Geology topics
Mark D. Zoback
Publications and source records attributed to Mark D. Zoback.
Deep hydraulic fracturing stress measurements in the KTB (Germany) and Cajon Pass (USA) Scientific Drilling Projects: A summary
The determination of in situ stresses along deep vertical profiles was declared a key issue for two scientific deep drilling projects: the German Continental Deep Drilling Project (KTB) situated in northeast Bavaria in an area of complex variscian thrust tectonics and the CAJON PASS Project, located only about 4 km from the San Andreas Fault Zone In southern California. In the KTB project 7 successful hydrofrac stress measurements could be completed in a 4 km deep pilot hole between 0.8 to 3.0 km. At the Cajon Pass site a series of 23 hydrofrac stress measurements was performed in 2 wells between 0.9 to 3.5 km.
Rapid intraplate strain accumulation in the New Madrid seismic zone
Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes >8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time.
Analysis of fractures from borehole televiewer logs in a 500m deep hole at Xiaguan, Yunnan province, Southwest China
Fractures from a 500 m deep hole in the Red River fault zone were analyzed using an ultrasonic borehole televiewer. Four hundred and eighty individual fractures were identified between 19 m and 465 m depth. Fracture frequency had no apparent relation to the major stratigraphic units and did not change systematically with depth. Fracture orientation, however, did change with stratigraphic position. The borehole intersected 14 m of Cenozoic deposits, 363 m of lower Ordovician clastic sediments, and 106 m of older ultramafic intrusions. The clastic sequence was encountered again at a depth of 484 m, suggesting a large fault displacement. Fractures in the top 162 m of the sedimentary section appear randomly distributed. Below that depth, they are steeply dipping with northerly and north-westerly strikes, parallel to the major active faults in the region. Fractures in the ultramafic section strike roughly eastwest and are steeply dipping. These orientations are confined to the ultramafic section and are parallel to an older, inactive regional fault set.
A transect across the Mesozoic accretionary margin of central California
No abstract available.
New evidence on the state of stress of the San Andreas fault system
Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.
In-Situ Stress Project, Technical Report Number 3; Analysis of fractures from borehole televiewer logs in a 500 m-deep hole at Xiaguan, Yunnan Province, China
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Interpretation of hydraulic fracturing data from Xiaguan, western Yunnan, China
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In-situ stress project; Technical Report Number 6; Results of hydraulic fracturing stress measurements at Jianchuan, western Yunnan Province, China
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In-situ stress measurements at Hi Vista, California; continuation of a deep borehole profile near the San Andreas Fault
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In-situ stress project, Technical Report Number 9; Interpretation of hydraulic fracturing data from Yongping, western Yunnan, China
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Structure, porosity and stress regime of the upper oceanic crust: Sonic and ultrasonic logging of DSDP Hole 504B
The layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries. Correlation of geophysical logs and cores recovered from DSDP Hole 504B provides some insight into the physical properties which control these gradient changes. Borehole televiewer logging in Hole 504B provides a continuous image of wellbore reflectivity into the oceanic crust, revealing detailed structures not apparent otherwise, due to the low percentage of core recovery. Physical characteristics of the crustal layers 2A, 2B and 2C such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic, borehole televiewer and electrical resistivity logs. A prediction of bulk hydrated mineral content, consistent with comparison to the recovered material, suggests a change in the nature of the alteration with depth. Data from the sonic, borehole televiewer, electrical resistivity and other porosity-sensitive logs are used to calculate the variation of porosity in the crustal layers 2A, 2B and 2C. Several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing. Interpretation of these observations suggests that there may be a fundamental pattern of cooling-induced structure in the oceanic crust.
Evidence for lower crustal ductile strain localization in southern New York
Historic triangulation data have been analysed to determine whether intraplate seismicity is associated with ongoing ductile deformation in the lower crust. The model we have attempted to test is basically analogous to strain accumulation and release along plate-boundary strike-slip faults like the San Andreas Fault in California. That is, beneath an elastic-seismogenic upper crust ???20 km thick, strain is preferentially localized within ductile shear zones in the lower crust due to broad-scale plate driving forces. The localized lower-crustal ductile strain causes stress and strain to accumulate elastically in the brittle crust which is eventually released in crustal earthquakes. At greater depths, this localized shear deformation probably develops into pervasive ductile flow. Numerous geodetic measurements along the San Andreas Fault confirm that earthquakes in the brittle upper crust are produced by the release of elastic strain that results from ongoing ductile shear or slip in the lower crust1,2. We have found evidence of high rates of crustal deformation in southern New York which suggest that localized ductile shear is occurring in the lower crust. ?? 1985 Nature Publishing Group.
Orientation of in situ stresses in the oceanic crust
Two in situ measurements of principal stress directions have been made in DSDP Holes 504B, south of the Costa Rica Rift on the Nazca plate, and 597C, west of the East Pacific Rise on the Pacific plate. In both cases, the orientations of in situ principal stresses determined from borehole breakouts are consistent with the stress directions inferred from intraplate earthquakes located near the sites. ?? 1984 Nature Publishing Group.
Report on televiewer log and stress measurements in core hole USW-G1, Nevada Test Site, December 13-22, 1981
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Index of stress data for the North American and parts of the Pacific plate
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Ultrasonic borehole televiewer investigation of oceanic crustal layer 2A, Costa Rice Rift
In situ lithostratigraphy and the distribution of fracture and void zones in oceanic layer 2A were examined with an ultrasonic borehole televiewer at IPOD sites 501 and 504B on the south flank of the Costa Rice Rift. These records indicate a decrease in the size of basalt pillows with depth and a corresponding increase in fracture and void density. The observed increase in P-wave velocity with depth in this hole, as layer 2A merges into 2B, is best explained by an increase in the degree to which clays and zeolites fill fractures and voids. This also explains the order-of-magnitude drop in permeability observed in hole 504B. An increase in alteration-filling with age could explain the transition from convective to conductive heat flow and the correlative disappearance of layer 2A on the flanks of oceanic spreading centres. ?? 1982 Nature Publishing Group.