648 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



ASIA 



Fig. 40.12. Gross elments of 

 orocline-sphenochasm concept of 

 origin of Arctic basin. Simpli- 

 fied by omission of transcurrent 

 movement. Reproduced from 

 Carey (1958). 



slip fault ( Spitzbergen to Severnaya Zemlya ) . Such a tension rift he calls 

 a sphenochasm. 



3. The bending of the orogenic belts of the western American Cor- 

 dillera in Alaska is believed to be the result of the rotation of the blocks 

 which opened up the "Arctic sphenochasm." The bent segment of the 

 orogenic belts is called the "Alaskan orocline." 



4. The Lomonosov Range is believed to be a stretched-out more viscous 

 part of the crust across the Arctic sphenochasm. The concept will be 

 understood if the following model anology is considered. Quoting from 

 Carey (1958, p. 195): 



If I break a slab of toffee which is cold and brittle except for one warm 

 spot, the slab will break cleanly except at the hot spot where a thread of toffee 

 will be drawn out across the rift. The thread will be straight or curved according 

 to the path of separation. If the isotherm at which fracture passes into flow is lo- 

 cally above the Mohorovicic discontinuity sialic material will rise into the rhom- 

 bochasm along with the rising mantel material and form a thread of sial across 

 the rhombochasm. In view of the density difference it will endure permanendy 

 as a submarine ridge on the ocean floor. For such threads the name nematath 

 [from Greek meaning, thread and stretched] is proposed. In practice I find that 

 such nemataths commonly join similar igneous centres across the rhombochasm, 

 giving support to the above hypothesis of their generation. Where the transition 

 from fracture to flow is below the Mohorovicic discontinuity, no nematath 

 results even if the isotherms are higher in some places than others. 



5. The Novaya Zemlya, the Pai Khoi, and Severnaya Zemlya orogenic 

 arc segments are considered oroclines and related to the "Iceland 

 megashear." Restored to original positions they form a continuous, 

 smoothly arcuate, Hercynian orogenic belt from the Urals to the North 

 Greenland-Ellesmere orogenic belt. Norway would lie along side east 

 Greenland and the Caledonian belts of each become parts of one broader, 

 original zone. The Alaskan orocline is the hub of all movements of the 

 northern hemisphere. 



6. Paleomagnetic polar wandering is considered in light of the Alaskan 

 orocline theory, and found to conform better to it than to other proposed 

 patterns of fragmental shifts or drifting. 



An idealized bold portrayal of the fracture and drift pattern of the 

 Alaskan orocline and related features is shown in Fig. 40.12. 



Rift Theory. Heezen, in November of 1956 (p. 1703), presented a 

 paper at the meetings of the Geological Society of America in which 

 the Mid-Atlantic rise and rift zone were postulated to extend to and 

 across the Arctic basin. Since then papers by Heezen and Tharp ( 1959 ) 

 and Ewing and Heezen (1957) have appeared which elaborate more on 

 the concept. Figure 40.13 is a map supplied the writer by Dr. Heezen 

 which shows the modern seismic activity of the Arctic and a new interpre- 

 tation of the bottom topography of the Greenland Sea basin and the 

 Arctic Ocean basin on the Eurasian side of the Lomonosov Range. He 

 divides the deep basin (C) into two longitudinal parts with a gentle 

 medial rise broken by a rift valley along the active seismic zone. The rift 

 topography is born out by the sonic depth profile of the Nautilus (Fig. 

 40.7). The deep trench across Nansen's sill is along the postulated rift 

 zone, and supports the concept of rifting, but it also favors drifting. 



The gentle rise and medial rift constitute a tectonic element com- 

 patible with oceanic crust, and if similar to the Mid-Atlantic rise, we 

 must postulate the zone to be one of volcanic activity. 



Heezen postulates an expanding earth and the widening of the ocean 

 basins as a result. The broad Mid-Atlantic rise and medial rift have 

 developed progressively as the expansion occurred. 



Conflicts and Problems. Assuming that the Arctic basin is underlain 

 by oceanic crust, which seems probable, then the postulated belts across 



