Warren Manspeizer

Chapter 3 - Triassic – Jurassic rifting and opening of the Atlantic: An overview

Abstract Events leading to the breakup of the Pangean plate and evolution of the Atlantic passive margins are recorded in the rock record of more than 40 offshore and onshore Late Triassic – Early Jurassic synrift basins that formed on the Variscan – Alleghanian orogen. The record shows that rifting took place along low-angle detachment faults, giving rise to half-grabens along a conjugate set of lower and upper plate margins that are noteably asymmetric. The American plate was marked by a broad belt of marginal plateaus with many northeast-trending detrital basins that were linked to eachother by transfer faults and displaced by cross faults. The Moroccan plate, on the other hand, was marked by few broadly subsiding evaporite basins. Typically each half-graben on the American plate was bordered by a hinged margin and one major basin-bounding fault, which delineated the surface trace of synthetic or antithetic listric faults on a seaward-dipping detachment zone. The American plate (during the Late Triassic) was dominated by high relief with high-altitude fluvial-lacustrine basins along the western part of the orogen, and by low-relief sea-level evaporite basins proximal to the future spreading axis. During detachment faulting, in the Late Triassic – Early Jurassic, the lower plate must have been uplifted isostatically into a broad central arch that migrated seaward, as the load of the overlying upper plate continued to be reduced by erosion and listric faulting. This had the consequence of elevating Late Triassic marine strata that lay near the proto-Atlantic axis. During the Lias, these marine basins were eroded and their strata reworked and transported landward toward the onshore basins of Morocco and North America. The topographic reversal is thought to reflect the easterly migration of upwelling asthenosphere, in response to tectonic thinning along the newly forming margin. It was a time of major crustal thinning with development of the postrift unconformity (COST G-2 cores), and adiabatic decompression on the upwelling asthenosphere. Whereas the earliest melts yielded off-axis alkaline-rich volcanics (as in Morocco), subsequent melts, which were derived from later partial melt derivatives, were tholeiitic (as in the Palisades). As the upwelling asthenosphere migrated eastward in response to tectonic thinning, the ‘abandoned’ rift-stage crust cooled and subsided, thereby ushering in the drifting phase of the margin. The Moroccan plate, by contrast, was a broad region of low relief throughout most of the Triassic and Liassic. It was distinguished by few detrital basins, and almost all of these occurred along the South Atlas fracture zone, as Triassic strike-slip basins in the High Atlas. Except for the offshore Essaouira basin, which is a seaward extension of the High Atlas Argana basin, the Moroccan margin (unlike the American) consists of few documented Triassic – Liassic rift basins. Triassic rifting of the Middle Atlas (e.g. at Bab-Bou-Idir and Berkane) broke the orogen into the Oranian and Moroccan mesetas, and is manifested by a thick carbonate sequence. The majority of the intraplate basins of North African occur on the mesetas, and are nonrift; typically they contain nonclastic, marine and fresh water evaporites of Liassic and younger strata that formed in broad, shallow, drift-type basins on a generally subsiding terrane of low relief, near the very end of synrift time.

Separation of Morocco and eastern North America: A Triassic-Liassic stratigraphic record

New primary data from northwest Africa show that the lower Mesozoic rocks of Morocco rest with profound unconformity on Hercynian metamorphic and/or Autunian sedimentary rocks and occur in three distinct, partially synchronous volcanic-sedimentologic provinces: the Oran Meseta, the High Atlas, and the Moroccan Meseta. The Oran Meseta of northwestern Morocco contains a Middle to Late Triassic andesite and carbonate-evaporite facies related genetically to the Tethys basin. The High Atlas province of southwestern Morocco consists of Late Triassic to Early Jurassic red beds and evaporites interbedded with tholeiite lavas. These tholeiites are underlain by the Minutosaccus - Patinasporites Concurrent Range Zone of middle Carnian age and yield an average isotopic age of about 196 m.y. They are time- and rock-stratigraphic correlatives of the First and Second Watchung-York Haven suite and the Quarryville basalts of Pennsylvania and New Jersey. The Moroccan Meseta of western and central Morocco consists of Lower Jurassic (Liassic) evaporites intercalated with low-alkali quartz tholeiites, yielding an isotopic mean age of 186 ± 8 m.y. This tholeiite is a rock- and time-stratigraphic correlative of the Rossville basalt of Pennsylvania. The tectonic model best explaining the chemical and stratigraphic distribution of lower Mesozoic rock now on the margins of the North Atlantic includes the following sequence: (1) Permian to Late Triassic uplift and crustal thinning along the axis of the future Atlantic Ocean; (2) Middle to Late Triassic strike-slip faulting and andesitic volcanism along east-trending fracture zones, followed by a westward advance of the Tethys Sea across northern Morocco; (3) Late Triassic rifting along the axis of the proto-Atlantic Ocean and shearing along east-west fracture zones, which had the combined effect of decoupling segments of the African and North American plates and providing a pathway for the marine transgression of the Tethys Sea across northern Morocco and south along the axis of rifting; and (4) Late Triassic to Early Jurassic crustal extension and extrusion of olivine and quartz tholeiites, followed by extrusion of subalkalic quartz tholeiites and collapse of the continental margins with the concomitant deposition of marine carbonates.

