Ma Rocks Research Articles

Gravity and Magnetic Modeling of Central Segment of Mid-Continent Rift in Iowa: New Insights into Its Stratigraphy, Structure, and Geologic History: ABSTRACT

Middle Proterozoic (approximately 1,000 Ma) rocks of the central segment of the Mid-Continent Rift (MCR) occupy an area of about 50,000 km{sup 2} of the Precambrian surface but are completely buried by Phanerozoic rocks. Gravity and magnetic modeling of the MCR, constrained by seismic reflection and well data, provides new insight into the stratigraphy, structure, and geologic history of this portion of the MCR. The MCR is dominated by the Iowa horst, a mafic volcanics-dominated axial horst reaching a maximum model thickness of at least 30 km and a maximum width of 55 km. It is segmented near its midpoint by an accommodation zone that displays about 20 km of left-stepping offset and is flanked by deep basins filled with up to 10 km of clastic rocks. The horst was apparently uplifted along high-angle reverse faults, over the clastic rock sequences, with as much as 5 km of horizontal overlap. Modeling indicates that the central horst is composed of two structurally distinct packages. The uppermost package is distinguished by remarkably continuous multiple parallel reflectors, interpreted as individual volcanic flows or groups of flows, that can frequently be traced across much of the horst. The flanking clastic-filled basins also appear tomore » be composed of two distinct lithologic packages, with the lower package displaying higher densities and more continuous reflectors than the upper. In transverse cross section, the Iowa horst and flanking basins have approximate bilateral symmetry, unlike the half-graben structure recently suggested for the western Lake Superior segment of the MCR. This symmetry suggests that at least the upper volcanic package was deposited in a uniformly subsiding graben.« less

Implications of U-Pb zircon ages for the tectonic settings of the Smartville and Slate Creek complexes, northern Sierra Nevada, California

U-Pb zircon ages combined with stratigraphic and structural data indicate that ca. 160 Ma arc rocks of the Smartville Complex were constructed across ca. 200 Ma arc rocks of the Slate Creek Complex, which were already juxtaposed against more easterly units in the northern Sierra Nevada. The Slate Creek Complex, an ophiolitic thrust sheet east of the Smartville Complex, is 200-210 Ma, which precludes previous correlations with ca. 160 Ma rocks of the Smartville Complex. The Slate Creek Complex tectonically overlies chert-rich Paleozoic-early Mesozoic units along a low-angle fault that is cut by 163-168 Ma and 158-161 Ma plutons. Seven new ages of 160-163 Ma for plutonic rocks in the Smartville Complex agree with previous ages from Smartville volcanic rocks, sheeted dikes, and other plutons. In addition, one tonalite is 198 Ma and is cut by a 163 Ma dike, thus providing the first evidence of the basement of the ca. 160 Ma rocks. The 198 Ma tonalitic basement is probably correlative with the Slate Creek Complex, which contains the only other ca. 200 Ma tonalite in the northern Sierra Nevada. Thrust emplacement of the Slate Creek Complex between 200 and 163 Ma was followed by locally extensional arc magmatismmore » in the Smartville Complex at about 160 Ma, then by Nevadan contractional deformation, a sequence of Jurassic deformation similar to that in the Klamath Mountains. Nevadan deformation could be part of a noncollisional deformation or a long-lived arc (Slate Creek Complex) collision.« less

Petrology, geochemistry and origin of a major Australian 1880-1840 Ma felsic volcano-plutonic suite: a model for intracontinental felsic magma generation

In Australia, a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 Ma, towards the end of the early Proterozoic. These volcanics and granites, covering at least 37 000 km 2, are compositionally uniform over large areas. The felsic magmas are predominantly I-type, being derived from infracrustal sources which have never been exposed to surficial weathering processes. When compared with early Archaean and Phanerozoic I-type granites, the 1880-1840 Ma suite has distinctively high levels of K 2O, Rb, Zr, La, Ce, Th, U, and low MgO and CaO contents even at relatively low SiO 2 levels of 60–65%. The 1880-1840 Ma rocks are characterized by low initial 87Sr/ 86Sr ratios, and negative ϵ Nd values. Chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle-derived sources that are around 2300 2000 Ma old. There is no evidence of a major Archaean component in these granites. The sources are interpreted to have formed as a result of polygonal small-scale mantle convection, which led to extensive underplating of Archaean crust. During this underplating event, large volumes of incompatible element-enriched mantle-derived material accreted to the base of the lower crust, and fractionated significantly. The more fractionated part of this source was remelted between 1880-1840 Ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every Proterozoic domain of Australia. The actual underplating event between 2300-2000 Ma, is believed to have triggered the formation of early Proterozoic sedimentary basins, which occur in a polygonal pattern, and appear to wrap around Archaean nuclei. The composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust: all are generated by processes of vertical accretion, in an intracontinental environment.

