ISSN:0094-8276    

DOI:10.1029/GL015i009p00931

年代:1988

数据来源: WILEY

摘要:

The Cajon Pass Scientific Drilling Project is a broad, interdisciplinary experiment involving over two dozen principal scientists. Phase I of drilling, coring and downhole experimentation began Dec. 8, 1986 and ended April 2, 1987 with the hole at a depth of 2115 m, 82 m of core recovered and a wide range of downhole experiments successfully completed. In this paper we briefly outline the scientific motivation for the project and provide an overview of the scientific program. We also indicate some of the varied research areas in the earth sciences where data obtained in this experiment will provide unique and important insight into activein situprocesses, regional geologic structure, and rock and fluid composition and origin.

ISSN:0094-8276    

DOI:10.1029/GL015i009p00933

年代:1988

数据来源: WILEY

摘要:

The drill hole for the Cajon Pass Scientific Drilling Project was designed uniquely to accomplish specific science experiments. Special consideration was given to drilling practices for hole size, spot coring, deviation from the vertical, chemical and physical properties of the drilling fluids and pipe lubricants, monitoring mud logging and drilling parameters, and procedures for handling of cores and cuttings samples. Phase I of drilling reached a depth of 2115 m.

ISSN:0094-8276    

DOI:10.1029/GL015i009p00937

年代:1988

数据来源: WILEY

摘要:

The Cajon Pass Deep Drillhole penetrates a late Tertiary basin developed on crystalline basement in the SW Mojave Desert, 4 km from the San Andreas fault. Cores, continuous cuttings and geophysical logs from phase I show great diversity in rock types, structure, and alteration. The hole encountered Cajon formation arkoses; granodiorite/tonalite; unusual megacrystic granite and augen gneiss; granitic and pelitic gneisses with quartzite; quartzofeldspathic orthogneiss cut by sheets of hornblende gabbro; and interlayered mafic and quartzofeldspathic orthogneisses with rare calcsilicate intervals. Foliation and compositional layering have low dips throughout the column and layered gneisses contain 10‐cm‐scale recumbent folds. Faults and alteration zones bound several rock units with low apparent dips. Basement cores are typically cut by steep fractures,<1 mm wide, that contain zeolites±calcite or chlorite‐epidote. Fractures and faults decrease in abundance with

ISSN:0094-8276    

DOI:10.1029/GL015i009p00941

年代:1988

数据来源: WILEY

摘要:

The 1795 m "Arkoma" Federal 1‐26 well, 48.5 m from the Cajon Pass Deep Drillhole (CPDDH), provides additional lithologic and structural data pertinent to that project. Basement surface was encountered 158 m deeper than in the CPDDH. Rock units correlate well between the holes; the offset persists to 1128 m. Below this depth both lithologic unit thicknesses and fault zones correlate between holes on subhorizontal projections. A combination of previously unrecognized high‐angle and low‐angle faults of several ages are required to explain the structures. Blind low angle faults may be regionally impo

ISSN:0094-8276    

DOI:10.1029/GL015i009p00945

年代:1988

数据来源: WILEY

摘要:

Basement rocks exposed near the drill site to the northeast of the San Andreas fault consist of Mesozoic granitoid intrusions and amphibolite facies migmatitic gneiss. A possible tongue or lobe‐shaped pluton of granodiorite is exposed on Cleghorn Ridge, Squaw Peak and Whale Mountain. Adjacent gneiss contains marble, calcsilicate rocks, quartzite, and aluminous schist of sedimentary origin. Metagabbro and dioritic gneiss of possible igneous origin are also present. Peak metamorphic temperatures during emplacement of granodiorite are estimated to have been between 600 to 750°C at a pressure between 3‐

ISSN:0094-8276    

DOI:10.1029/GL015i009p00949

年代:1988

数据来源: WILEY

摘要:

The geologic structure near the Cajon Pass drill site is controversial. The Squaw Peak fault was previously interpreted as the edge of a thrust plate of large displacement. This may be incorrect based on the style of deformation and the presence of similar granodiorite on both sides of the fault. Alternatively, mid‐Miocene crustal extension on a west‐facing listric normal fault may have formed the basin in which the Cajon Formation was deposited. Post Miocene crustal shortening normal to the San Andreas fault may have folded the upper plate of this fault and thrust it northeastward. By this interpretation, steeply inclined beds at the drill site terminate on a fault ramp, and Squaw Peak is the corner of the initially stable block that was broken off and ramped upw

ISSN:0094-8276    

DOI:10.1029/GL015i009p00953

年代:1988

数据来源: WILEY

摘要:

Geochemical well logs recorded in the Cajon Pass Scientific drillhole provide an estimate of the elemental concentrations of Si, Al, Fe, Ca, K, Th, Ti, S, and the HREE Gd. The continuous geochemical profiles were used to define a lithostratigraphy in the cystalline basement from 501 to 1,829 m. The interval between the faulted top of basement and a thrust fault at 689 m consists of six alternating slivers of granite and granodiorite. Interlayered granitic and granodioritic units were then encountered to another fault at 1290 m, whereas gneisses of granitic to tonalitic composition were logged in the basal portion of the hole. Linear discriminant analysis of major element chemistry, as well as variations in Gd, Th, Ti, and S, were used to demonstrate that each of the major lithostratigraphic units is chemically distinct. Different trace element signatures and variable degrees of metamorphism indicate a complex thermal and hydrological history for the basement rocks encountered in the Cajon Pass well.

ISSN:0094-8276    

DOI:10.1029/GL015i009p00957

年代:1988

数据来源: WILEY

摘要:

Chemical analyses of cores from the Cajon Pass Deep Drillhole confirm vertical lithologic diversity of the basement rocks. They are metaluminous and calcalkaline; weak major element correlations with wide‐ranging SiO2suggest they are not a simple cogenetic suite. Quartz contents correlate with the thermal gradient profile, supporting conduction as the principal thermal transport mechanism. Abundances of heat producing elements are variable and somewhat lower than average crustal values. Cross‐calibration with other measures of Th, U and K are underway. Ages of surface granitoids and core 17 are late Cretaceous; other plutonic rocks appear older but late Mesozoic. Initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the h

ISSN:0094-8276    

DOI:10.1029/GL015i009p00961

年代:1988

数据来源: WILEY

摘要:

An analysis of downhole geophysical logs, and their relation to surface geological data constrain the structure in the first 700 m of the Cajon Pass scientific drillhole. Several angular folds, mapped in a small canyon 150 m to the southeast of the drillsite, are associated with a steep northeast dipping reverse fault at depth. This fault is inferred to cut the scientific drillhole in basement at 700 m, and to intercept a nearby oil‐exploration well, Arkoma Federal 1‐26, within the sediments at 350 m. The folds mapped at the surface are thought to have developed in Late Miocene to Early Pliocene time, during the Squaw Peak thrusting event. The inferred reverse fault offsets the sediment‐basement contact 180 m between the two

ISSN:0094-8276    

DOI:10.1029/GL015i009p00965

年代:1988

数据来源: WILEY