Autunian and Carnian Palynoflorules Contribution to the Chronology and Tectonic History of the Moroccan Pre-Atlantic Borderland

Abstract A palynoflorule from the Moroccan Meseta north of Khenifra closely matches fossil pollen assemblages from the European Autunian Series, as described by Daubinger (1974) and from the Pictou Group of Nova Scotia (Barss and Haquebard, 1967). The age of these assemblages is latest Carboniferous to earliest Permian A second palynoflorule from the Central High Atlas south of Marrakech is of mid-Carnian age based upon the concurrence of age-diagnostic pollen species that have been reported from the Swiss and English middle Keuper, type Carnian of Austria, and North American Triassic beds in Virginia, North Carolina, Pennsylvania, New Jersey, Texas, New Mexico, and Arizona. These age determinations serve to document a large-scale unconformity that encompasses virtually all the Permo-Triassic on the Moroccan Meseta and High Atlas. This episode of pronounced crustal thinning preceded Triassic rifting, and ultimately led to the fragmentation of Pangaea.

A foreland-type fold and related structures in the Newark rift basin

Abstract The Jacksonwald synformal syncline at the eastern end of the Narrow Neck (connecting the Newark and Gettysburg basins) folds Carnian to Hettangian synrift strata. Like other en echelon folds in Mesozoic rift basins, the synform has characteristics of folds in the foreland of fold and thrust belts. The shallow plunging (15° towards 305°), upright synform has a subangular hinge and straight planar limbs, typical of chevron folds. Early formed clastic dikes were rotated towards the hinge, while mud cracks were stretched parallel to the hinge during buckling. An axial planar, spaced solution cleavage is well developed in siltstones and maintains a constant orientation throughout the fold, suggesting formation late in the fold history. Fibrous quartz and calcite were precipitated in veins (that consistently cut cleavage) oriented subperpendicular to the fold axis. The structural family associated with the Jacksonwald synform records shortening at a high angle to the basin border. Three sets of steeply dipping, predominantly strike-slip faults (on map to hand-sample scales) also deform the synrift strata. West-northwest-striking sinistral faults along the border and in the basin appear to be coeval with north-striking minor faults (showing both senses of shear). The early fault sets are cut by northeast-striking sinistral and dextral faults, both in and along the basin. The population of minor faults produced bulk plane strain with subhorizontal northeast shortening and northwest extension, essentially coaxial with the fold structures. A wide variety of tectonic models (including sinistral transtension, simple wrenching, and sinistral transpression) are considered which could produce the structural geometries in this part of the Narrow Neck.

