176 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Merrimack Synclinorium. East of the Bronson Hill anticline and 

 northwest of the Rockingham anticlinorium is a large area of Littleton 

 formation, all in the sillimanite zone of metamorphism. Inasmuch as this 

 band of the Littleton formation is bordered on either side by older strata, 

 it must occupy a synclinorium. This structural feature is called the Mer- 

 rimack synclinorium, because much of it is drained by the Merrimack 

 River and its tributaries. 



Throughout much of western New Hampshire the western limb of the 

 Merrimack synclinorium is invaded by large bodies of the New Hamp- 

 shire plutonic series. These relations are well shown on sections A-A' and 

 B-B'-B" of Fig. 11.26. 



Thrust Faults. The Ammonoosuc thrust is marked generally by Am- 

 monoosuc volcanics being thrust over the Littleton formation with a 

 stratigraphic displacement of 7000 feet. The fault dips from 32 to 50 

 degrees westerly. It is younger than the regional metamorphism. 



The Northey Hill thrust predates the metamorphism because there is 

 no break in grade of metamorphism across it. This feature renders recog- 

 nition of the fault a little difficult, yet mapping shows the Littleton forma- 

 tion lies in contact with several different formations along it, and a 

 maximum stratigraphic displacement of 12,000 feet may be measured. A 

 steep dip characterizes much of its length, and this is believed due to 

 later deformation. 



The Monroe thrust is about as long as the Ammonoosuc (85 miles). 

 It is nearly vertical throughout most of its length, but in places dips 

 southeasterly. It is mostly older than the regional metamorphism, but later 

 deformation steepened it and also caused some renewed movements 

 along it. 



Magma Series 



Plutonic rocks are abundant and varied in form and composition. Four 

 magma series have been worked out (Billings, 1937). The oldest is known 

 as the Highlandcroft magma series and is probably of late Ordovician 

 age. See chart, Fig. 11.21 and map, Fig. 11.20. Some time after the Lower 

 Devonian, probably in Mid- and Late Devonian time, other large 



quantities of magma invaded the region. The Oliverian magma 

 series preceded the folding and was followed by the New Hampshire 

 magma series, the earlier members of which were contemporaneous with 

 the main period of folding, and the later members of which were slightly 

 younger than the folding. The White Mountain magma series is the 

 youngest of the plutonic rocks, and it appears less extensive than the 

 others. It is probably early Mississippian in age (Billings, 1945). 



The Highlandcroft magma series is represented by the Highlandcroft 

 granodiorite and small bodies of diorite, quartz diorite, and quartz 

 monzonite. The Oliverian magma series is represented by the pink Owls 

 Head granite in the Littleton area and by other units in the Rumney, Mt. 

 Cube, and Mascoma quadrangles. Many sills in the Ammonoosuc vol- 

 canics are of this series. 



The White Mountain magma series is characterized by ring-dikes, 

 stocks, a batholith, and by eruptive differentiates. According to Billings, 

 1945: 



Much of the magma of the White Mountain magma series was erupted on 

 the surfaces to from the Moat volcanics. Tuffs, breccias, and lavas, composed 

 chiefly of rhyolite, andesite, and basalt, but also including some trachyte, are 

 typical. Rhyolite is by far the most common; trachyte is rare. 



The intrusive rocks range in composition from gabbro to granite, and a great 

 variety of intermediate types are developed. Chapman and Williams, in a 

 careful, detailed study, have shown that the mafic rocks are the oldest and the 

 felsic are the youngest. They have also determined the areal extent of the 

 plutonic rocks and calculated the percentage of each compared to the whole 

 magma series. The order of intrusion, from oldest to youngest, and the percent- 

 age of each as exposed at the surface, are gabbro, norite, diorite, and quartz 

 diorite (0.5 per cent); monzodiorite and monzonite (1.5 per cent); syenite, in- 

 cluding some nepheline-sodalite syenite (9 per cent); quartz syenite (10 per 

 cent); granite and granite porphyry (79 per cent). Although the rocks in gen- 

 eral became more siliceous as differentiation progressed, this is not true in 

 detail. Especially important is the fact that the Albany quartz syenite is younger 

 than the granite porphyry. This is significant in considering the tectonic evolu- 

 tion of the area. 



Chapman and Williams have also shown that fractional crystallization con- 

 trolled the evolution of the series, but that abyssal assimilation played an im- 

 portant role. 



The Moat volcanics, in large part contemporaneous with the granite por- 

 phyry, are older than the Albany type of quartz syenite, but their age relative 

 to the more mafic plutonic rocks is uncertain. 