A Paleocene North American paleomagnetic pole incorporating declination‐Only data

A problem in determining apparent polar wander paths for the continents is how to incorporate the valid declinations from paleomagnetic studies that have measured shallowly biased inclinations (i.e. inclination error). It is proposed that in such cases pole positions and confidence limits be determined using a method previously applied only to paleomagnetic data from oceanic plates. Applying this method to paleomagnetic data from 67‐55 Ma rocks of North America, a paleomagnetic pole, 78.7°N, 185.1°E, and 95% confidence ellipse, 6.0° major semiaxis striking 105° clockwise from north and minor semiaxis of 3.7°, were determined. An analysis of the information content of the data shows that the single declination‐only datum, from the Nacimiento Formation (New Mexico), contains 31% of the information, which would have been wasted by conventional paleomagnetic analysis if excluded or caused a bias if the inclination had also been included.

Reconstruction of the early Proterozoic stratigraphy of the Gawler Craton, South Australia

Abstract Detailed structural mapping on NE Eyre Peninsula, South Australia, has led to a revised stratigraphy and model of sedimentation for Early Proterozoic metasediments of the Gawler Craton. Four stages of deformation have been recognised; three stages are associated with the Kimban Orogeny (c. 1820–1580 Ma) and a fourth stage is known as the Wartakan Event (c. 1500–1450 Ma). The recognition of major D2 folds has shown the previously used stratigraphy to be incorrect and has necessitated its revision. At the base of the sequence, unconformably overlying a 2300 Ma or older basement, is the Warrow Quartzite. A transgressive cycle of schist, dolomite (Katunga Dolomite) and iron formation (Lower Middleback Jaspilite) overlies the quartzite, and this is overlain in turn by a regressive semipelitic unit containing local amphibolites (Cook Gap Schist), and another transgressive iron‐formation bearing cycle (Upper Middleback Jaspilite). At the top of the sequence is the Yadnarie Schist. All units overlying the older basement to the top of the Yadnarie Schist are defined collectively as the Hutchison Group. The Middle‐back ‘Group’ consisting of units from the top of the Warrow Quartzite to the base of the Yadnarie Schist is redefined as the Middleback Subgroup. Sediments of the Hutchison Group were probably derived from 2300+ Ma rocks on western Eyre Peninsula and deposited on a shallow platform now oriented approximately N‐S.

Palaeomagnetic study of the (Late Precambrian) West Greenland kimberlite-lamprophyre suite: definition of the Hadrynian Track

Kimberlite and potassic lamprophyre dykes were intensively intruded into the early Proterozoic Nagssugtoqidian mobile belt of West Greenland during an important phase of brittle reactivation in Late Precambrian-Early Cambrian times (ca. 580-570 Ma) and during at least one other minor phase. Thermal and alternating field demagnetisation studies of 52 of these dykes identify primary components residing in the critical blocking temperature range distributed between shallow westerly and steep positive directions. Near the axis of the Proterozoic shear belt the dykes (predominantly lamprophyres) have closely grouped shallow directions with a reversal; near the margins of the shear belt dykes (predominantly kimberlites) have steeper and distributed directions. The cleaned components of magnetisation appear to be single, and the distribution of directions is interpreted to record a migration of the palaeofield axis which intersecting relationships show to have been from shallow to steep. The dyke directions are grouped to define representative mean palaeopoles of 215°E 3°N (LK1, A 95 = 3.9°), 213°E 18°N (LK2, A 95 = 6.1°), 203°E 46°N (LK3, A 95 = 10.4°) and 259°E 54°N (LK4, A 95 = 11.0°); a subsidiary direction recorded in five dykes near the southern margin of the shear belt (LK5, palaeopole at 297°E 16°S (A 95 = 12.5°)) is derived entirely from lamprophyres and is possibly Silurian in age. An RbSr isochron on three lamprophyres of 1227 Ma and agreement of the remanence direction with ca. 1220 Ma rocks from elsewhere in Greenland suggests that the LK1 component is wholly or partly of that age. The remaining sequence of palaeopoles falls along the Hadrynian Polar Track and the age data relating to this track are re-evaluated. Evidence for a pre-800 Ma age is no longer valid and the new data from West Greenland confirm that the track is latest Precambrian to Early Cambrian in age. It is shown to connect poles of Late Precambrian and Lower Cambrian age and to embrace other data from the Laurentian shield. The rapid passage of the shield across the South Pole is consistent with the sedimentation sequences, and suggests a high-latitude origin for the tillite horizon of this age. The Hadrynian Track is also compared with the contemporaneous record from Gondwanaland and it is shown that the two shields were in juxtaposition in the identical reconstruction to the Proterozoic Supercontinent until earliest Cambrian times. This discovery links the Lower Cambrian marine transgression and the widespread ca. 580-560 Ma alkaline province in the Gondwanaland, Laurentian and Fennoscandian shields to major continental break up, and it conforms with evidence that the Iapetus Ocean did not open until Cambrian times.