A stratigraphic record from Morocco and North America of rifting, drifting and Tethyan transgressions of the Central proto-Atlantic

Abstract The stratigraphic record from 35 onshore and offshore basins, along the Central Atlantic margins of Africa and North America, document substantially different rifting and rifting histories for the formerly contiguous landmasses, Whereas both sets of basins rest with major unconformity on deeply eroded Paleozoic basements, the basins of Morocco are structurally and lithologically more varied, encompass a greater time-stratigraphic record, a have a different subsidence and orogenic history than those of North America. Comparable syn-rift basins occur only in the High Atlas, along the South Atlas F.Z. There, as in North America, Late Triassic extensional basins developed along strike-slip and re-activated thrust faults, High-standing coastal ranges with high-altitude lacustrine basins bordered narrow salt-filled grabens that lay astride the proto-Atlantic axis where they were trangressed by an arm of the Tethys Sea as far south as Georges Bank. However, in the Early Jurassic, as the thinned and cooled crust broadly subsided throughout most of Morocco and along the northwestern margin off Canada, it was widely transgressed by Teethyan waters, ushering in the drift phase of the passive margins. Subsequent orogenic uplift has exposed these terranes in Morocco, while the post-rift margin of North Anerica continues to subside and is overlain by as much s 15 km of drift strata.

Dead Sea Rift: Impact of Tectonics and Climate on Patterns of Sedimentation: ABSTRACT

The Dead Sea Rift, a classic strike-slip basin, occurs along a transform that connects the Red Sea, where sea-floor spreading is occurring, to the Taurus Mountains, where plate convergence is occurring. The rift formed primarily from left-lateral displacement of about 105 km (65 mi) since the Miocene, producing uplift and normal faulting along its shoulders. Sedimentation within the transform occurs primarily in elongate, asymmetric pull-apart basins such as the Dead Sea, as transform segments pass each other along the zone of strike slip. Pleistocene and Recent patterns of sedimentation were mapped on a scale of 1:50,000 along the west bank of the Dead Sea for a distance of 50 km (30 mi). Three sedimentologic units are recognized: an older sequence of debris flows and shallow-water fans; a medial unit of fan deltas interfingering with shallow-to-moderately deep-water lacustrine deposits; and an upper unit comprised of beach gravels, deltaic sands, and playa deposits. Their combined thickness is about 3,500 m (11,500 ft) along the western border fault, where they exhibit repetitive small-scale cyclical patterns of deposition within a general fan delta complex that prograded into the Dead Sea; there, geophysical studies show that the prograding subsea fans have been intruded by salt diapirs. Such patterns of deposition clearly are related to recurrent movement along the border faults, producing rhomb-shaped basins, high-relief topography, and a unique rift climate. As the moist air rises over the shoulders of the rift, it cools adiabatically yielding as much as 800-1,000 mm (31-39 in.) of rain per year to high discharge ephemeral streams that transport huge quantities of coarse clastics into the basin. Conversely, as the air descends into the basin, it warms adiabatically, evaporating more than 2,000 mm (80 in.) of water per year, thus causing a concomitant drop in the Dead Sea level, precipitation of evaporites, change in the base level, and progradation of fans into deeper water. End_of_Article - Last_Page 503------------

En Echelon Folds: A Case History from Jacksonwald Syncline of Foreland Folds in Newark Rift Basins of Eastern North America: ABSTRACT

The Jacksonwald syncline of the Newark rift basin, like other en echelon folds in both the onshore and offshore Triassic-Jurassic basins, has characteristics typically found in foreland fold and thrust belts. The syncline, comprising Carnian to Sinemurian strata, is a shallow plunging synform (15° toward N55°W) with an axial surface dipping 83° toward S35°W. The fold has a subangular hinge with straight planar limbs, typical of buckle folds. Axial planar, spaced solution cleavage is well developed in siltstones. Although early clastic dikes in the border fanglomerates are rotated, the cleavage is unfanned and maintains a constant orientation. Quartz and calcite, dissolved along cleavage seams, were precipitated in extensional veins oriented perpendicul r to the fold axis and to elongate mud cracks. Conjugate shear fractures near the postulated border fault strike N79°W and N5°W, and show sinistral and dextral slip, respectively. Displacement along these shears indicates that sinistral slip is 5 times greater than dextral. Along the basin margin, tectonic layer-parallel shortening by pressure solution exceeds compaction shortening by 2:1. The fold is intruded by an undeformed Sinemurian diabase dike of Rossville composition, parallel to the extension veins. The data show a consistent strain pattern, from the Upper Triassic through the Lower Jurassic, of northeast shortening and northwest extension, and suggest that en echelon folds formed along an east-trending sinistral shear couple. This stress field is consistent with dextral oblique slip along the Flemington fault, observed by Burton and Ratcliffe. End_of_Article - Last_Page 1440------------