of strata formed the uplifts. This kind of movement is incompatible with structures not of impact origin with which they have been compared. The structural style of cryptoexplosion structures, together with features that suggest shock deformation, supports the belief that they are the eroded roots of impact craters.

Science↗